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Award type：Award from publisher, newspaper, foundation, etc.  Country：Japan

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Award type：Award from publisher, newspaper, foundation, etc.  Country：Japan

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Award type：Award from publisher, newspaper, foundation, etc.  Country：Japan

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Award type：Award from Japanese society, conference, symposium, etc.  Country：Japan

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Award type：Award from publisher, newspaper, foundation, etc.  Country：Japan

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Award type：Award from publisher, newspaper, foundation, etc.  Country：Japan

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Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Publisher：2024 IEEE International Magnetic Conference - Short Papers, INTERMAG Short Papers 2024 - Proceedings  

We present the research on the fabrication of RuO2 and the change in dynamic magnetic properties of RuO2/Co-Fe-B stack films with various Co-Fe-B thicknesses (tCFB). The damping constant (α) and effective magnetization (4π Ms,eff) in these stacking films were evaluated by using the broadband ferromagnetic resonance (FMR) measurement technique. 4π Ms,eff decreases with the decrease of tCFB, which might be attributed to the increase of the interfacial anisotropy energy. α increases with the decrease of tCFB which might be attributed to contribution of the spin pumping, and/or the magnetic inhomogeneity in the stacking films.

DOI： 10.1109/INTERMAGShortPapers61879.2024.10576749

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.jpcc.2c08982

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Authorship：Lead author,　Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

File： 1_2023_JJAP_62_SG0811_invited.pdf

DOI： 10.35848/1347-4065/accaed

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DOI： 10.35848/1347-4065/acb062

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DOI： 10.1103/PhysRevMaterials.7.014405

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Language：English   Publishing type：Research paper (scientific journal)  

Authorship：Lead author,　Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/UT49729.2023.10103425

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Language：English   Publisher：The Japan Society of Vacuum and Surface Science  

<p>Introduction</p><p>(La,Sr)MnO3 (LSMO), a perovskite manganese oxide has been drawing significant interest in its potential applications as a next generation spintronic material owing to its ferromagnetic and half-metallic properties at room temperature. Along this purpose, recent studies on (La,Sr)(Mn,Ru)O3 (LSMRO) thin films, where about 10% of Mn at the B site of LSMO is substituted with Ru, have been one of the attempts to further modulate the properties of LSMO. These films, different from the parent LSMO, exhibit perpendicular magnetic anisotropy while maintaining room-temperature ferromagnetism ^[1]. On the other hand, in the research field of magnetic metal alloys, compositionally-graded structures, where the composition continuously varies from the interior to surface of a material, have been extensively investigated to lead to the discovery of unique magnetic properties that had never been found in the corresponding homogeneous compositions ^[2]. However, there are few reports on the fabrication of such graded-composition structures for magnetic oxides such as LSMO. This study thus focuses on the fabrication of compositionally-graded epitaxial thin films of LSMRO and investigates possible influences of the graded structures on the magnetic properties.</p><p></p><p>Experimental section</p><p>All thin films, with their thickness of 30 nm, were fabricated on (LaAlO₃)_0.3-(SrAl_0.5Ta_0.5O₃)_0.7 (LSAT) (001) substrates, employing pulsed laser deposition with a rapid beam deflection (RBD-PLD: Pascal co.) system [3]. The Ru composition and its gradient structure were controlled by nanoscale alternating ablation of two sintered ceramics targets of LSMO and Ru15%: LSMRO. The oxygen partial pressure was maintained to be 200 mTorr throughout this process, while the substrate temperature was set to 675 °C. The laser conditions were set to a fluence of 0.6 J cm⁻² and a repetition rate of 2 Hz, respectively.</p><p></p><p>Results and discussion</p><p>A compositionally-graded Ru-LSMO epitaxial thin film, with its Ru-gradient ranging from 0% to 15% along the growing direction (referred to as “UP-graded film”), was confirmed to coherently grow in the (001) orientation on an LSAT (001) substrate. From reciprocal space mapping measurement (Fig. (a)), it exhibited that the peak shape extended over the peak positions of homogeneous Ru 0% and 15% epitaxial thin films, a distinct peak which is characteristic of the compositionally-graded structure ^[4]. SIMS measurements also confirmed successful implementation of the designed compositional gradient structure. <i>M-H </i>measurements at 100 K (Fig. (b)) revealed that the introduction of Ru substitution, irrespective of whether it was in the homogeneous or gradient mode, resulted in the disappearance of magnetic anisotropy and the enhancement in coercivity. However, there were also found some different magnetic behaviors between them. In the homogeneous films, the coercive force more increased with higher Ru%, while the saturation magnetization showed a monotonic decrease: 3.6 μB/u.c. for Ru 0%, 3.2 μB/u.c. for Ru 7.5%, and 2.7 μB/u.c. for Ru 15%. In contrast, UP-graded film still maintained a high saturation magnetization value of 3.6 μB/u.c., which was very close to that of the Ru 0% film, and its coercive force values were 150 Oe (in-plane) and 250 Oe (out-of-plane), as similarly observed in the Ru 7.5% homogeneous film (Note: the Ru composition was set to the same as the average one in UP-graded film for comparison). From these results, the emphasis should be placed on such unique magnetic properties of UP-graded film distinct from those of the homogeneous Ru-substituted films.</p><p>View PDF for the rest of the abstract.</p>

DOI： 10.14886/jvss.2023.0_1ga01

CiNii Research

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

The influence of the sidewall damage on the thermal stability factor (Δ) of quad-interface magnetic tunnel junctions (quad-MTJs) was investigated through a string method-based micromagnetic simulation. The quad-MTJs consist of a reference layer/MgO-barrier/CoFeB/middle-MgO/CoFeB/MgO-cap, which has four CoFeB/MgO interfaces to enhance the interfacial perpendicular magnetic anisotropy for large Δ. Experimentally obtained magnetic parameters at room temperature [e.g., saturation magnetization ( M_s), stiffness constant ( A_s), interfacial perpendicular magnetic anisotropy constants ( K_i), and exchange coupling ( J_ex)] in blanket multilayer films of the quad-MTJs were used in micromagnetic simulation. The influence of the sidewall damage on the quad-MTJs, which is difficult to be analyzed in the experimental way, was investigated. The quad-MTJs without damaged layers having relatively higher K_i show the split of the energy barrier into two, resulting in a decrease in Δ. When the decrease in magnetic anisotropy energy ( E_ani) is more than the increase in the static magnetic energy ( E_sta), the antiferromagnetically (AF) coupled state of two free layers is formed at the midpoint to minimize the total energy ( E_all). This causes the split of the energy barrier. The sidewall damage plays a role in lowering K_i in each layer, consequently avoiding the formation of the AF state. Note that the value of Δ with the sidewall damage, which shows the unified energy barrier, is comparable to non-damaged Δ, which shows the split of the energy barrier; these quad-MTJs have the same volume of free layers.

File： 5.0112741.pdf

DOI： 10.1063/5.0112741

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DOI： 10.1109/IEDM45625.2022.10019412

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

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Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

Graphene on [Formula: see text]-FePd(001), which has been experimentally studied in recent years, is a heterogeneous interface with a significant lattice symmetry mismatch between the honeycomb structure of graphene and tetragonal alloy surface. In this work, we report on the density functional study of its atomic-scale configurations, electronic and magnetic properties, and adsorption mechanism, which have not been well understood in previous experimental studies. We propose various atomic-scale models, including simple nontwisted and low-strain twisted interfaces, and analyze their energetical stability by performing structural optimizations using the van der Waals interactions of both DFT-D2 and optB86b-vdW functionals. The binding energy of the most stable structure reached [Formula: see text] eV/atom for DFT-D2 ([Formula: see text] eV/atom for optB86b-vdW). The calculated FePd-graphene spacing distance was approximately 2 Å, which successfully reproduced the experimental value. We also find out characteristic behaviors: the modulation of [Formula: see text]-bands, the suppression of the site-dependence of adsorption energy, and the rise of moiré-like corrugated buckling. In addition, our atomic structure is expected to help build low-cost computational models for investigating the physical properties of [Formula: see text] alloys/two-dimensional interfaces.

DOI： 10.1063/5.0101703

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/TMAG.2022.3151562

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A crystallographically heterogeneous interface was fabricated by growing hexagonal graphene (Gr) using chemical vapor deposition (CVD) on a tetragonal FePd epitaxial film grown by magnetron sputtering. FePd was alternately arranged with Fe and Pd in the vertical direction, and the outermost surface atom was identified primarily as Fe rather than Pd. This means that FePd has a high degree of L10-ordering, and the outermost Fe bonds to the carbon of Gr at the interface. When Gr is grown by CVD, the crystal orientation of hexagonal Gr toward tetragonal L10-FePd selects an energetically stable structure based on the van der Waals (vdW) force. The atomic relationship of Gr/L10-FePd, which is an energetically stable interface, was unveiled theoretically and experimentally. The Gr armchair axis was parallel to FePd [100]L10, where Gr was under a small strain by chemical bonding. Focusing on the interatomic distance between the Gr and FePd layers, the distance was theoretically and experimentally determined to be approximately 0.2 nm. This shorter distance (≈0.2 nm) can be explained by the chemisorption-type vdW force of strong orbital hybridization, rather than the longer distance (≈0.38 nm) of the physisorption-type vdW force. Notably, depth-resolved X-ray magnetic circular dichroism analyses revealed that the orbital magnetic moment (Ml) of Fe in FePd emerged at the Gr/FePd interface (@inner FePd: Ml = 0.16 μB → @Gr/FePd interface: Ml = 0.32 μB). This interfacially enhanced Ml showed obvious anisotropy in the perpendicular direction, which contributed to interfacial perpendicular magnetic anisotropy (IPMA). Moreover, the interfacially enhanced Ml and interfacially enhanced electron density exhibited robustness. It is considered that the shortening of the interatomic distance produces a robust high electron density at the interface, resulting in a chemisorption-type vdW force and orbital hybridization. Eventually, the robust interfacial anisotropic Ml emerged at the crystallographically heterogeneous Gr/L10-FePd interface. From a practical viewpoint, IPMA is useful because it can be incorporated into the large bulk perpendicular magnetic anisotropy (PMA) of L10-FePd. A micromagnetic simulation assuming both PMA and IPMA predicted that perpendicularly magnetized magnetic tunnel junctions (p-MTJs) using Gr/L10-FePd could realize 10-year data retention in a small recording layer with a circular diameter and thickness of 10 and 2 nm, respectively. We unveiled the energetically stable atomic structure in the crystallographically heterogeneous interface, discovered the emergence of the robust IPMA, and predicted that the Gr/L10-FePd p-MTJ is significant for high-density X nm generation magnetic random-access memory (MRAM) applications.

File： 1_2022_ACSNano_16_4139_Gr-FePd.pdf

DOI： 10.1021/acsnano.1c09843

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

Dynamic magnetic properties of Ta-O/Co₂₀Fe₆₀B₂₀ bilayer films are strongly influenced by the oxidation condition of the Ta-O layer. The oxidation of the Ta-O layer by a slight amount of oxygen with a pressure ( P_Oxygen) of 0.03 Pa decreases in-plane damping constant ( α_IP), and increases the effective magnetization (4π M_s,eff). Then, both α_IP and 4π M_s,eff maintain their values by increasing P_Oxygen up to 0.3 Pa. The out-of-plane damping constant ( α_OP) showed a similar tendency to that of α_IP against P_Oxygen, although α_OP is much smaller than α_IP in every P_Oxygen. α_OP reaches to 0.0033 for sample oxidized at 0.03 Pa. It was suggested that α_IP consists of both the intrinsic damping and the extrinsic damping, while α_OP is closer to the intrinsic damping. The control of α_OP and α_IP by the oxidation would be beneficial in designing the high frequency spintronic devices.

File： 2022_AIP_Adv_12_035133_SOT.pdf

DOI： 10.1063/9.0000297

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/TMAG.2021.3083575

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/TMAG.2021.3078710

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Language：English   Publishing type：Research paper (scientific journal)  

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

File： 2_2021_ACS_ApplElectronMater_3_4836_BFO.pdf

DOI： 10.1021/acsaelm.1c00688

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File： 2_2021_APR_8_031307_hBN.pdf

DOI： 10.1063/5.0049792

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Authorship：Lead author   Language：English   Publishing type：Research paper (international conference proceedings)  

Advanced quad-interfaces perpendicular magnetic tunnel junction (Quad-MTJ) was developed by engineering a low effective damping constant (αeff) material in free layer with high perpendicular magnetic anisotropy (PMA), and low resistance area product (RA) in MgO layers, and stable reference layer. The advanced 18 nm Quad-MTJ fabricated by the developed low-damage 300 mm fabrication process exhibited following performances over those of Double-MTJ; (a) 1.77 times larger thermal stability factor (Δ), (b) 0.83 times smaller writing current (Ic) at 10 ns, (c) 2.1 times higher write efficiency (Δ/Ic) at 10 ns. Thanks to the above excellent MTJ stack design, it is the first time beyond 2X nm generation that the advanced 18 nm Quad-MTJ achieves at least 6×1011 endurance with 10 years retention. Consequently, the advanced Quad-MTJ technologies have broken out the dilemma issue of retention and endurance even under scaling of 2X nm.

File： 1_2021_VLSI2021_Techonology_T12-4.pdf

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Language：English   Publishing type：Research paper (scientific journal)  

We successfully developed 25-nm quad CoFeB/MgO-interfaces perpendicular magnetic tunnel junction (quad-MTJ) with enough thermal stability. To fabricate the quad-MTJ, a physical vapor deposition (PVD) process for depositing novel free layer and low resistance-area (RA) product MgO layer and low-damage fabrication processes were developed. The developed quad-MTJ technology and advanced process bring better tunnel magneto resistance (TMR) ratio and RA to quad-MTJ than those of double-interface MTJ (double-MTJ), even though quad-MTJ has an additional MgO layer. Scaling down the MTJ size to 25 nm, we demonstrated the advantages of quad-MTJ compared with double-MTJ as follows: 1) two times larger thermal stability factor ( $\Delta $ ), which results in over ten years retention; 2) superiority of large $\Delta $ in the measuring temperature range up to 200 °C; 3) 1.5 times higher write efficiency; 4) lower write current at short write pulse regions at less than 100 ns; and e) excellent endurance of over 1011 thanks to higher write efficiency, which results from the reduced voltage owing to low RA and the low damage integration process technology. As a result, the developed quad-MTJ technologies will open the way for the realization of high-density STT-MRAM with low power, high speed, high reliability, and excellent scalability down to $2\times $ nm node.

File： 09422110.pdf

DOI： 10.1109/TED.2021.3074103

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Language：Japanese   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2021.1.0_1264

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

We investigated the effects of surface modification treatment (SMT) on the perpendicular magnetic anisotropy (PMA) and the thermal tolerance of top-pinned magnetic tunnel junctions (MTJs) with a Co/Pt synthetic ferrimagnetic coupling reference layer. Applying an SMT to the bottom Pt layer increased the PMA of the overlying Co/Pt multilayer. X-ray diffraction spectrum analysis revealed that the SMT resulted in a higher crystallinity and smaller lattice spacing in the Co/Pt multilayer in the thinner bottom Pt layer, which may have increased the PMA in the Co/Pt multilayer. The tunnel magnetoresistance (TMR) ratio of the MTJ with SMT increased as the annealing temperature was increased up to 400 °C; conversely, the TMR ratio of the MTJ without SMT decreased at an annealing temperature of 400 °C. Evaluation of the m-H loops revealed that, after annealing at 400 °C, the reference layers in the MTJs after SMT possessed better magnetic properties than those in the MTJs without an SMT; this is attributable to the higher PMA of the reference layers with SMT. EDX line analysis revealed that SMT suppressed Pt diffusion to the MgO barrier, resulting in a higher thermal tolerance and larger PMA of the reference layer.

File： 2_2021_AIP Advances 11_025211_MRAM.pdf

DOI： 10.1063/9.0000047

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File： 2011_AIP Advances_11_ 025231_MRAM.pdf

DOI： 10.1063/9.0000048

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File： 2_2021_JAP_129_034101_BFO.pdf

DOI： 10.1063/5.0032428

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Language：English   Publishing type：Research paper (scientific journal)  

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We fabricated a quadruple-interface perpendicular magnetic tunnel junction (MTJ) (Quad-MTJ) down to 33 nm using physical vapor-deposition, reactive ion etching, and damage-control integration process technologies that we developed under a 300-mm process. We demonstrated the greater scalability and higher writing speed of Quad-MTJ compared with double-interface perpendicular MTJ: 1) it has twice the thermal stability factor-1X nm Quad-MTJ can achieve 10 years retention-while maintaining a low resistance-area product and high tunnel magnetoresistance ratio; 2) smaller overdrive ratio of write voltage to obtain a sufficiently low write-error rate; 2) smaller pulsewidth dependence of the switching current; and 4) more than double the write efficiency at 10-ns write operation down to 33-nm MTJ. The effective suppression of the switching current increase for higher write speeds was explained by the spin-transfer-torque model using the Fokker-Planck equation. Our 33-nm Quad-MTJ also achieved excellent endurance (at least 10(11)) owing to its higher write efficiency and low-damage integration-process technology. It is thus a promising method for low power, high speed, and reliable STT-MRAM with excellent scalability down to the 1X nm node.

File： 2020_IEEE_TED_67_5368_STT-MRAM.pdf

DOI： 10.1109/TED.2020.3025749

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

File： 2_2020_AEM.pdf

DOI： 10.1021/acsaelm.0c00718

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Publisher：IEEE Transactions on Magnetics  

We investigated the influence of hard mask (HM) materials on tunnel magnetoresistance (TMR) properties and magnetic properties for the magnetic tunnel junctions with pependicular easy axis (p-MTJs) with double CoFeB/MgO interface annealed at 400 °C for 0.5-10 h. After annealing at 400 °C for 5 h, the TMR ratio of the MTJ with TiN HM was significantly decreased, whereas the TMR ratio of the MTJ with Ta-HM was maintained at a value of more than 120%. The analysis results by secondary-ion mass spectrometry (SIMS) and energy-dispersive X-ray spectroscopy (EDX) revealed that Ru in Ru-cap for the MTJ with TiN-HM diffused into the free layer through the MgO-cap layer, resulting in the degradation of the magnetic and TMR properties of the MTJ with TiN-HM. In contrast, Ru did not diffuse into the free layer for the MTJ with Ta-HM, which results from suppression of Ru diffusion into the free layer due to the formation of Ta-Ru alloy. Furthermore, in both the MTJs with TiN-HM and Ta-HM annealed at 400 °C for 10 h, EDX analysis revealed Pt diffusion into the CoFeB reference layer, which could degrade the magnetic properties of the reference layer, resulting in the degradation of TMR properties. To obtain a double CoFeB/MgO interface p-MTJ with higher thermal tolerance, it is necessary to design an HM material that suppresses the diffusion of the cap material into the free layer in addition to suppressing the Pt diffusion of the reference layer.

DOI： 10.1109/TMAG.2020.3004576

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DOI： 10.1063/5.0007499

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File： 2020_AIP_10_075106.pdf

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File： 2020_JAP_Growht mechanism and domain structure on epitaxial BFO film grown on LSAT_JAP.pdf

DOI： 10.1063/5.0005672

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

We have firstly fabricated quad-interface perpendicular MTJ (Quad-MTJ) down to 33 nm with our developed PVD, RIE and damage control integration process technologies under 300 mm process. Secondly, we demonstrated scalability merit as well as high speed writing of Quad-MTJ compared with double-interface p-MTJ (Double-MTJ) as follows
(a) two times larger thermal stability factor δ(1X nm Quad- MTJ is extrapolated to achieve 10 years retention.), (b) lower write voltage at short write pulse regions at less than 30 ns, (c) in scaled MTJ, effective suppression of write current increase for higher write speed, (d) more than 2 times higher write efficiency at 10ns write operation down to 33 nm MTJ. Finally, we revealed that our developed 33 nm Quad-MTJ achieve excellent endurance of more 1011 thanks to higher write efficiency and low damage integration process technology. These results show that the Quad-MTJ technology is one of promising way for low power, high speed and enough reliable STT -MRAM with excellent scalability down to 1X nm node.

DOI： 10.1109/VLSITechnology18217.2020.9265104

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File： 09123981.pdf

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：{AIP} Publishing  

File： 1_2020_APL_116, 173101_FePdonGF.pdf

DOI： 10.1063/1.5143567

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Language：Japanese   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2020.1.0_1253

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

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Lattice mismatch-induced biaxial strain effect on the crystal structure and growth mechanism is investigated for the BiFeO3 films grown on La0.6Sr0.4MnO3/SrTiO3 and YAlO3 substrates. Nano-beam electron diffraction, structure factor calculation and x-ray reciprocal space mapping unambiguously confirm that the crystal structure within both of the BiFeO3 thin films is rhombohedral by showing the rhombohedral signature Bragg's reflections. Further investigation with atomic resolution scanning transmission electron microscopy reveals that while the ~1.0% of the lattice mismatch found in the BiFeO3 grown on La0.6Sr0.4MnO3/SrTiO3 is exerted as biaxial in-plane compressive strain with atomistically coherent interface, the ~6.8% of the lattice mismatch found in the BiFeO3 grown on YAlO3 is relaxed at the interface by introducing dislocations. The present result demonstrates the importance of: (1) identification of the epitaxial relationship between BFO and its substrate material to quantitatively evaluate the amount of the lattice strain within BFO film and (2) the atomistically coherent BFO/substrate interface for the lattice mismatch to exert the lattice strain.

File： 2_2019_Scientific_Reports_9_6715_BFO.pdf

DOI： 10.1038/s41598-019-42998-x

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Language：English   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2019.1.0_1215

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DOI： 10.7567/JJAP.57.11UF10

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Language：English   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2018.2.0_1305

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DOI： 10.11470/jsapmeeting.2018.2.0_1335

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DOI： 10.7567/JJAP.57.0902BC

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DOI： 10.7567/JJAP.57.090201

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DOI： 10.7567/JJAP.57.0902A5

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Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of Visualized Experiments  

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LAO thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. Although electrostatic mechanisms have been proposed in the past to describe the existence of this critical thickness, the importance of chemical defects has been recently accentuated. Here, we describe the growth of metal/LAO/STO heterostructures in an ultra-high vacuum (UHV) cluster system combining pulsed laser deposition (to grow the LAO), magnetron sputtering (to grow the metal) and X-ray photoelectron spectroscopy (XPS). We study step by step the formation and evolution of the q2DES and the chemical interactions that occur between the metal and the LAO/STO. Additionally, magnetotransport experiments elucidate on the transport and electronic properties of the q2DES. This systematic work not only demonstrates a way to study the electrostatic and chemical interplay between the q2DES and its environment, but also unlocks the possibility to couple multifunctional capping layers with the rich physics observed in two-dimensional electron systems, allowing the fabrication of new types of devices.

DOI： 10.3791/56951

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Nature Publishing Group  

Comprehensive crystal structural study is performed for BiFeO3 (BFO) film grown on KTaO3 (KTO) substrate using transmission electron microscopy (TEM) and x-ray diffraction (XRD). Nano-beam electron diffraction (NBED) combined with structure factor calculation and high resolution TEM images clearly reveal that the crystal structure within BFO thin film is rhombohedral BFO, i.e., bulk BFO phase. Epitaxial relationship found by NBED indicates the BFO film grows in a manner that minimizes lattice mismatch with KTO. It further suggests BFO film is under slight biaxial tensile stress (~0.35%) along in-plane direction. XRD reveals BFO lattice is under compressive stress (~1.6%), along out-of-plane direction as a result of the biaxial tensile strain applied along in-plane direction. This leads to Poisson's ratio of ~0.68. In addition, we demonstrate (1) why hexagonal notation rather than pseudocubic one is required for accurate BFO phase evaluation and (2) a new XRD method that shows how rhombohedral BFO can readily be identified among other phases by measuring a rhombohedral specific Bragg's reflection.

File： 2_2018_SciRep_8_893_BFO構造 on KTO.pdf

DOI： 10.1038/s41598-018-19487-8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Institute of Physics Inc.  

We present a time-resolved study of the magnetization dynamics in a microstructured Cr|Heusler|Pt waveguide driven by the spin-Hall-effect and the spin-transfer-torque effect via short current pulses. In particular, we focus on the determination of the threshold current at which the spin-wave damping is compensated. We have developed an alternative method based on the temporal evolution of the magnon density at the beginning of an applied current pulse at which the magnon density deviates from the thermal level. Since this method does not depend on the signal-to-noise ratio, it allows for a robust and reliable determination of the threshold current which is important for the characterization of any future application based on the spin-transfer-torque effect.

DOI： 10.1063/1.5011721

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)  

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

We investigate the amplification of externally excited spin-waves via the spin-transfer-torque (STT) effect in combination with the spin-Hall effect (SHE) resulting from short current pulses. In the case of overcompensation of the spin-wave damping, a strong nonlinear shift of the spin-wave frequency spectrum occurs. In particular, this shift limits spin-wave amplification from the SHE-STT effect. However, it allows for the realization of a spin-wave switch. At the corresponding working point, efficient spin-wave excitation is possible only in the presence of the SHE-STT effect with a spin-wave intensity that is a factor of 20 larger than in the absence of the SHE-STT effect.

DOI： 10.1109/LMAG.2018.2803737

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The voltage noise characteristic and sensitivity of magnetic tunnel junction sensors are crucial for ultralow field detection. In this work, we used a soft magnetic material electrode Co70.5Fe4.5Si15B10 as a sensing layer to improve the sensitivity. Then, a bias field along the easy axis of a free layer was applied to improve the linearity and manipulate the sensitivity of magnetic tunnel junction sensors. More importantly, random telegraph noise was suppressed by the bias field, resulting in hysteresis-free performance. The highest sensitivity of 3.9%/Oe and the best field detectivity of 4.5nT/ root Hz at 10 Hz without hysteresis have been achieved. The sensors showed excellent performance with CoFeSiB electrodes, indicating that it is an effective way to improve the performance of sensors by introducing the bias field. Published by AIP Publishing.

DOI： 10.1063/1.4990478

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Spin-torque ferromagnetic resonance (ST-FMR) spectra of nano-scaled CoFeB/MgO/CoFeB full perpendicular magnetic tunnel junctions (p-MTJs) were investigated, especially in detail at low dc-bias voltage region. Usually in in-plane magnetized MTJs (i-MTJs), the ST-FMR spectrum line shape reverses its symmetry as switching dc-bias voltage polarities; however, it is found that in the p-MTJs the line shape reversal behaves differently, that not only the spectrum shows anti-symmetric line shape at zero dc bias but also the dependence of reversal symmetry on dc bias is broken. Based on the framework of homodyne-detected ST-FMR, we extracted the parameters characterizing the spectra and discussed the possible factors resulting these differences.

DOI： 10.1109/TMAG.2017.2707081

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 and SrTiO3 has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LaAlO3 thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. In this paper, the chemical, electronic, and transport properties of LaAlO3/SrTiO3 samples capped with different metals grown in a system combining pulsed laser deposition, sputtering, and in situ X-ray photoemission spectroscopy are investigated. The results show that for metals with low work function a q2DES forms at 1-2 uc of LaAlO3 and is accompanied by a partial oxidation of the metal, a phenomenon that affects the q2DES properties and triggers the formation of defects. In contrast, for noble metals, the critical thickness is increased above 4 uc. The results are discussed in terms of a hybrid mechanism that incorporates electrostatic and chemical effects.

DOI： 10.1002/adma.201700486

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Crystal and electronic structures of similar to 380 nm BiFeO3 film grown on LaAlO3 substrate are comprehensively studied using advanced transmission electron microscopy (TEM) technique combined with first-principles theory. Cross-sectional TEM images reveal the BiFeO3 film consists of two zones with different crystal structures. While zone II turns out to have rhombohedral BiFeO3, the crystal structure of zone I matches none of BiFeO3 phases reported experimentally or predicted theoretically. Detailed electron diffraction analysis combined with first-principles calculation allows us to determine that zone I displays an orthorhombic-like monoclinic structure with space group of Cm (=8). The growth mechanism and electronic structure in zone I are further discussed in comparison with those of zone II. This study is the first to provide an experimentally validated complete crystallographic detail of a highly strained BiFeO3 that includes the lattice parameter as well as the basis atom locations in the unit cell.

File： 2_2017_SciRep_7_46498_BFO構造 on LAO.pdf

DOI： 10.1038/srep46498

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Language：English   Publishing type：Research paper (scientific journal)  

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We present the temperature dependence of the thermal magnon density in a thin ferromagnetic layer. By employing Brillouin light scattering and varying the temperature, an increase of the magnon density accompanied by a lowering of the spin-wave frequency is observed with increasing temperature. The magnon density follows the temperature according to the Bose-Einstein distribution function, which leads to an approximately linear dependency. In addition, the influence of this effect in spin-transfer-torque-driven systems is presented. In particular, the increase in the magnon density with temperature sets the limit for a suppression of magnons in charge current-driven systems. Hence, the maximum possible suppression of thermal magnons occurs at a finite current.

DOI： 10.1109/LMAG.2017.2734773

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

The spin-orbit interaction couples the electrons' motion to their spin. As a result, a charge current running through a material with strong spin-orbit coupling generates a transverse spin current (spin Hall effect, SHE) and vice versa (inverse spin Hall effect, ISHE). The emergence of SHE and ISHE as charge-to-spin interconversion mechanisms offers a variety of novel spintronic functionalities and devices, some of which do not require any ferromagnetic material. However, the interconversion efficiency of SHE and ISHE (spin Hall angle) is a bulk property that rarely exceeds ten percent, and does not take advantage of interfacial and low-dimensional effects otherwise ubiquitous in spintronic hetero- and mesostructures. Here, we make use of an interface-driven spin-orbit coupling mechanism the Rashba effect in the oxide two-dimensional electron system (2DES) LaAlO3/SrTiO3 to achieve spin-to-charge conversion with unprecedented efficiency. Through spin pumping, we inject a spin current from a NiFe film into the oxide 2DES and detect the resulting charge current, which can be strongly modulated by a gate voltage. We discuss the amplitude of the effect and its gate dependence on the basis of the electronic structure of the 2DES and highlight the importance of a long scattering time to achieve efficient spin-to-charge interconversion.

File： 2016_NatMat_15_1261_LAOSTO.pdf

DOI： 10.1038/NMAT4726

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We have systematically investigated the Gilbert damping constant a for L1(0)-FePd films using the time-resolved magneto-optical Kerr effect (TRMOKE). The field angle dependence of TRMOKE signals was measured and analyzed. The field angle dependence of the lifetime of magnetization precession was explained by evaluating extrinsic contributions such as the anisotropy distribution and two-magnon scattering. The crystalline uniaxial perpendicular magnetic anisotropy constant K-u and alpha values were evaluated for FePd films for various L1(0) order parameters S. K-u values of approximately 15 Merg/cm(3) were obtained for films with large-S values (i.e., over 0.8). In addition, alpha for the low-S film was found to be approximately 0.007 and decreased with increasing S. Smaller values of alpha (of 0.002-0.004) were obtained for films with S values of approximately 0.6-0.8. Results revealed that FePd films have both large-K-u and small-alpha values, which is a useful property for low-power magnetization switching while maintaining high thermal stability in spin-transfer-torque magnetoresistive random access memory applications.

DOI： 10.1103/PhysRevB.94.174425

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

All electrical manipulation of magnetization is crucial and of great important for spintronics devices for the sake of high speed, reliable operation, and low power consumption. Recently, widespread interests have been aroused to manipulate perpendicular magnetization of a ferromagnetic layer using spin-orbit torque (SOT) without field. We report that a commonly used antiferromagnetic material IrMn can be a promising candidate as a functional layer to realize field-free magnetization switching driven by SOT in which IrMn is employed to act as both the source of effective exchange bias field and SOT source. The critical switching current density within our study is J(c) = 2.2 x 10(7) A/cm(2), which is the samemagnitude as similar materials such as PtMn. A series of measurements based on anomalous Hall effect was systematically implemented to determine the magnetization switching mechanism. This study offers a possible route for IrMn application in similar structures. Published by AIP Publishing.

File： 2_2016_APL_109_ 132402_spinhall.pdf

DOI： 10.1063/1.4963235

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE INC  

Mn-poor LaSrMnO3 (LSMO) epitaxial films were grown on SrTiO3 (100) substrates by radio frequency magnetron sputtering in an argon and oxygen gas mix, and then the samples were annealed in air at various temperatures (T-a). 2 theta-chi X-ray diffraction mapping, nano-beam diffraction analysis through transmission electron microscopy, and electron back scatter diffraction through scanning electron microscopy revealed that -the crystal symmetry of the LSMO films changed from monoclinic/orthorhombic to rhombohedral on annealing in air. Curie temperature (T-C) of the LSMO films was found to increase with increasing T-a, and become higher than the room temperature at T-a &gt;= 861 degrees C, indicating that the cause of these changes was the filling of oxygen and the transition of the crystal symmetry into rhombohedral. (C) 2016 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.matchar.2016.05.002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We investigated the tunneling magnetoresistance (TMR) properties for the magnetic sensor application in MgO-based magnetic tunnel junctions (MTJs) using a perpendicularly magnetized [Co/Pd]-based reference layer and an in-plane magnetized CoFeB sensing layer with various thicknesses (t(CoFeB)). Linear TMR curves to an out-of-plane magnetic field were successfully obtained with a dynamic range of more than 600 Oe, corresponding to the coercivity of the [Co/Pd]-based reference layer. The MTJs showed the highest sensitivity of 0.026%/Oe for t(CoFeB) = 1.8 nm and the smallest nonlinearity of 0.11% full scale for t(CoFeB) = 3 nm. We clarified that the sensitivity and the nonlinearity in the MTJs are significantly associated with tCoFeB, which is attributed to the change in the anisotropy field of the CoFeB sensing layer.

DOI： 10.1109/TMAG.2016.2518188

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The exchange bias (EB) is an effective fundamental and applicational method to realize magnetic hysteresis loop shifting. However, further manipulation of EB unidirectional anisotropy is difficult after setup using either field deposition or post-annealing. In this work, we experimentally show a new approach to control the magnetic hysteresis loop bias in a[ Co(0.2)/Pd(1)](5)/CoFeB orthogonal exchange-spring (ES) coupling system, where the direction and strength of unidirectional anisotropy can be easily manipulated by applying an external magnetic field. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/APEX.9.063003

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Language：Japanese   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2016.1.0_2047

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

Crystal and electronic structures of BiFeO3 thin films (∼10 and ∼300 nm) grown on SrTiO3 substrate have been investigated in terms of BiFeO3 film thickness dependence using the advanced transmission electron microscopy (TEM) technique. Electron diffraction patterns of both BiFeO3 thin films acquired along [011]SrTiO3 cross sections clearly exhibited the existence of extra Bragg's reflections which are absent in that from SrTiO3. Structure factor calculations unambiguously revealed that the electron diffraction pattern corresponds to the [211] net pattern of rhombohedral BiFeO3. High-resolution TEM images combined with multislice simulation also demonstrated that the crystalline structure of both BiFeO3 films is rhombohedral. Electron energy loss spectroscopy results for both BiFeO3 thin films showed spectra with the characteristics of bulk BiFeO3, i.e., rhombohedral. The lattice mismatch of &lt
2.5% between BiFeO3 and SrTiO3 found in a particular epitaxial relationship is considered to be the reason that BiFeO3 can grow by maintaining its bulk crystalline, i.e., rhombohedral, structure.

DOI： 10.1103/PhysRevB.93.064115

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

The Co40Fe40B20(CFB)/MgO/CFB/MgO/CFB-based multilayer was prepared by conventional magnetron sputtering and utilised in the fabrication of double magnetic tunnel junctions (DMTJs) for which the middle CFB layers were island-shaped. By analysing the magnetic property of the CFB islands with Langevin's equation, it was possible to identify their diameters of 7.6, 8.9 and 11.0nm; accordingly submicron-scaled DMTJs were fabricated to investigate single-spin transport phenomena. The coulomb staircase and the oscillatory tunnel magnetoresistive (TMR) were able to be observed at 6K, where the TMR ratio was enhanced up to 60%, which is the highest value ever achieved in this structure.

DOI： 10.1080/14786435.2015.1131343

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The cause of the time scale for ultrafast demagnetization induced by irradiation using a fs pulse laser remains an open issue. Spin-flip mediated by electron-phonon scattering due to spin-orbit interactions is one major theory proposed to explain ultrafast demagnetization. Ultrafast demagnetization in Ni, FePd, and FePt films was investigated in order to systematically study the influence of heavy elements on the demagnetization time. The ultrafast demagnetization in these systems was analyzed using the microscopic three temperature model, which is a theory based on spin-flip mediated by electron-phonon scattering. The spin-flip probability (a(sf)) values for the Ni, FePd, and FePt films were evaluated in the low pump fluence regime. It was found that the a(sf) value for the Ni film was larger than that for the FePd and FePt films. Thus, there is no correlation between the a(sf) value and the spin-orbit coupling strength. Fast demagnetization of the FePd film was also observed due to large electron-phonon scattering. In addition, it was found that the a(sf) values decreased with increasing magnetization quenching for all the films.

DOI： 10.1088/0022-3727/49/3/035002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Magnetic tunneling junctions with Co40Fe40B20/Co70.5Fe4.5Si15B10 composite free layer have been fabricated and annealed at different temperatures to obtain the highest tunneling magnetoresistance. The field and temperature dependences of the low frequency noise have been measured to understand the origin of noise. The random telegraph noise has been observed at low magnetic field, and the corresponding fluctuating moment is estimated to be 4.8 x 10(5) mu(B) with an effective area of 240 nm(2). The dependence of noise on temperature was coincident with the thermally activated kinetics model above 30 K while it deviated from this model below 30 K. Studying the origin of the low frequency noise is helpful to reduce the noise level in MTJs. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2015.09.026

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Searching new approaches to combine spintronic devices and microwave applications is a fascinating topic. On one hand, the application of spintronic devices opens new roads to generate and detect microwave in wide frequency range, on the other hand, applying/injection microwave also modulates the spintronic devices properties. In this report, we present our recent work on effects of MW injection on the switching properties of magnetic tunneling junctions (MTJs). Magnetic tunneling junction is a promising device cell choice for spintronic applications, such as magnetic sensors, nonvolatile magnetic random access memories, and magnetic logical. As the MTJ cell size approaches to nanoscale, magnetic materials with large magnetic anisotropy are usually adopted as MTJ magnetic electrodes to keep the thermal stability. This usually inevitably increases magnetization switching field or spin transfer torque switching current density. Therefore, searching methods to assist MTJ magnetization switching becomes important for practical applications. Here, we show that directly injecting microwave current into MTJ and taking advantage of microwave current induced spin transfer torque (STT) effect can assist magnetization switching effectively. Since microwave current rather than MW magnetic field is utilized, it enables us to control the switching assistance electrically and eliminates cross-talking between neighboring cells.

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Crystallographic Society of Japan  

Despite extensive studies on crystal structure of thin film BiFeO₃ (BFO) thin film, it remains debated primarily due to its structural complexity as well as stress effect from underlying substrates. We have examined comprehensive crystal structure analysis for BFO thin layer (30 nm) grown on SrTiO₃ (STO) substrate using cross-sectional transmission electron microscopy technique along three different zone axes. Nano-beam electron diffraction (NBED) patterns combined with structure factor (SF) calculations and high-resolution transmission electron microscopy images unambiguously reveal that BFO thin layer grows with rhombohedral structure that is identical to its bulk form. No evidence of monoclinic and/or tetragonal distortion is found. The rhombohedral BFO thin layer is found to grow onto STO by maintaining an epitaxial relationship in a manner that can minimize lattice mismatch at BFO/STO interface. Our current work clearly demonstrates that multiple-zone axes NBED combined with SF calculation is highly effective for precise crystal structure analysis of thin film BFO.

DOI： 10.5940/jcrsj.58.96

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Other Link： https://jlc.jst.go.jp/DN/JLC/20022528894?from=CiNii

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We fabricated magnetic tunnel junctions (MTJs) with a perpendicularly magnetized Co-Fe-B sensing layer for magnetic sensor applications exhibiting a linear tunneling magnetoresistance behavior, and systematically investigated correlation between sensitivity and nonlinearity in the MTJs. The experimental results in the MTJs annealed at different temperatures with various thicknesses and compositions of the Co-Fe-B sensing layer were compared with the simple calculation based on the Stoner-Wohlfarth model, which predicts the tradeoff relationship between the sensitivity and the nonlinearity. We found the clear tradeoff correlation between them regardless the annealing temperature and the composition of the Co-Fe-B sensing layer. These results show us a guideline for designing the sensing properties of magnetic sensors based on MTJs.

DOI： 10.1109/TMAG.2015.2448723

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

MgO magnetic tunnel junction (MTJ) sensors with spin-valve-like sensing layers of Ir22Mn78 (6)/Ni80Fe20 (t(NiFe) = 20-70)/Ru (0.9)/Co40Fe40B20 (3) (unit: nm) have been fabricated. A linear field dependence of magnetoresistance for these MTJ sensors was obtained by carrying out a two-step field annealing process. The sensitivity and linear field range can be tuned by varying the thickness of NiFe layer and annealing temperature, and a high sensitivity of 37%/mT has been achieved in the MTJ sensors with 70 nm NiFe at the optimum annealing temperature of 230 degrees C. Combining the spin-valve-like sensing structure and a soft magnetic NiFe layer, MTJ sensors with relatively wide field sensing range have been achieved and could be promising for showing high sensitivity magnetic field sensing applications. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4927840

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

In perpendicular magnetic anisotropy (PMA) multilayers electrode system, the low spin polarization and large lattice mismatch between traditional PMA electrodes ([Co/Pt]&lt
inf&gt
n&lt
/inf&gt
, [Co/Pd]&lt
inf&gt
n&lt
/inf&gt
) and spin-filter barrier MgO make it difficult to obtain large tunneling magneto-resistance (TMR) ratio [1]. One of several possible solutions is to insert a magnetic electrode between MgO barrier and PMA multilayers electrodes, forming composite magnetic electrodes [2]. Therefore, it is critical to understand magnetic behavior of composite magnetic electrodes. Cain et al early found exchange bias like effect in NiFe/TbCo (FM-FM) bilayers [3]. They explained that tilt of spins at the interface between TbCo and NiFe is responsible for the observed exchange bias like effect. Recently, by applying a large in-plane magnetic field, B. Dieny et al initialized exchange bias in NiFe/[Pt/Co] (FM-FM) bilayer [4] and explained exchange bias like effect using closure domain model [5, 6]. People believe that closure domain formed at the interface is responsible for observed exchange bias. But few works were studied to figure out how perpendicular magnetic field affect closure domain and exchange bias like effect.

DOI： 10.1109/INTMAG.2015.7156581

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Publisher：2015 IEEE International Magnetics Conference, INTERMAG 2015  

Magnetic tunneling junctions (MTJ) have been widely used in magnetic random access memory (MRAM) and magnetic sensors for the large magnetoresistance (TMR). [1, 2] One of the major issues for high sensitivity sensors is to obtain a low switching field (Hsw). Recently various ferromagnetic metals with low Hsw, such as NiFe [3, 4] and amorphous CoFeSiB [5], have been selected as the sensing layer in MTJ sensors. Moreover, CoFeSiB has a lower saturation magnetization, which is also beneficial for reducing Hsw. [6] With CoFeSiB as the sensing layer, the sensitivity up to 40%/Oe has been achieved in MTJ sensors. [5] Besides the high sensitivity, the noise level of the sensor devices in the low frequency regime is an important parameter, and nanotesla-scaled detectivity has been achieved in MTJ sensors. In this work, the low frequency noise in MTJs with CoFeB/CoFeSiB composed free layer has been studied.

DOI： 10.1109/INTMAG.2015.7157499

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DOI： 10.1063/1.4917334

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

This article is devoted to the evaluation of the material parameters of the Heusler compound Co2Mn0.6Fe0.Si-4 via Brillouin light scattering spectroscopy. Recently, cobalt-based Heusler compounds and, in particular, the compound Co2Mn0.6Fe0.4Si have attracted huge interest in the fields of spintronics and magnon spintronics. Thus, evaluation of the material parameters that govern spin dynamics in the gigahertz regime is essential to develop and understand advanced experimental scenarios as well as potential technical applications. We demonstrate the evaluation of these parameters based on wavevector as well as time-resolved Brillouin light scattering spectroscopy. The focus of our study is the determination of the spin-wave damping in an individual microstructure as wells as of the exchange constant of Co2Mn0.6Fe0.4Si-parameters, that are difficult to estimate with alternative techniques.

DOI： 10.1088/0022-3727/48/16/164015

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Nowadays, a wealth of information on ultrafast magnetization dynamics of thin ferromagnetic films exists in the literature. Information is, however, scarce on bulk single crystals, which may be especially important for the case of multi-sublattice systems. In Heusler compounds, representing prominent examples for such multi-sublattice systems, off-stoichiometry and degree of order can significantly change the magnetic properties of thin films, while bulk single crystals may be generally produced with a much more well-defined stoichiometry and a higher degree of ordering. A careful characterization of the local structure of thin films versus bulk single crystals combined with ultrafast demagnetization studies can, thus, help to understand the impact of stoichiometry and order on ultrafast spin dynamics.
Here, we present a comparative study of the structural ordering and magnetization dynamics for thin films and bulk single crystals of the family of Heusler alloys with composition Co2Fe1-xMnxSi. The local ordering is studied by Co-59 nuclear magnetic resonance (NMR) spectroscopy, while the time-resolved magneto-optical Kerr effect gives access to the ultrafast magnetization dynamics. In the NMR studies we find significant differences between bulk single crystals and thin films, both regarding local ordering and stoichiometry. The ultrafast magnetization dynamics, on the other hand, turns out to be mostly unaffected by the observed structural differences, especially on the time scale of some hundreds of femtoseconds. These results confirm hole-mediated spin-flip processes as the main mechanism for ultrafast demagnetization and the robustness of this demagnetization channel against defect states in the minority band gap as well as against the energetic position of the band gap with respect to the Fermi energy. The very small differences observed in the magnetization dynamics on the picosecond time-scale, on the other hand, can be explained by considering the differences in the electronic structure at the Fermi energy and in the heat diffusion of thin films and bulk crystals.

DOI： 10.1088/0022-3727/48/16/164016

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Co-based full-Heusler alloys, such as Co2MnSi and Co2MnGe, are expected to be used as half-metallic ferromagnetic material, which has complete spin polarization. They are the most promising materials for realizing half-metallicity at room temperature due to their high Curie temperature. The optimization of the magnetic damping constant of ferromagnetic materials is extremely important for achieving high-speed magnetization switching and reducing critical current density for spin torque transfer switching. We have systematically investigated the magnetic damping constant in Co-based full Heusler alloy epitaxial films. We found that the Gilbert damping constant seems to be roughly proportional to the total density of states at the Fermi level (E-F) by first principle calculation. A very small magnetic damping constant of 0.003 in the Co2Fe0.4Mn0.6Si epitaxial film was demonstrated. The small magnetic damping constant in Co2FexMn1-xSi films with x &lt; 0.6 can be attributed to the half-metallicity of Heusler alloys. Co-based full Heusler alloys with both half-metallicity and small magnetic damping will be very useful for future applications based on spintronic devices.

DOI： 10.1088/0022-3727/48/16/164012

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

A (001)-epitaxial-BiFeO3/CoFe2O4 bilayer was grown by self-assembly on SrTiO3 (100) substrates by just coating a mixture precursor solution. The thickness ratio of the bilayer could be controlled by adjusting the composition ratio. For example, a BiFeOx : CoFe2Ox = 4 : 1 (namely Bi4CoFe6Ox) mixture solution could make a total thickness of 110 nm divided into 85-nm-thick BiFeO3 and 25-nm-thick CoFe2O4. Self-assembly of the bilayer occurred because the perovskite BiFeO3 better matched the lattice constant (misfit approximately 1%) and crystal symmetry of the perovskite SrTiO3 than the spinel CoFe2O4 (misfit approximately 7%). The magnetic domains of the hard magnet CoFe2O4 were switched by the polarization change of BiFeO3 due to an applied vertical voltage, and the switched magnetic domain size was approximately 100 nm in diameter. These results suggest that self-assembled BiFeO3/CoFe2O4 bilayers are interesting in voltage driven nonvolatile memory with a low manufacturing cost.

File： 2_2015_SciRep_5_09348_BFO-CFO.pdf

DOI： 10.1038/srep09348

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Comprehensive crystal structure analysis was performed for a BiFeO3 thin layer (similar to 30 nm) grown on a SrTiO3 substrate using cross-sectional transmission electron microscopy along three different zone axes. Nano-beam electron diffraction patterns combined with structure factor calculations and high-resolution transmission electron microscopy images unambiguously revealed that the BiFeO3 thin layer grew with a rhombohedral structure identical to its bulk form. No evidence of monoclinic and/or tetragonal distortion was found. The rhombohedral BiFeO3 thin layer was found to grow onto SrTiO3 by maintaining an epitaxial relationship in a manner minimizing the lattice mismatch at the BiFeO3/SrTiO3 interface. (C) 2015 The Japan Society of Applied Physics

File： 2_2015_APEX_8_031501_BFO構造解析.pdf

DOI： 10.7567/APEX.8.031501

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Language：English   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2015.1.0_1967

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Ultraviolet photoemission spectroscopy (UPS) is a powerful tool to study the electronic spin and symmetry features at both surfaces and interfaces to ultrathin top layers. However, the very low mean free path of the photoelectrons usually prevents a direct access to the properties of buried interfaces. The latter are of particular interest since they crucially influence the performance of spintronic devices like magnetic tunnel junctions (MTJs). Here, we introduce spin-resolved extremely low energy photoemission spectroscopy (ELEPS) to provide a powerful way for overcoming this limitation. We apply ELEPS to the interface formed between the half-metallic Heusler compound Co2MnSi and the insulator MgO, prepared as in state-of-the-art Co2MnSi/MgO-based MTJs. The high accordance between the spintronic fingerprint of the free Co2MnSi surface and the Co2MnSi/MgO interface buried below up to 4 nm MgO provides clear evidence for the high interface sensitivity of ELEPS to buried interfaces. Although the absolute values of the interface spin polarization are well below 100%, the now accessible spin-and symmetry-resolved wave functions are in line with the predicted existence of non-collinear spin moments at the Co2MnSi/MgO interface, one of the mechanisms evoked to explain the controversially discussed performance loss of Heusler-based MTJs at room temperature.

DOI： 10.1038/srep08537

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

0.9BiFeO(3)-0.1BiCoO(3) (BFCO) films (t = 100 nm) were prepared on orthorhombic YAlO3 (YAO) (100) substrates by r.f. magnetron sputtering. Film flatness, crystallinity, crystal symmetry, and secondary phase formation are strongly affected by the pressure of the sputtering gasses. Ar and O-2. Phi-scan measurements showed that the films were epitaxially grown on the substrates, with the crystal relation [101](p)(101)(p) BFCOII[101](p)( 101)(p) YAO. X-ray reciprocal space mapping revealed that the crystal symmetry of the BECO films was a pseudo-cubic-like monoclinic structure, with M-C phase, rather than the Cm symmetry of the bulk BFCO. Cross-sectional transmission electron microscopy analysis revealed that the film had, as a result of a lattice misfit of 7%, strong compressive strain less than 10 nm from the interface, which relaxed monotonically with increasing distance from the interface. Magnetic measurements show that strained monoclinic BFCO has smaller magnetization compared to rhombohedral BFCO. (C) 2014 Elsevier B.V. All rights reserved

DOI： 10.1016/j.jcrysgro.2014.09.044

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Spin dynamics excited by spin-polarized current in magnetic tunnel junctions (MTJs) is potentially useful in nanoscale electrical oscillation sources and detection devices. A spin oscillator/detector should work at a high frequency, such as that of a millimeter-wave, where the quality of a semiconductor device is restricted by carrier mobility, the CR time constant, and so on. Developers of spin systems for practical use need to find out how to excite spin dynamics (i) in the millimeter-wave region, (ii) with low power consumption (ex: no external magnetic field, low damping material), and (iii) for broad frequency modulation. Here L1(0)-ordered FePd alloy with perpendicular magnetocrystalline anisotropy (PMA) and a low damping constant, 0.007, was used for the free layer in the MTJs, and a homodyne-detected ferromagnetic resonance (FMR) signal was obtained at around 30 GHz together with the possibility of one-octave frequency modulation. The FMR signal in out-of-plane magnetized L10-ordered FePd free layer could be excited without an external magnetic field by injecting in-plane spin polarized alternating current. This study shows the potential utility of L1(0)-ordered alloy materials such as FePt, CoPt, MnAl, and MnGa in a variety of millimeter-wave spin devices.

File： 1_2015_NanoLetters_15_623_FePd.pdf

DOI： 10.1021/nl504114v

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By properly choosing material parameters and geometry of a magnetic nanowire, pinned domain wall oscillations at a local constriction can be obtained at low current densities. The thickness modulation of the wire gives an additional uniaxial magnetic anisotropy, which modifies the transverse anisotropy energy that defines the critical current density required for the onset of domain wall oscillations. Broadband microwave signals are obtained with these values of current density.

DOI： 10.1109/LMAG.2014.2379629

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Language：English   Publisher：IEEE  

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Language：Japanese   Publisher：Japanese Society for Medical and Biological Engineering  

MTJ is a small size device, working in room temperature with low power consumption, and is expected to enable detection of bio-magnetic field without liquid He. For the purpose of practical realization of MTJ bio-magnetic sensor, this study evaluated the signal and noise with various MgO barrier thicknesses in MTJ to reduce 1/f noise in frequency domain. Figures show MgO thickness dependence of signal voltage, noise voltage and S/N ratio measured from 18 Hz, 120 nT_p-p input signal. Both signal and noise voltage increased with increasing MgO thickness. From this relation of signal and noise, maximum 154 S/N ratio was acquired by 2.2 nm MgO thickness.

DOI： 10.11239/jsmbe.53.S187_03

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Language：Japanese   Publisher：Japanese Society for Medical and Biological Engineering  

Magnetic tunnel junctions (MTJs) have great advantages for the magnetic field sensor applications. However, a significant improvement of tunnel magneto-resistance (TMR) ratio is needed to detect a small bio-magnetic field. In this study, we fabricated MTJs with half-metallic Co₂Fe_0.4Mn_0.6Si(CFMS) Heusler alloy which are expected to increase TMR ratio. The fabricated MTJswere annealed twice to achieve sensor-type TMR curves. Figure shows the 2nd annealing temperature dependence of TMR curves. In MTJ annealed at 200℃, TMR curve showed a linear resistance response, which is required for sensor applications. This work was supported by the S-Innovation program, Japan Science and Technology Agency (JST).

DOI： 10.11239/jsmbe.53.S187_01

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Language：English   Publisher：IEEE  

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Language：Japanese   Publisher：The Japan Institute of Metals and Materials  

DOI： 10.2320/materia.54.383

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Other Link： https://jlc.jst.go.jp/DN/JLC/20013609670?from=CiNii

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The nonlinear decay of propagating spin waves in the low-Gilbert-damping Heusler film Co2Mn0.6Fe0.4Si is reported. Here, two initial magnons with frequency f(0) scatter into two secondary magnons with frequencies f(1) and f(2). The most remarkable observation is that f(1) stays fixed if f(0) is changed. This indicates, that the f(1) magnon mode has the lowest instability threshold, which, however, cannot be understood if only Gilbert damping is present. We show that the observed behavior is caused by interaction of the magnon modes f(1) and f(2) with the thermal magnon bath. This evidences a significant contribution of the intrinsic magnon-magnon scattering mechanisms to the magnetic damping in high-quality Heusler compounds.

DOI： 10.1103/PhysRevLett.113.227601

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We fabricated (001)-oriented epitaxial four-layered thin films of Cr/Co2MnSi/Cu/Co50Fe50 by dc magnetron sputtering in order to investigate the physical characteristics of the transverse spin current. The penetration depth lambda(T) characterizes the transverse spin current generated by the spin pumping effect that takes place in a film subjected to an external magnetic field. By analyzing the dependence on the Co50Fe50 thickness of the peak-to-peak line widths of ferromagnetic resonance, we determined AT within the Co50Fe50 layer and found it to be lambda(T) = 0.9 nm. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2014.05.043

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

High-quality L1(0) ordered 20 nm-thick FePd epitaxial thin films with a large perpendicular magnetic anisotropy were fabricated using a SrTiO3 substrate. The uniaxial crystalline magnetic anisotropy constant Ku evaluated for the films annealed above 500 degrees C was 14 Merg/cm(3). A very low effective damping constant, alpha(eff) = 0.007, was observed for FePd thin films annealed at 500 degrees C. This value is smaller than that of other Fe-based ordered alloys with a large perpendicular magnetic anisotropy. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4897547

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Soc. of Med. Electronics and Biol. Engineering  

Since MTJ sensor is an element of room temperature operation, low power consumption, low cost and small device size, thus it is expectable to spread widely the medical diagnostics and basic research using bio-magnetic fields. In this study, in order to reduce 1/f noise, 100×100 MTJ sensor arrays were fabricated. Figure shows the system of imperceptible magnetic filed detection. The magnetic field was generated with the one turn coil, using the sine wave of 123 Hz. The result is shown in figure. In this study, the detection of a magnetic field of 0.29 nT was demonstrated with fabricated 100×100 MTJ sensor array.

DOI： 10.11239/jsmbe.52.O-505

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Soc. of Med. Electronics and Biol. Engineering  

Currently, SQUID is the most sensitive of the magnetic sensor and is used for the measurement of biological fields. However, dissemination of the device to the medical field realistically is very strict due to the big size of the device, in terms of introduction and maintenance costs. In particular, a dewar for holding a low temperature is required in order to immersion in liquid helium for operating the SQUID element. The distance of the dewar from the head surface, and also the dewar surface from the SQUID element, act as restraint for accurate measurement. This paper proposes a sensor for the biomagnetic field measurementwith magnetic tunnel junction (MTJ) devices. It can be operated at room temperature and is brought it into close contact with the head. We describe the necessary technical challenges toward its realizationand the feasibility in the future.

DOI： 10.11239/jsmbe.52.OS-33

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Current-induced microwave spectra were measured in small-sized giant magnetoresistance devices composed of a NiFe vortex free layer and an out-of-plane magnetized Co/Pd multilayer polarizer. The influence of a large direct current (DC) and a bias field on the excited mode of the free layer is systematically investigated. For small current values, microwave spectra due to the vortex core oscillation were observed around 1 GHz, while the frequency abruptly changed to 4-4.5 GHz at certain DC values. The experimental data were reproduced by micromagnetic simulation, which indicates that the mode change of the vortex core oscillation in the free layer is dominated by the Oersted field from the large DC. (C) 2014 AIP Publishing LLC.

File： 2_2014_APL_105_052407_vortex.pdf

DOI： 10.1063/1.4892077

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

BiFeO3 (BFO) and La(x)Sr(y)MnzO(3) (LSMO) films were epitaxially grown on SrTiO3 (100) substrates by r.f. magnetron sputtering with various oxygen gas flow ratios (F-O2). Compositional ratios of each atom in both of BFO and LSMO could be controlled kept to around 10 at.% by changing F-O2. Adjusting the compositional ratio to La0.35Sr0.15Mn0.5O3 not only increase T-c of LSMO but also produces sufficient oxygen to form a perovskite lattice. For an LSMO/BFO heterostructure, detailed observation by cross sectional transmission electron microscopy (TEM) revealed that the lattice of rhombohedral (SG: R-3c) LSMO was shrank by a clamping effect from the SrTiO3 substrates, and then the BFO was grown in two layers: (i) an interfacial BFO layer (7 nm thick) with evenly shrunk a-axis and c-axis, and (ii) an upper BFO layer (25 nm thick) expanded along the c-axis. Neither misfit strain nor dislocations appeared at the interface between the shrunken BFO and LSMO layers, and these heterostructures did not show exchange bias. These results suggest that BFO is suitable for a tunneling barrier combine with LSMO electrode. (C) 2014 Author(s).

DOI： 10.1063/1.4893998

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We have studied thin film samples of Co2FeSi and Co2MnSi with different degrees of chemical ordering using the time-resolved magneto-optical Kerr effect to elucidate the influence of defects in the crystal structure on magnetization dynamics. Surprisingly, we find that the presence of defects does not influence the optically induced magnetization dynamics on the ultrashort timescale (some 100 fs). However, we observe a second demagnetization stage with a timescale of tens of picoseconds in Co2MnSi for low chemical ordering; that is, a large number of defects. We interpret this second demagnetization step as originating from scattering of mostly thermalized majority electrons into unoccupied minority defect states.

DOI： 10.1088/1367-2630/16/6/063068

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

A production of polycrystalline phase with glassy precursor, i.e., glass-ceramics processing, has a great potential to fabricate the sophisticated materials. In this study, we have prepared the Bi2O3-Fe2O3-BaO-B2O3 system precursor, in which BiFeO3 phase is crystallizable, and examined the phase formation and physical properties in resulting glass-ceramics. We found both ferroelectricity and ferromagnetism in the glass-ceramics consisting of BiFeO3 phase, i.e., multiferroic glass-ceramics. This study has demonstrated an alternative method for synthesis of polycrystalline BiFeO3 material. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4881138

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The magnetization dynamics of both Ta/CoFeB/MgO and Ta/CoFeB/Ta films were investigated using an all-optical pump-probe method. The magnetic field strength and the applied field direction dependencies of the precession frequency and the relaxation time were explained well by the Landau-Lifshitz-Gilbert equation when taking the magnetic anisotropy distribution in the film into account. The thickness dependence of the a values obtained for both stacked films was also discussed. The a values increased linearly with increasing inverse CoFeB thickness (t(CoFeB)). The slope of the a vs 1/t(CoFeB) characteristic for Ta/CoFeB/MgO films was smaller than that for Ta/CoFeB/Ta films, implying that the enhancement of a was caused by the CoFeB/Ta interface. Comparison of the annealing temperature dependence of a and the perpendicular magnetic anisotropy constant Ku revealed no correlation between a and Ku.

DOI： 10.1103/PhysRevB.89.174416

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The spin-dependent transport behavior in organic semiconductors (OSs) is generally observed at low temperatures, which likely results from poor spin injection efficiency at room temperature from the ferromagnetic metal electrodes to the OS layer. Possible reasons for this are the low Curie temperature and/or the small spin polarization efficiency for the ferromagnetic electrodes used in these devices. Magnetite has potential as an advanced candidate for use as the electrode in spintronic devices, because it can achieve 100% spin polarization efficiency in theory, and has a high Curie temperature (850 K). Here, we fabricated two types of organic spin valves using magnetite as a high efficiency electrode. C-60 and 8-hydroxyquinoline aluminum (Alq(3)) were employed as the OS layers. Magnetoresistance ratios of around 8% and over 6% were obtained in C-60 and Alq(3)-based spin valves at room temperature, respectively, which are two of the highest magnetoresistance ratios in organic spin valves reported thus far. The magnetoresistance effect was systemically investigated by varying the thickness of the Alq(3) layer. Moreover, the temperature dependence of the magnetoresistance ratios for C-60 and Alq(3)-based spin valves were evaluated to gain insight into the spin-dependent transport behavior. This study provides a useful method in designing organic spin devices operated at room temperature. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4870154

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DOI： 10.1063/1.4855016

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We investigated the static and dynamic magnetic properties of thin films of Mn-Co-Ga Heusler compound. Gilbert damping and exchange stiffness constants of the films were evaluated by using the ferromagnetic resonance technique in the X-band regime (f = 9.4 GHz). By analyzing the experimental spectra, magnetic parameters of the films such as the line width and the Gilbert damping were deduced, and the exchange stiffness constant was estimated from the perpendicular standing spin-wave resonance. The Gilbert damping constant was estimated to be 0.017 in a specific film composition. The exchange stiffness constant showed a linear dependence on the film composition. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4864250

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

Co and Mn co-substituted BiFeO3 films were prepared on Pt/Ti/SiO2/Si(100) substrates, and the effect of co-substitution on the structural, electric, and magnetic properties of the prepared films was systematically investigated. Cross-sectional transmission electron microscopy observations showed that the films were homogeneous; moreover, no magnetic secondary phases were observed in the films. Nano-beam energy-dispersive X-ray spectroscopy, carried out at various points in the films, revealed that Co and Mn were distributed in the films. Further, the average concentration of Bi, Co, Mn, and Fe was 59, 2, 2, and 37 at.%, respectively; these values were almost consistent with the nominal composition of the precursor solution used. X-ray diffraction profiles of the films showed that the (012)-d-spacing decreased and (104) and (110) peaks merged into a single peak with increasing co-substitution content. The leakage current in films under high electric fields drastically decreased upon co-substitution without any degradation of ferroelectricity; moreover, this effect was also observed for single Mn-substituted materials. Saturation magnetization of the films monotonically increased with the co-substitution content, and this increase was quantitatively identical to that in the case of single Co-substituted materials. These results indicate that Co and Mn play significant roles in determining the properties of co-substituted BiFeO3 films. (C) 2014 Published by Elsevier B.V.

DOI： 10.1016/j.tsf.2014.03.046

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Language：Japanese   Publisher：Japanese Society for Medical and Biological Engineering  

Although the biomagnetic field is useful to observe an organic activity and the superconducting quantum interference device (SQUID) is used to detect at the moment, there is a problem of needing a liquid helium in order to operate this SQUID. This research focuses attention on the ferromagnetic tunnel junction (MTJ) device which is operable in a room temperature, and aims at the reduction of the elements for using this MTJ device as a biomagnetic field sensor and the reduction of the circuit system noise.

DOI： 10.11239/jsmbe.52.O-503

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Language：Japanese   Publisher：Japanese Society for Medical and Biological Engineering  

In magnetic tunnel junctions (MTJs), the resistance changes by external magnetic field through tunnel magnetoresistance (TMR) effect and MTJs can be applied to magnetic field sensors. To detect a small bio-magnetic field, we have to develop MTJs with high sensitivity (=TMR/2<I>H</I><SUB>k</SUB>, <I>H</I><SUB>k</SUB>: magnetic anisotropy field) of more than 100%/Oe, which is two digit larger than that of actual devices. In this work, MTJs with a low <I>H</I><SUB>k</SUB> CoFeSiB amorphous electrode was fabricated to realize such a highly sensitive magnetic sensor. After optimizing the preparation condition of CoFeSiB, a very high sensitivity of 40%/Oe was obtained. The result came much closer to our goal.

DOI： 10.11239/jsmbe.52.O-504

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

In order to enhance the magnetoresistance (MR) of perpendicular magnetic tunnel junctions (pMTJs) based on MnGa alloys, a single ferromagnetic layer such as Fe and Co was previously inserted between MnGa and MgO barrier. In this study, to further enhance the spin-filter effect, we introduced a Fe/Co bilayer as an interlayer in the MnGa/MgO interface. Compared to the single Co interlayer, an apparent MR ratio enhancement was obtained when Fe layer thickness was around 0.3 nm for pMTJs with MnGa compositions of Mn57Ga43, Mn62Ga38, and Mn70Ga30, and the maximum MR ratio reaches 50% at room temperature. In addition, inverted magnetoresistance loops were observed due to the antiparallel alignment of the magnetic moments of Co and MnGa layers separated by the thin Fe layer. (C) 2013 AIP Publishing LLC.

DOI： 10.1063/1.4828483

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We present an experimental study identifying structural reasons that degrade spin-polarization of Co2MnSi thin films deposited on MgO(001) substrates. Through the fabrication of magnetic tunnel junctions, we measure a range of values for tunneling magneto-resistance (TMR) ratios following post-deposition annealing and epitaxial crystallization of the Heusler film. These TMR ratios reflect qualitatively the change in spin polarization of the Co2MnSi thin films. Low-temperature annealing results in low spin-polarization due to a high fraction of an amorphous phase. As annealing temperatures increase, the fraction of L2(1) and B2 chemically ordered phases increases, thus improving significantly the spin-polarization. However, for samples annealed at higher temperatures, significant degradation in the cubic magneto-crystalline anisotropy is observed, which we attribute to the detection of manganese diffusion into the MgO substrate. This Mn diffusion is manifested in a reduction of the value of the TMR ratio, namely, the spin polarization. Additionally, the maximum TMR ratio measured here, approximately 65% at room-temperature, is limited because the semi-coherent interface of Co2MnSi with the MgO substrate terminates with a Mn-Si layer. (C) 2013 AIP Publishing LLC.

DOI： 10.1063/1.4826908

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The magnetic tunnel junctions (MTJs) with a soft magnetic CoFeSiB amorphous electrode and a MgO barrier layer were fabricated. A double annealing process was carried out to obtain the linear resistance response to the external magnetic field. The effect of the annealing temperature on the sensitivity of magnetic sensors was systematically investigated. We achieved a high sensitivity of 40%/Oe, where the sensitivity is defined as TMR/(2H(k)), where TMR is the tunnel magnetoresistance ratio and H-k is the magnetic anisotropy field of the free layer of MTJs. (C) 2013 The Japan Society of Applied Physics

DOI： 10.7567/APEX.6.103004

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The effects of annealing temperature on the structural and magnetic properties of CoFeB/FePd bilayer were systematically investigated. A thin (0.5 nm) CoFeB layer was inserted between the FePd and MgO layers and then annealed at different temperatures. The magnetic anisotropy field increased with an increase in the annealing temperature owing to the enhancement of the interfacial perpendicular magnetic anisotropy (PMA) by crystallizing the thin CoFeB layer. The thermal annealing of the deposited layers promoted L1(0) ordering and reduced the surface roughness. It was found that a thinner FePd layer requires a higher annealing temperature in order to achieve PMA. After annealing at 350 degrees C, a PMA of 7.1 Merg/cc was obtained even for a thin FePd film with a thickness of 2.0 nm.

DOI： 10.1109/TMAG.2013.2251612

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The magnetoresistance effects of the perpendicular magnetic tunnel junctions (p-MTJs) based on Mn-Ga ordered alloys are reported. By tuning the Mn-Ga composition, the MTJs based on L10 and D022 structured Mn-Ga alloys were achieved in the MTJ stack structure of Cr(40)/MnxGa100 - x(30)/Mg(0.4)/MgO(2.2)/CoFeB(1.2)/Ta(5)/ Ru(7) (nm). The values of magnetoresistance (MR) ratio at room temperature for different Mn-Ga composition are around 5%. In order to enhance the MR ratio, a thin CoFeB layer was introduced between the Mn-Ga and the MgO barrier. The MR effect shows a strong Mn-Ga composition dependent as CoFeB interlayer thickness increases. An MR ratio of 50% was obtained at room temperature when the CoFeB thickness is 1.5 nm for Mn62 Ga38 based MTJs. © 2013 IEEE.

DOI： 10.1109/TMAG.2013.2242861

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Fast magnetization precession was observed in L1(0)-FePt thin films with different L1(0) order parameter values by all optical pump-probe technique. Precession frequency was varied widely for the films with different order parameter, which is due to large difference in perpendicular magnetic anisotropy. Gilbert damping constant (alpha) was estimated from relaxation time as apparent damping. Clear difference in alpha was not observed with different perpendicular magnetic anisotropy. (C) 2013 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.52.073002

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We succeeded in fabricating L1(0)-ordered MnAl films with a high perpendicular magnetic anisotropy energy of 10(7) erg/cm(3) and a small average film roughness of 0.4nm by using a molten Mn-Al sputtering alloyed target and optimizing the substrate temperature. In addition, we investigated the tunnel magnetoresistance (TMR) effect in magnetic tunnel junctions (MTJs) with the prepared L1(0)-ordered MnAl electrode. The TMR effect was observed at RT in an MTJ with a very thin Co50Fe50 layer inserted into the MnAl electrode and MgO tunneling barrier interface. This is the first observation of the TMR effect in MTJs with an L1(0)-ordered MnAl electrode. (C) 2013 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.52.063003

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Insertion of a thin 3d ferromagnetic metal/alloy layer between the barrier layer and the perpendicularly magnetized ferromagnetic electrode is an effective method to enhance the magnetoresistance (MR) ratio in perpendicular magnetic tunnel junctions (p-MTJs). In the present paper we systematically studied the structural and magnetic properties as well as the spin-dependent transport in p-MTJs with a core structure MnGa/FM/MgO/CoFeB (FM = Fe, Co), with the MnGa being the L1(0) MnGa alloy (Mn57Ga43, Mn62Ga38) and the D0(22) MnGa alloy (Mn70Ga30). The insertion of the Fe and Co layers enhances the MR ratio significantly as well as the MnGa composition dependence of the MR ratio. In addition, opposite magnetic properties and MR(H) curves of MTJs with Fe and Co interlayers are observed, naturally suggesting the ferromagnetic and antiferromagnetic exchange coupling for MnGa/Fe(bcc) and MnGa/Co(bcc), respectively. By considering the exchange coupling between the FMand MnGa, we successfully simulated the MR(H) curves of the samples with Fe and Co interlayers based on a simple model. Furthermore, the interlayer effect on the transport properties are discussed based on the temperature dependence of the MR ratio by using the magnon excitation model modified with impurity-induced hopping. It shows that the FM interlayer restrains the impurity induced hopping and the magnon excitation; and furthermore, the Co is more effective in restraining the impurity diffusion and magnon excitation as compared to Fe.

DOI： 10.1103/PhysRevB.87.184426

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Rhombohedral (R-) and tetragonal (T-) Bi(Fe, Co)O-3 (BFCO) films were epitaxially grown on the SrTiO3 (100) substrates, and the optical properties of the BFCO films were evaluated by spectroscopic ellipsometry. It was revealed that the refractive indexes of R- and T-BFCO epitaxial films were 2.93 and 2.86 at wavelength of 600 nm, and 2.65 and 2.59 at 1550 nm, respectively, which are comparable to the pure BiFeO3. The refractive index of the R-BFCO film was totally larger than that of the T-BFCO film; it might be caused by structural strain and local symmetry breaking. It was confirmed that the extinction coefficients of both films were almost zero at wavelengths larger than 600 nm. In addition, the optical band gaps of the R- and T-BFCO films were estimated to be 2.78 and 2.75 eV, respectively. It can expect that the BFCO film has a possibility to use optical-magnetic field sensor working at room temperature. (C) 2013 American Institute of Physics.

DOI： 10.1063/1.4794878

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The tunnel magnetoresistance (TMR) effect and the bias voltage dependence of tunnelling conductance spectra were measured in L1(0) Mn-Ga/Fe/MgO/CoFe magnetic tunnel junctions (MTJs). The TMR ratio and bias-voltage dependences on annealing conditions and Fe-layer thickness were investigated. The TMR ratio showed an increase subsequent to Fe-layer deposition under the optimum annealing condition, and a maximum value of 24% was achieved in an MTJ with a perpendicularly magnetized Fe layer of thickness of 1.1 nm at room temperature; this corresponded to a 57% TMR ratio in the case of completely antiparallel magnetization configuration. In the tunnelling conductance spectra, an anomalous dip, the so-called zero-bias anomaly, was observed for all the samples. The zero-bias anomaly is speculated to have appeared because of an increase in magnon excitation at the magnetic layer/barrier interfaces according to the model by Zhang et al (1997 Phys. Rev. Lett. 79 3744). In our experiments magnitude of the zero-bias anomaly depended on both the annealing condition and the Fe-layer thickness. We discuss our observation of the variation in the Curie temperature of the magnetic layer at the barrier layer interface depending on the the preparation conditions of the Fe insertion layer, which caused the change in the magnitude of the zero-bias anomaly.

DOI： 10.1088/0022-3727/46/15/155001

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Arrays of 100 × 100 magnetic tunnel junctions (MTJs) connected in parallel and series were fabricated. A synthetic antiferro-coupled bottom free layer with a NiFe/Ru/CoFeB structure and MgO tunneling barrier were used to realize a high sensitivity, which is defined as TMR/2Hk, where, TMR is the tunneling magnetoresistance ratio and Hkis the magnetic anisotropy field of the free layer. To obtain a linear response of tunneling resistance against an applied external magnetic field, a double annealing process was carried out. From R-H curve measurements, the sensitivity of the 100 × 100 integrated MTJs was lower (8%/Oe) than that of a single MTJ (25%/Oe). However, a 1/30 decrease in noise power density was realized in the integrated MTJs. Consequently, a very small magnetic field of 0.29 nT was detected with the integrated MTJs. © 2013 The Japan Society of Applied Physics.

DOI： 10.7567/JJAP.52.04CM07

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Structural and magnetic properties of epitaxial L10-FePd/MgO films on GaAs and InP lattice mismatched substrates are investigated at different MgO and FePd growth temperatures. While c-axis lattice constants of MgO and FePd show similar values on both substrates, the remanent magnetization becomes larger on GaAs than that on InP. Since the ratio of FePd (002) tetragonal ordered phase and FePd (200) cubic disordered phase follows similar growth temperature dependence to the remanent magnetization and the long range chemical order parameter, the perpendicular magnetic anisotropy grown on the lattice-mismatched semiconductors is strongly affected by formation of the disordered phase. © 2013 American Institute of Physics.

DOI： 10.1063/1.4795443

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Magnetic Heusler materials with very low Gilbert damping are expected to show novel magnonic transport phenomena. We report nonlinear generation of higher harmonics leading to the emission of caustic spin-wave beams in a low-damping microstructured Co2Mn0.6Fe0.4Si Heusler waveguide. The source for the higher harmonic generation is a localized edge mode formed by the strongly inhomogeneous field distribution at the edges of the spin-wave waveguide. The radiation characteristics of the propagating caustic waves observed at twice and three times the excitation frequency are described by an analytical calculation based on the anisotropic dispersion of spin waves in a magnetic thin film. DOI: 10.1103/PhysRevLett.110.067201

DOI： 10.1103/PhysRevLett.110.067201

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Language：English   Publishing type：Part of collection (book)   Publisher：Springer Netherlands  

We demonstrated that a large TMR ratio of 753 % has been observed at 2 K in a MTJ using a Co2MnSi Heusler alloy electrode and a crystalline MgO tunnel barrier. At room temperature (RT), we also have observed a large TMR ratio of 217 %, which value at RT is much larger than that of MTJs using an amorphous Al-oxide tunnel barrier. However, the temperature dependence of the TMR ratio was still large. In order to improve the interface, we investigated the TMR effect in Co2MnSi/CoFeB(0-2 nm)/MgO/CoFe MTJs. TMR ratio was enhanced by inserting a thin CoFeB layer at the Co2MnSi/MgO interface. The MTJ with CoFeB thickness of 0.5 nm exhibited the highest TMR ratio. From the conductance-voltage measurements for the fabricated MTJs, we inferred that the highly spin polarized electron created in Co2MnSi can conserve the polarization through the 0.5 nm thick FeB layer.

DOI： 10.1007/978-90-481-3832-6_17

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The integration of organic semiconductors and magnetism has been a fascinating topic for fundamental scientific research and future applications in electronics, because organic semiconductors are expected to possess a large spin-dependent transport length based on weak spin-orbit coupling and weak hyperfine interaction. However, to date, this length has typically been limited to several nanometres at room temperature, and a large length has only been observed at low temperatures. Here we report on a novel organic spin valve device using C 60 as the spacer layer. A magnetoresistance ratio of over 5% was observed at room temperature, which is one of the highest magnetoresistance ratios ever reported. Most importantly, a large spin-dependent transport length of approximately 110 nm was experimentally observed for the C 60 layer at room temperature. These results provide insights for further understanding spin transport in organic semiconductors and may strongly advance the development of spin-based organic devices. © 2013 Macmillan Publishers Limited. All rights reserved.

DOI： 10.1038/ncomms2423

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The post annealing temperature and Co thickness dependence of tunnel magnetoresistance (TMR) ratio was investigated for L1(0)-MnGa/Co(t(Co))/MgO/CoFeB perpendicular magnetic tunnel junctions (MTJs). The MnGa/MgO interface optimization by Co insertion was shown to be an effective way for improving the TMR ratio. The TMR value increases with the annealing temperature, (T-a), and exhibits maxima at 325 degrees C for t(Co) = 1.0 - 5.0 nm. Thermal annealing process improves the structure of MTJs, but also causes elements diffusion. In this work, the annealing effect on MTJs with different Co thicknesses was discussed in detail along with a fast annealing method.

DOI： 10.1109/TMAG.2012.2196420

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

A series of MgO perpendicular magnetic tunneling junction (p-MTJ) films with soft/hard composite electrodes is prepared by RF-sputtering, where amorphous CoFeB is chosen as soft layer. The modulation of MTJs film magnetic properties on CoFeB thickness is investigated. The critical thickness for composite free layer transform from rigid magnet (RM) to exchange spring system (ES) is indentified. Besides, an unexpected in-plane exchange bias is observed which is attributed to the formation of closure domains. The evolution of domain structures on CoFeB is examined by magnetic force microscope. Coexistence of two distinguished magnetic domains of different sizes is observed. It is found that the evolution of domain morphology as varying CoFeB thickness is due to the modulation of effective anisotropy.

DOI： 10.1109/TMAG.2012.2198795

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SrTiO3 (100) sub/BiFeO3/CoFe/Ru/CoFe/Cu/CoFe/Ta structure was prepared by a combination of chemical solution deposition and sputtering method, and followed by a systematical investigation for the structural, magnetic and magnetoresistance properties at room temperature (RT) as a function of CoFe and Ru thicknesses. It was revealed that introduction of synthetic CoFe/Ru/CoFe as a pinning layer increased the giant magentoresistance (MR) ratio to 8.3% at RT. This enhancement of MR ratio might be attributed to (i) the increase of pinning field, and (ii) suppression of the influence of the surface roughness of BiFeO3 by inserting the synthetic CoFe/Ru/CoFe layer. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4745504]

DOI： 10.1063/1.4745504

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DOI： 10.1063/1.4737000

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This letter investigates the microstructure and mean inner potential (MIP) profile of Ta/CoFeB/MgO/CoFeB/Ta magnetic tunnel junctions (MTJs) by high resolution transmission electron microscopy (HRTEM) and electron holography, respectively. The inconspicuous crystallization of MgO barrier is confirmed by HRTEM in the post-annealed sample at 250 degrees C. An obvious MIP difference is displayed in the Ta layers between the top and bottom of the MTJ, and elemental content difference of them is confirmed by energy dispersive spectroscopy. These results imply that the chemical diffusion can also give rise to a lower tunnel magnetoresistance ratio besides the inconspicuous crystallization of MgO barrier. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4732463]

DOI： 10.1063/1.4732463

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BiFeO3-CoFe2O4 composite thin films were formed on Pt/Ti/SiO2/Si(100) substrates by chemical solution deposition from a mixed precursor solution. X-ray diffraction and transmission electron microscopy analyses confirmed that CoFe2O4 nanoparticles less than 10 nm were uniformly distributed in the BiFeO3 matrix. The BiFeO3-CoFe2O4 composite films exhibited the same ferroelectric switching charge as BiFeO3 thin films, although a larger applied electric field was necessary. However, the magnetic properties were significantly improved by incorporation of CoFe2O4 nanoparticles into BiFeO3; a saturated magnetization of 80 emu/cm(3) and a magnetic coercive field of 450 Oe were attained at 300 K. Furthermore, the composite films did not show superparamagnetic behavior in zero-field-cooling and field-cooling measurements, which suggest that the thermal fluctuation of CoFe2O4 nanoparticles was suppressed by exchange coupling with BiFeO3. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729831]

DOI： 10.1063/1.4729831

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We report the preparation of (Bi,Pr)(Fe,Mn)O-3(BPFM)/SrRuO3 (SRO)-Pt/CoFe2O4(CFO) layered film structure on (100) SrTiO3 substrate by pulsed laser deposition method and their structural and electrical properties. Favorable ferroelectric properties of BPFM were observed in the layered film structure with (100)-oriented growth of BPFM, SRO, and CFO. Variation of polarization vs electric field loops of BPFM by applying DC magnet field was observed, and the remnant polarization was found to increase by 3% with increasing the applied magnetic field from 0 to 10 kOe. The magnetoelectric coefficient in the present structure was estimated to be 1.5 V/(cmOe). (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4730334]

DOI： 10.1063/1.4730334

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

Mn- and Co-BiFeO3 polycrystalline films were prepared by a chemical solution deposition (CSD) method and their structures were investigated by grazing incidence X-ray diffraction (GIXRD) and theta-2 theta XRD in conjunction with transmission electron microscopy (TEM). GIXRD measurement revealed that the Mn-BiFeO3 film contains the Bi2Fe4O9 pyrochlore phase, which might be attributed to the low magnetization of the Mn-BiFeO3 film. For the Co-BiFeO3 film, secondary phases were not observed by structural analyses and the composition was almost homogeneous. From the analytic aspect of the structure, the magnetization enhancement in the Co-BiFeO3 films is considered to be associated with Co substitution for Fe in BiFeO3. (c) 2012 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.51.061501

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BiFeO3/CoFe bilayers with different BiFeO3 (BFO) crystalities were fabricated by chemical solution deposition and sputtering method. Exchange bias has been successfully induced in these bilayers by post-annealing. The annealing temperature dependence of exchange bias as well as coercivity was investigated. Two kinds of annealing temperature dependence behaviors were observed. It is found that, similar to the conventional antiferromagnet/ferromagnet system, the temperature dependence of exchange bias is dominated by direct interface coupling, and the crystality of BFO has no profound effect on exchange bias. (C) 2012 American Institute of Physics. [doi:10.1063/1.3673435]

DOI： 10.1063/1.3673435

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Dependence of spin-transfer switching characteristics on the interlayer exchange coupling strength in the MTJs with synthetic free layers of Co90Fe10/Ru/Co40Fe40B20 with strong coupling strength was investigated. In the MTJ with the relatively weakly coupled synthetic ferrimagnetic free layer, larger thermal stability (Delta(0)) and lower intrinsic critical current density (J(c0)) than those of the MTJ with the single free layer were observed. Meanwhile, in the MTJs with the strongly coupled synthetic ferri- or ferromagnetic free layers, very large Delta(0) and high J(c0) were observed probably due to high effective magnetic energy barrier. It was found that the MTJ with the relatively weakly coupled synthetic ferrimagnetic free layer is suitable for the STTRAM application. (C) 2012 American Institute of Physics. [doi:10.1063/1.3672240]

DOI： 10.1063/1.3672240

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Structural and magnetic properties of MnAl thin films with different composition, growth temperature, and post-annealing temperature were investigated. The optimum condition for fabrication of L1(0)-MnAl perpendicularly magnetized thin film deposited on Cr-buffered MgO single crystal substrate was revealed. The results of x ray diffraction indicated that the MnAl films annealed at proper temperature had a (001)-orientation and L1(0)-ordered structure. The L1(0)-ordered films were perpendicularly magnetized and had a large perpendicular anisotropy. In addition, low surface roughness was achieved. For the optimized fabrication condition, the saturation magnetization M-s of 600 emu/cm(3) and perpendicular magnetic anisotropy K-u of 1.0 x 10(7) erg/cm(3) was obtained using the Mn48Al52 target at deposition temperature of 200 degrees C and post-annealing temperature of 450 degrees C. (C) 2012 American Institute of Physics. [doi:10.1063/1.3676428]

DOI： 10.1063/1.3676428

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The magnetic films of Co with Si/SiO2/Pt/Co/molecule structure were fabricated and their structural properties and magnetic anisotropy were investigated by varying both Co (0.5-1.8 nm) thickness and molecular capping layers of 5,6,11,12-tetraphenylnaphthacene (rubrene) and copper phthalocyanine (CuPc), respectively. The crystal structures were characterized using x-ray diffraction (XRD) and the magnetization curves were measured using vibrating sample magnetometer with an applied field both in parallel and perpendicular to a film plane. It was found that the thickness of Co for the maximum perpendicular magnetic anisotropy (PMA) is around 0.7 nm for both group films. However, the estimated effective magnetic anisotropy energy for Co was 2.9 +/- 0.3 x 10(6) erg/cc for rubrene-capped sample, which was smaller than the value of 4.9 +/- 0.4 x 10(6) erg/cc for CuPc-capped sample. The XRD patterns showed the crystal structure of rubrene layer was of amorphous structure and CuPc layer was polycrystalline. The different interface effects of Co/CuPc and Co/rubrene were discussed to analyze the change of PMA. (C) 2012 American Institute of Physics. [doi:10.1063/1.3676240]

DOI： 10.1063/1.3676240

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Magnetic tunnel junctions with a Ni80Fe20/Ru/Co40Fe40B20 synthetic antiferro-coupled bottom free layer and an MgO barrier layer have been fabricated. Double annealing process was carried out in order to obtain linearity against magnetic field with hysteresis-free resistance response. The effect of the annealing temperature and NiFe thickness in the free layer on the magnetic field sensor performance was investigated. We have observed a very high sensitivity of 25.3%/Oe while keeping linearity. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3677266]

DOI： 10.1063/1.3677266

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The L1(0)-ordered FePd thin films were prepared using an ultrahigh vacuum magnetron sputtering system on MgO(001) substrates at 300 degrees C. The crystallographic and magnetic properties and the surface morphology of films with and without a very thin amorphous CoFeB intermediate layer inserted between the FePd and the MgO layers were systematically investigated as a function of the thickness of the FePd layer. The perpendicular anisotropy of the samples was increased by inserting the thin CoFeB as an intermediate layer below the FePd with a thickness of 4.0 nm. The reason for the enhancement by inserting the amorphous CoFeB layer is attributed to: (i) the promotion of the L1(0) ordering of the FePd due to the reduction of the lattice mismatch between the MgO and FePd, and (ii) the fact that thin CoFeB has a perpendicular anisotropy at the interface of the MgO, which superposed the perpendicular anisotropy of the L1(0)-FePd. (C) 2012 American Institute of Physics. [doi:10.1063/1.3673409]

DOI： 10.1063/1.3673409

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The tunnel magnetoresistance (TMR) ratio in a magnetic tunnel junction (MTJ) with an L1(0)-Mn62Ga38/Fe/MgO/CoFe structure was considerably improved by Fe layer insertion. A maximum TMR ratio of 24% was observed in an MTJ with a Fe thickness of 1.1nm at room temperature, which corresponded to a 57% TMR ratio in the case of a complete antiparallel magnetization configuration. Fe layer thickness dependences of the magnetization curve and TMR effect were also investigated. It was revealed that the magnetization of Fe on 30-nm-thick MnGa could be fixed in a perpendicular direction when the thickness of the Fe was below 2.0 nm. (C) 2012 The Japan Society of Applied Physics

DOI： 10.1143/APEX.5.043003

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We report on the investigation of spin-wave propagation in a micro-structured Co2Mn0.6Fe0.4Si (CMFS) Heusler waveguide. The reduced magnetic losses of this compound compared to the commonly used Ni81Fe19, allow for the observation of spin-wave propagation over distances as high as 75 mu m via Brillouin light scattering (BLS) microscopy. In the linear regime, a maximum decay length of 16.7 mu m of the spin-wave amplitude was found. The coherence length of the observed spin-wave modes was estimated to be at least 16 mu m via phase-resolved BLS techniques. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3693391]

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Language：Japanese   Publisher：公益社団法人 応用物理学会  

DOI： 10.11470/jsapmeeting.2012.1.0_2128

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Mn-Ga binary alloys show strong magnetism and large uniaxial magnetic anisotropy even though these alloys do not contain any noble, rare-earth metals or magnetic elements. We investigate the composition dependence of saturation magnetization M(S) and uniaxial magnetic anisotropy K(u) in epitaxial films of M(n-x)Ga(1-x) alloys (x similar to 0.5-0.75) grown by magnetron sputtering. The M(S) values decrease linearly from approximately 600 to 200 emu/cm(3) with increasing x, whereas the K(u) values decrease slightly from approximately 15 to 10 Merg/cm(3) with increasing x. These trends are distinct from those for known tetragonal hard magnets obtained in a limited composition range in Mn-Al and Fe-Pt binary alloys. These data are analyzed using a localized magnetic moment model.

DOI： 10.1103/PhysRevB.85.014416

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Materials Research Society of Japan  

The electronic structures of BiFe_0.96Mn_0.04O₃ and BiFe_0.96Co_0.04O₃ thin films have studied by X-ray absorption spectroscopy and soft-X-ray emission spectroscopy. The BiFe_0.96Mn_0.04O₃ thin film has the mixed valence states of Mn³⁺ and Mn⁴⁺, although the BiFe_0.96Co_0.04O₃ thin film has the valence state of Co³⁺. The conduction band consists of the Fe 3<i>d</i> state. The valence band is mainly composed of the O 2<i>p</i> state hybridized with the Fe 3<i>d</i> state. The bandwidth of valence band depends on the valence state of doped element. The energy gap of BiFe_0.96Mn_0.04O₃ thin film is smaller than that of BiFe_0.96Co_0.04O₃ thin film. These findings may indicate that the leakage current of thin film is closely related with the electronic structure and energy gap.

DOI： 10.14723/tmrsj.37.77

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The composition dependence of the tunnel magnetoresistance (TMR) effect in Mn-Ga/MgO/CoFe magnetic tunnel junctions (MTJs) for Mn54Ga46, Mn62Ga38, and Mn71Ga29 (at. %) electrodes was investigated. An MTJ with a Mn62Ga38 electrode showed a maximum TMR ratio of 23% at 10 K and high annealing endurance up to 375 degrees C. The bias voltage dependence of the TMR ratio was distinct among MTJs with different Mn-Ga compositions. Here, we discuss this dependence on the basis of the difference in the Delta(1) band dispersions for Mn-Ga alloys calculated by first principles. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3659484]

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Fully epitaxial current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices with a Co2Fe0.4Mn0.6Si/Ag/Co2Fe0.4Mn0.6Si structure were fabricated. The bottom and top Co2Fe0.4Mn0.6Si layers had good crystallinity and an L2(1)-ordered structure. In addition, we found from scanning transmission electron microscopy (STEM) measurements that both Co2Fe0.4Mn0.6Si/Ag and Ag/Co2Fe0.4Mn0.6Si interfaces were very flat and sharp. The magnetoresistance (MR) ratio at room temperature was 74.8%, the largest to date for CPP-GMR devices. CPP-GMR devices with Co2Fe0.4Mn0.6Si electrodes would be very useful for the next generation of hard disk drive (HDD) read heads. (C) 2011 The Japan Society of Applied Physics

DOI： 10.1143/APEX.4.113005

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The perpendicular magnetic anisotropy (PMA) of cobalt (0.5-1.8 nm) films capped separately by pentacene (Pc), fullerene (C(60)), and 8-hydroxyquinoline-aluminum (Alq(3)) are investigated. For all three series, the thickness of Co is around 0.7 nm for maximum out-of-plane coercivity. It is found that the coercivity of C(60)-capped films is nearly equal to that for Alq(3)-capped samples, although both are smaller than for Pc-capped films. The different interface effects of Co/molecules are discussed to explain this observation. This work highlights the PMA of ferromagnetic metal, which can be markedly infected depending on the nature of organic molecule. (C) 2011 American Institute of Physics. [doi:10.1063/1.3651766]

DOI： 10.1063/1.3651766

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Effects of Pt doping on magnetic properties and Gilbert damping are investigated for Ni80Fe20 permalloy films to compare with damping in alloy films containing Pt with a large perpendicular anisotropy. Gilbert damping constant alpha and g-factor g for (Ni80Fe20)(100-x)Pt-x (x = 0-34 at. %) are evaluated from out-of-plane angular variations of ferromagnetic resonance (FMR) linewidth and resonance field with an analysis based on the Landau-Lifshitz-Gilbert equation. Data of angular dependence of the FMR linewidth are fitted reasonably well by a theoretical model without having to take into account any extrinsic influences on linewidth, thereby allowing us to determine precise values of alpha. The alpha values show variation with increasing Pt concentration rising by similar to 0:06 at a Pt concentration of 34 at. %, which is very close to those in perpendicularly magnetized CoCrPt and FePt film reported recently. Nevertheless, Gilbert damping rate G for the Pt doped permalloy films is smaller than those in CoCrPt and FePt films. These experimental results are discussed with a spin-orbit torque theory. (C) 2011 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.50.103003

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DOI： 10.1109/TMAG.2011.2143392

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Fast precessional dynamics of magnetization are reported in very thin films of CoCrPt alloy with large perpendicular magnetic anisotropy. The films are fabricated with different substrate temperatures, and magnetization dynamics are investigated using time-resolved magneto-optical Kerr effect. The Kittel's equation well describes a dependency of magnetization precession frequency on an applied magnetic field direction for all the films, only when the field is relatively large. An inverse lifetime of magnetization precession, a counterpart of a line width in ferromagnetic resonance (FMR), depends strongly on the field direction even at the applied field of around 10 kOe. The Gilbert damping constant alpha is estimated to be similar to 0.05 for all the films using a model taking account of an influence of perpendicular magnetic anisotropy dispersion.

DOI： 10.1109/TMAG.2011.2154357

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We fabricated a spin transistor structure that consisted of two magnetic tunnel junctions with half-metallic Co2MnSi electrodes. Transient responses were observed by applying pulsing gate voltage. Output currents were controlled by both the source-drain and gate voltage and magnetic configuration of the Co2MnSi. The drain current increased around 3000 times at a source-drain voltage of 0.01 V and anti-parallel magnetic configuration, when a gate voltage of 1 V peak-to-peak was applied. In addition, the maximum magnetocurrent ratios were 215% at 6 K. Expected operation properties are observed in our proposed spin transistor. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3645637]

DOI： 10.1063/1.3645637

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

(BiNd0.05)(Fe0.97Mn0.03)O-3 (BNFM)/Pt/CoFe2O4 (CFO) layered thin film was fabricated on (100) SrTiO3 substrate by pulsed laser deposition. BNFM. Pt, and CFO layers were epitaxially grown on the substrate. Almost no increase of leakage current due to the formation of heteroepitaxial structure was found, and well-saturated hysteresis loops in the polarization vs electric field and magnetization vs magnetic field curves coexist at room temperature. The remnant polarization and remnant magnetization values were 55 mu C/cm(2), and 70-145 mA/m, respectively. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tsf.2011.05.067

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We have fabricated a polycrystalline BiFeO3 (BFO) film with a thickness of 650 nm on a Pt/Ti/SiO2/Si substrate by a chemical solution deposition method, and its optical and thermooptic properties were evaluated by spectroscopic ellipsometry. The optical band gap energy of the BFO film was 2.95 eV at 50 degrees C and monotonically decreased with increasing temperature to 2.79 eV at 530 degrees C. The extinction coefficient was significantly low at a wavelength longer than 600 nm in the temperature range from 50 to 530 degrees C. The refractive index of the BFO film was estimated to be 2.96 at 600 nm and 2.68 at 1550 nm at 50 degrees C using a single Gaussian oscillator, and a thermooptic coefficient of 0.8 x 10(-4) K-1 was obtained at a wavelength of 1550 nm. Although the refractive index decreased with increasing temperature at all wavelengths, the variation was larger at shorter wavelengths. This seems to be caused by the combination of broadening in the Gaussian oscillator vibration and thermal expansion. (C) 2011 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.50.09NB02

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Magnetic anisotropy of Co40Fe40B20 thin films sandwiched between Ta and MgO layers was investigated. Magnetic properties of CoFeB layers deposited on top and bottom of MgO layer are different. The thickness of the CoFeB layer and annealing temperature are the critical parameters to achieve and keep the perpendicular magnetic anisotropy. The phase diagram of perpendicular anisotropic strength of CoFeB layers on annealing temperatures and thicknesses of CoFeB layers is observed. According to phase diagrams, perpendicular CoFeB/MgO/CoFeB tunnel junctions were fabricated, and tunneling magnetoresistance (TMR) ratio was higher than 30% at low temperatures. (C) 2011 American Institute of Physics. [doi:10.1063/1.3605564]

DOI： 10.1063/1.3605564

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Effects of metallic Mg insertion on tunnel magnetoresistance (TMR) effect were investigated in D0(22)-Mn2.4Ga/Mg/MgO/CoFe magnetic tunnel junctions (MTJs). The thickness of Mg (d(Mg)) was varied from 0 to 1.4 nm. TMR ratio exhibited maximum value of 22% with the d(Mg) = 0.4 nm and a negative value of 14% with the d(Mg) = 1.4 nm at 10 K. The dependence of resistance area products (R x A) on the d(Mg) showed similar trend compared with those of what reported in conventional CoFeB-MgO based MTJs. Bias voltage dependences of differential conductance (dI/dV) and TMR ratio exhibited asymmetry with respect to the zero-bias. The difference of the zero-bias anomaly in the dI/dV spectra was also discussed, and an indication of reducing the inelastic tunneling process was found, implying the improvement of barrier/magnetic-layer interfaces by the Mg insertion. Considering a theoretical work done by Wang et al. [Phys. Rev. B 82, 054405 (2010)], the inversion of the sign and the asymmetric bias voltage dependence of TMR ratio were inferred to be attributed to the minority spin tunneling via a quantum well state in the thin metallic Mg layer between Mn2.4Ga and MgO. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3603034]

DOI： 10.1063/1.3603034

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MDPI AG  

The 10 at.% Co-substituted BiFeO3 films (of thickness 50 nm) were successfully prepared by radio frequency (r.f.) magnetron sputtering on SrTiO3 (100) substrates with epitaxial relationships of [001](001)Co-BiFeO3//[001](001)SrTiO3. In this study, a single phase Co-substituted BiFeO3 epitaxial film was fabricated by r.f. magnetron sputtering. Sputtering conditions such as Ar, O-2 gas pressure, annealing temperature, annealing atmosphere, and sputtering power were systematically changed. It was observed that a low Ar gas pressure and low sputtering power is necessary to suppress the formation of the secondary phases of BiOx. The Co-substituted BiFeO3 films were crystalized with post-annealing at 600 degrees C in air. The process window for single phase films is narrower than that for pure BiFeO3 epitaxial films. By substituting Fe with Co in BiFeO3, the magnetization at room temperature increased to 20 emu/cm(3). This result suggests that Co-substituted BiFeO3 films can be used in spin-filter devices.

DOI： 10.3390/ma4061087

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The xBiCoO(3)-(1-x)BiFeO3 [0 &lt;= x &lt;= 58 at. %] solid solution films were epitaxially grown on SrTiO3 (100) substrates, and the dependence of the magnetic properties on the BiCoO3 concentration was systematically investigated. The remanent magnetization (M-r) and coercive field (H-c) increased with the BiCoO3 in the case of rhombohedral composition (x &lt;= 18 at. %), and it maximally enhanced at morphotropic phase boundary (MPB) (20 &lt;= x &lt;= 25 at. %). The magnetization decreased in the case of a tetragonal structure (30 at. % &lt;= x). The magnetoelectric (ME) effect at room temperature expect to observe for the BiCoO3-BiFeO3 solid solution films in the case of the rhombohedral structure and for MPB. (C) 2011 American Institute of Physics. [doi:10.1063/1.3561776]

DOI： 10.1063/1.3561776

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Multiferroic BiFeO3 epitaxial films were fabricated on SrTiO3 (100) substrates by a chemical solution deposition method. Magnetic layers of Co, Py, Co40Fe40B20, Co75Fe25, and Co50Fe50 were then deposited by sputtering under a magnetic field. Despite employing a chemical process, a clear exchange bias was observed for all the magnetic materials. The temperature dependence of Hex was evaluated for a Co50Fe50/BiFeO3 bilayer having a relatively large Hex and high squareness. The Hex of Co50Fe50/BiFeO3 bilayer increased between 10 to 250 K. (c) 2011 American Institute of Physics. [doi: 10.1063/1.3563061]

DOI： 10.1063/1.3563061

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

The tunnel magnetoresistance (TMR) effect with a perpendicularly magnetized D0(22)-Mn3-delta Ga (delta = 0.6) electrode was investigated in epitaxially grown D0(22)-Mn3-delta Ga (30)/Mg (d(Mg))/MgO (2)/CoFe (2.5) (nm) magnetic tunnel junctions (MTJs). The maximum TMR ratio of 9.8% (22.1%) was achieved at 300 K (10 K) with d(Mg) = 0: 4 nm. The bias voltage dependence of differential conductance spectra suggests the existence of a coherent tunneling process in the MTJs. First principles calculations of band dispersion relations and tunneling transmittance in a Mn3Ga/MgO/Mn3Ga structure were also performed. The results revealed the existence of Delta(1)-bands in Mn3Ga and demonstrated the possibility of a coherent tunneling process existing in the MTJ. (C) 2011 The Japan Society of Applied Physics

DOI： 10.1143/APEX.4.043002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Spin precession with frequencies up to 280 GHz is observed in Mn(3-delta)Ga alloy films with a perpendicular magnetic anisotropy constant K(u) similar to 15 Merg/cm(3). The damping constant alpha, characterizing macroscopic spin relaxation and being a key factor in spin-transfer-torque systems, is not larger than 0.008 (0.015) for the delta = 1.46 (0.88) film. Those are about one-tenth of alpha values for known materials with large K(u). First-principles calculations well describe both low alpha and large K(u) for these alloys.

DOI： 10.1103/PhysRevLett.106.117201

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

The structural and magnetic properties of Co2MnSi films on different oriented n-Ge substrates were compared. The orientation of the substrates changed the growth of the film. Therefore, a significant change of magnetization of Co2MnSi films was noticed depending on the orientation of the Ge substrates. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssa.201026569

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Fast magnetization precession is observed in L1(0)-FePt alloy epitaxial thin films excited and detected by all-optical means. The precession frequency varies from 45 to 65 GHz depending on the applied magnetic field strength and direction, which can be explained by a uniform precession model taking account of first- and second-order uniaxial magnetic anisotropy. The lowest effective Gilbert damping constant has a minimum value of 0.055, which is about half that in Co/Pt multilayers and is comparable to Ni/Co multilayers with perpendicular magnetic anisotropy. (C) 2011 American Institute of Physics. [doi:10.1063/1.3549704]

DOI： 10.1063/1.3549704

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Magnetic tunnel junctions (MTJs) with a CoFeB/Ru/Ni80Fe20 synthetic ferrimagnetic free layer and an MgO barrier layer were fabricated. The effect of the shape and thickness of the free layer on the magnetic field sensor characteristics was systematically investigated. We achieved a high sensitivity of 4.8%/Oe in the MTJ with a 70-nm-thick Ni80Fe20 layer and an aspect ratio of 1.0. Here, sensitivity is defined as TMR/(2H(k)), where TMR is tunnel magnetoresistance ratio in the MTJ and H-k is a magnetic anisotropy field of the free layer. Furthermore, we successfully increased the detection field range up to 230 Oe while keeping high sensitivity and linearity. (c) 2011 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.50.013001

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DOI： 10.1143/APEX.4.013005

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DOI： 10.1143/APEX.4.019201

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DOI： 10.1063/1.3512958

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We investigated the polarized Raman spectra of a strain-free, unipolar-axis oriented tetragonal Pb(Zr,Ti)O-3 thick epitaxial film. We evaluated the single crystal-like selection rules of the A(1)- and E-symmetry components, and found an anomalous behavior in the angular dependence of the A(1)(1TO)-mode intensity similar to that observed in high-T-c superconductor single crystals. Raman tensor analyses of the A(1)(1TO) mode revealed complex phases may exist between two independent Raman-tensor components even in the single 180 degrees domain state. (C) 2010 American Institute of Physics. [doi:10.1063/1.3488015]

DOI： 10.1063/1.3488015

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CERAMIC SOC JAPAN-NIPPON SERAMIKKUSU KYOKAI  

A measurement system with a high driving frequency of 100 kHz is effective for measuring the ferroelectricity of leaky ferroelectric materials such as multiferroic BiFeO3 films. A high driving frequency reduces the measurement time, leading to a drastic reduction in the influence of the leakage current density on ferroelectric hysteresis loops. Phase-delay compensation is essential in a system with a driving frequency of 100 kHz; in this study a standard capacitor was used for phase-delay compensation. The value of remanent polarization of a BiFeO3 film measured by the 100-kHz system was confirmed by a positive, up, negative and down measurement. (C) 2010 The Ceramic Society of Japan. All rights reserved.

DOI： 10.2109/jcersj2.118.656

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The magnetic damping constant in a series of Co2MnAlxSi1-x and Co2FexMn1-xSi Heusler alloy epitaxial films were systematically investigated by using ferromagnetic resonance technique. The determined magnetic damping constant is roughly proportional to the density of states at the Fermi energy of the first principle calculation. The result is consistent with the theoretical prediction when taking spin-orbit interaction into account. The small Gilbert damping constant for the fabricated films other than the Co2FexMn1-xSi film with x&gt;0.6 can be originated in the half-metallic electronic structure of Heusler alloys. (C) 2010 American Institute of Physics. [doi:10.1063/1.3456378]

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The magnetic properties of CoFeB buffered [Co(0.3 nm)/Pd(x nm)](6) multilayer films have been investigated. It is found that the magnetic properties of the multilayer depended on the Pd thickness and for thickness below 0.5 nm no perperdicular anisotropy is observed. Magnetization dynamics in perpendicularly magnetized CoFeB-[Co/Pd] multilayer films are investigated using time-resolved magneto-optical Kerr effect (TRMOKE). The variation of precession frequency with external magnetic field for different palladium thickness is quantitatively understood using the macrospin approximation of the Landau-Lifshitz-Gilbert equation of motion. The Gilbert damping constant alpha, in the range 0.04-0.1, varying with the palladium thickness is reported. The observed alpha value is comparable to the damping coefficient of bulk Ni, and much lower than the reported values for perpendicularly magnetized films. The CoFeB buffer layer with in-plane anisotropy appears to significantly affect the precession frequency and thus the damping constant of the films.

DOI： 10.1109/TMAG.2009.2038929

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The influence of annealing temperature and film thickness on structural and magnetic properties of Mn2.5Ga films were investigated in this work. The annealing temperature of 400 degrees C was found to be the optimum condition to obtain the films with high perpendicular magnetic anisotropy (PMA) (K-u(eff) = 7.8 x 10(6) erg/cm(3)) and smooth surface (R-a approximate to 0.15 nm). The PMA property was maintained in the 5 nm thick film, and deterioration of the PMA properties with decreasing film thickness can be ascribed to the tensile strain existed in the thin Mn2.5Ga films.

DOI： 10.1109/TMAG.2010.2045108

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

In this work, we have presented the electrical characteristic of Co2MnSi/MgO/n-Si junctions as a function of the doping concentration of Si. Films were fabricated by dc sputtering and post annealed at 400 degrees C for 1 h without breaking the vacuum. The Co2MnSi/MgO/n-Si junctions exhibited diode like characteristics at low doping concentration 10(16)/cc. This can be attributed due to oxide charges or interface traps close to the silicon interface, which causes the bend bending and forms the large extended depletion region. The junction characteristic was found to change with the increase of doping concentration and became symmetric at doping concentration of 10(19)/cc. Therefore, with the same thickness of MgO barrier, the junction characteristic was changed from Schottky to symmetric tunneling with the doping density of Si. The origin of the change of junction characteristic might be due to the change of the depletion width with the doping density.

DOI： 10.1109/TMAG.2010.2043223

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Magnetic tunnel junctions (MTJs) of perpendicularly magnetized [formula omitted] structure were fabricated. In-plane x-ray diffraction measurements and element specific evaluation of magnetic properties using soft x-ray magnetic circular dichroism were carried out to examine quite thin [formula omitted] (CMS) inserted layer between CoPt and MgO interface. Ordered [formula omitted] was successfully fabricated onto [formula omitted] as thin as 1 nm thick, and CMS layer shows perpendicular magnetic anisotropy below 3 nm thick via exchange coupling with CoPt layer. In the MTJ-stacking, epitaxial growth was confirmed except for partial misalignment in the upper FePt layer, and coercive field difference clearly appeared between the bottom and the top ferromagnetic electrodes. © 2010, American Institute of Physics. All rights reserved.

DOI： 10.1063/1.3358239

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To investigate the correlation between perpendicular magnetic anisotropy and intrinsic Gilbert damping, time-resolved magneto-optical Kerr effect was measured in Pt/Co(d(Co))/Pt films. These films showed perpendicular magnetization at d(Co)=1.0 nm and a perpendicular magnetic anisotropy energy K(u)(eff) that was inversely proportional to d(Co). With an analysis based on the Landau-Lifshitz-Gilbert equation, the intrinsic Gilbert damping constant alpha was evaluated by parameter-fitting of frequency and lifetime expressions to experimental data of angular variations in spin precession frequency and life-times. The alpha values increased significantly with decreasing d(Co) but not inversely proportional to d(Co).

DOI： 10.1063/1.3396983

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The structure and magnetic properties of Co2MnSi films on n-Ge(111) substrate were investigated for the motivation of spin injection. The 50-nm-thick Co2MnSi films were fabricated on n-Ge(111) substrates by dc sputtering and postannealed about 1 h at the various temperatures ranging from 200-500 degrees C. In the annealing temperature range of 300-400 degrees C, the Co2MnSi were grown epitaxially at the vicinal surface of n-Ge(111) substrates. The epitaxial layer of Co2MnSi films was about 18 nm thick at 350 degrees C and exhibited ferromagnetic behavior. Interestingly, there was no diffusion of Ge inside the epitaxial layer and atomically flat interface was obtained.

DOI： 10.1063/1.3378986

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One of the features of spin-transfer torque (STT)-based magnetic random access memories (spin-RAMs) is a fast write cycle; however, switching properties in the subnanosecond regime for MgO-based magnetic tunnel junctions (MTJs) are still unclear. In this work, we demonstrated subnanosecond magnetization switching by STT for MgO-based MTJs. We also discuss the thermal effect on subnanosecond STT switching, as well as the subnanosecond pulse width that is dependent on the remarkable plateau that switching probability has under a zero-bias field.

DOI： 10.1063/1.3380595

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Language：Japanese   Publisher：公益社団法人 応用物理学会  

DOI： 10.11470/jsapmeeting.2010.1.0_2054

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We report electrical transport properties of perpendicular magnetized Mn-Ga epitaxial films with various thicknesses. The maximum extraordinary Hall resistivity and Hall angle is 11.5 mu cm and 5.7%, respectively, which is comparable to the highest value reported in amorphous Fe0.79Gd0.21 alloy. In the low temperature region, resistivity was proportional to T-2.9 owing to the unconventional one-magnon scattering processes, indicating high spin polarization of this material.

DOI： 10.1063/1.3298363

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

BiFeO3 (BFO) films were formed on (001), (110), and (111) La-doped SrTiO3 single-crystal substrates. All the films were epitaxially grown and had mainly rhombohedral structure. The BFO film formed on the (001) substrate had only a (001) component whereas the BFO films on (110) and (111) substrates had ((1) over bar 10) and ((1) over bar 11) components, respectively. The (001), (110), and (111) epitaxial BFO films showed the remanent polarizations of 63, 84, and 106 mu C/cm(2), respectively. It seems that the ((1) over bar 10) and ((1) over bar 11) components were changed to (110) and (111) ones, respectively, by applying an external voltage. The (001) epitaxial BFO film showed marked asymmetry in its electrical properties. It was found that the (001) epitaxial BFO film had a thin tetragonal layer with spontaneous polarization fixed in the downward direction near the substrate. Finally, we concluded that this tetragonal layer caused the accumulation of space charges at the interface, causing a downward built-in field to be generated. The downward build-in field facilitated the switching of upward polarization and caused asymmetric relaxation. (C) 2010 The Japan Society of Applied Physics

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We report for the first time that the dynamic magnetic intermediate state (DMI) was observed at a speed of several ns during spin transfer switching for MgO-based magnetic tunnel junctions (MTJs). The DMI was observed as slow resistance oscillation at the center of the parallel to anti-parallel state by single shot time domain measurements. The DMI is observable at certain current amplitudes. The outbreak probability decreases with further current increase. We concluded that the DMI originates from inhomogeneous magnetization behavior. On the other hand, previous single shot time domain measurements have shown only for single-domain-like magnetization behavior. (C) 2010 The Japan Society of Applied Physics

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We have investigated magnetic field strength (up to 10 kOe) and angle dependences of spin dynamics in 4-nm-thick films of CoCrPt alloys with perpendicular magnetic anisotropy using the all-optical time-resolved magneto-optical Kerr effect (TRMOKE). The comprehensive TRMOKE measurements have indicated the Gilbert damping constant alpha of 0.05 for the alloy film with low coercivity. The experiments also indicated that alpha values for the alloy films deposited at higher temperatures with higher coercivities were also no greater than 0.06. (C) 2010 The Japan Society of Applied Physics

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Ceramic Society of Japan  

Amorphous Bi–Fe–O<i>_x</i> films prepared on SrTiO₃ (100) substrates using a conventional r.f. magnetron sputtering system were crystallized by post-annealing at 873 K in an atmosphere. Microstructural observations by X-ray diffraction and cross-sectional transmission electron microscopy revealed that the crystallized Bi–Fe–O<i>_x</i> films were well-epitaxially BiFeO₃ fabricated without interfacial layer although as-prepared film was amorphous structure with excess Bi. The crystallized BiFeO₃ films have fairly epitaxial compatibly ([001](001)BiFeO₃ // [001](001)SrTiO₃). These results indicate that (1) BiFeO₃ has good epitaxial compatibility with SrTiO₃ and (2) crystallizing amorphous Bi–Fe–O<i>_x</i> is one possible method that can be used to fabricate high-quality multiferroic barriers for tunnel junctions.

DOI： 10.2109/jcersj2.118.648

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We investigated the structure and magnetic properties of Co2MnSi/MgO/n-Si(100) films with the goal of achieving efficient spin injection. The Co2MnSi films were fabricated by dc sputtering and post annealed at 400 degrees C for 1 h. They were oriented along the &lt; 100 &gt; direction with a MgO barrier layer. A strong chemical reaction was observed between the Co2MnSi thin films and n-Si substrates in the absence of the MgO barrier. The diffusion or chemical reaction may occur up to a MgO layer thickness of 2 nm. With a MgO layer that is 2 nm thick, Co2MnSi possessed large saturation magnetization and low surface roughness at room temperature. The electrical (I-V) characteristics of Co2MnSi/MgO(2 nm)/n-Si(100) obtained at various junction sizes were symmetric, suggesting that MgO was an effective tunnel barrier. (C) 2009 American Institute of Physics. [doi:10.1063/1.3260253]

DOI： 10.1063/1.3260253

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The structural, magnetic and electrical properties of Co2MnSi thin films grown onto n-doped Si(110) and n-Si(100) substrates were studied. The structure and magnetic properties of Co2MnSi thin films were found to depend strongly on the annealing temperature (T-A). At T-A = 275-350 degrees C, the Co2MnSi films were of B2 phase with &lt; 100 &gt; texture and possessed magnetic moment on both substrates. The saturation magnetization (M-S) of Co2MnSi thin films was found maximum at T-A = 300 degrees C. Chemical reaction might occur between Co2MnSi and Si above T-A = 350 degrees C which caused nearly zero M-S value. The current-voltage (I-V) characteristic of the Co2MnSi thin films onto n-Si substrates was obtained linear suggesting the contacts were ohmic nature.

DOI： 10.1109/TMAG.2009.2024320

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The atomic ordering of Co2MnSi (CMS) full-Heusler film and the interface structure of CMS/MgO/CMS magnetic tunnel junctions (MTJs) were investigated by high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM). We observed the atomic ordering of L2(1) and B2 structures of CMS from the atomic number (Z) contrast STEM images. We also confirmed that the interface structure consists of the layer next to the Co layer terminating in the CMS to MgO layer from the layer periodicity along the [001] direction, however, site-disorder exists between two atomic layers at the termination of CMS, including locally L2(1)-ordered MnSi terminated structure. (C) 2009 The Japan Society of Applied Physics

DOI： 10.1143/APEX.2.093001

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The optical properties of the chemical solution deposition (CSD)-derived BiFeO3 (BFO) film and the pulsed laser deposition (PLD)-derived (Bi,Nd)(Fe,Mn)O-3 (BNFM) film were evaluated by spectroscopic ellipsometry, and their optical constants (n, k) and band gaps were determined. At a wavelength of 600 nm, the refractive indexes of 3.22 and 2.87 were estimated for the BFO and BNFM films, respectively, although the existence of a refractive index gradient was suggested in the BFO film. In addition, at a wavelength of 1550 nm, which is generally used for optical communication, the refractive indexes of 2.91 and 2.59 were estimated for the BFO and BNFM films, respectively. The band gaps of the BFO and BNFM films were estimated to be 2.79 and 2.72 eV, respectively, and it was confirmed that the extinction coefficients of both films were almost zero at wavelengths larger than 600 nm. These results suggest that the BFO-system multiferroic films have a high potential as an optical material with a high refractive index. (C) 2009 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.48.09KB01

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CoFeB/MgO/CoFeB/MgO/CoFeB double barrier magnetic tunnel junctions (DBMTJs) with thin middle layers were fabricated on SiO2/Si(001) substrates by r.f. magnetron sputtering. We successfully obtained a large tunnel magnetoresistance of 1056% at room temperature in a DBMTJ with a middle CoFeB layer thickness of 1.2 nm that was fabricated at a relatively low post-deposition annealing temperature of 350 degrees C. This DBMTJ also realized sharp magnetization switching in the free middle CoFeB layer, which is attributed to strong antiferromagnetic coupling between the exterior CoFeB and PtMn layers. These favorable magnetoresistive properties offer interesting possibilities for developing practical spintronics applications and noble magnetotransport physics. (C) 2009 The Japan Society of Applied Physics

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The structural and magnetic properties of CO2MnSi thin films grown on n-doped Si(110) and Si(100) substrates were studied and observed to have a strong dependence on annealing temperature (T-A). At T-A = 275-350 degrees C, the CO2MnSi films exhibited the B2 phase with a &lt; 100 &gt; orientation and a magnetic moment on both substrates. The saturation magnetization (M-S) of CO2MnSi thin films was observed to reach a maximum at T-A = 300 degrees C, above which it was found to decrease. We consider that at T-A similar or equal to 300 degrees C, the CO2MnSi thin films on Si substrates exhibited the (100) orientation, a high M-S and a low roughness which might promote spin injection. (C) 2009 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.48.083002

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Tunnel magnetoresistance (TMR) effect was investigated in Co2MnSi/CoFeB(0-2 nm)/MgO/CoFe magnetic tunnel junctions (MTJs). TMR ratio was enhanced by inserting a thin CoFeB layer at the Co2MnSi/MgO interface. The MTJ with CoFeB thickness of 0.5 nm exhibited the highest TMR ratio. From the conductance-voltage measurements for the fabricated MTJs, we infer that the highly spin polarized electron created in Co2MnSi can conserve the polarization through the 0.5-nm-thick CoFeB layer. Furthermore, by insertion of the thin CoFeB layer, the temperature dependence of the TMR ratio was improved because of the suppression of the fluctuation of the magnetic moment at the Co2MnSi/MgO interface.

DOI： 10.1063/1.3156858

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The applicability of perpendicularly magnetized CoCrPt films to the MgO-based magnetic tunnel junctions (MTJs) was investigated. For this study, CoCrPt films deposited on the Ru buffer exhibited hcp(0002)-oriented growth by sputtering method using the low substrate temperature of 250 degrees C, low saturation magnetization of around 360 emu/cm(3), and high magnetic anisotropy field of 6 kOe, which is sufficient to retain the thermal stability of the magnetization direction. The MgO-based MTJs with a synthetic ferrimagnetlike structure were fabricated: CoFe was coupled magnetically with CoCrPt through the thin Ru layer. Transport properties with a magnetic field applied perpendicular to the film plane revealed a tunnel magnetoresistance ratio of about 6% at room temperature. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3062816]

DOI： 10.1063/1.3062816

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f An interfacial epitaxial BiMnO3 layer was fabricated by chemical solution deposition on SrTiO3 (100) substrate, and the microstructure of the film was analyzed by x-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM). The TEM observation revealed the epitaxial growth of BiMnO3 on the SrTiO3 substrate as follows: ([110] (001)) BiMnO3 parallel to[0-10] x (001) SrTiO3. XRD and TEM analyses revealed that the mismatch between the epitaxial BiMnO3 and the SrTiO3 substrate causes a distortion in lattice parameters of BiMnO3 and, consequently, a large compressive strain in the BiMnO3 layer. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3074096]

DOI： 10.1063/1.3074096

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

BiFeO3-based composite films with excess iron content were fabricated, and the dependence of their magnetic and ferroelectric properties on the excess iron content was systematically investigated. Structural and magnetic analyses indicated that the specimens might be composed of large antiferromagnetic BiFeO3 grains and small ferromagnetic component. When the iron content was increased, saturation magnetization (M-s) increased and remanent polarization decreased. Antiferromagnetic coupling (H-ex) between the antiferromagnetic BiFeO3 grains and the ferromagnetic grains was observed at 10 K. It was revealed that grain growth is the key to increasing M-s and P-r and observing H-ex at room temperature in BiFeO3-based composite films. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3055284]

DOI： 10.1063/1.3055284

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The influence of Pb and La contents on the lattice configuration in La-substituted Pb(Zr-0.65,Ti-0.35)O-3 (La-PZT) films was systematically investigated. La-PZT films with various La and Pb contents were fabricated on Pt/Ti/SiO2/Si substrates by chemical solution deposition (CSD). In the La-PZT films with a Pb content ratio of 125% relative to a stoichiometric value, La ions were substituted for not only A-site ions but also B-site ions at La contents greater than 3 mol%. La substitution for B-site seems to cause larger reduction of the unit cell size. In addition, we found that in the La-PZT films with a La content of 3 mol%, the Pb content of 116 mol% (120% relative to a stoichiometric value) was optimum from the viewpoint of site occupancy. This indicates that excess Pb prevented the A-site substitution of La ions.

DOI： 10.1109/TUFFC.2009.1091

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We report on epitaxial growth and magnetic properties of Mn2.5Ga thin films, which were deposited on Cr/MgO single crystal substrates by magnetron sputtering. X-ray diffraction results revealed the epitaxial relationships as Mn2.5Ga(001)[100]parallel to Cr(001)[110]parallel to MgO(001)[100]. The presence of (002) and (011) superlattice peaks indicates that the films were crystallized into DO22 ordered structures. The perpendicular magnetic anisotropy (PMA) properties were found to be related to the extent of DO22 chemical ordering. A giant PMA (K-u(eff)=1.2x10(7) erg/cm(3)) and low saturation magnetization (M-s=250 emu/cm(3)) can be obtained for the film with highest chemical ordering parameter (S=0.8).

DOI： 10.1063/1.3108085

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Transport properties in magnetic tunnel junctions (MTJs) with Co2FexMn1-xSi (CFMS, x=0-1.0)/Al-O/Co75Fe25 structure and Gilbert damping constant in the epitaxial CFMS films were investigated. The tunnel magnetoresistance ratio is as high as 75% in MTJs with x=0.6 at room temperature. The Gilbert damping constant is minimal at x=0.4. Relations between half-metallicity and the Gilbert damping constant in CFMS films were examined, revealing that the damping constant is small in half-metallic CFMS films.

DOI： 10.1063/1.3105982

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

xBiCoO(3)-(1-x)BiFeO3 films were deposited by metalorganic chemical vapor deposition. Although the film composition changed with deposition temperature, the composition could be adjusted by varying the input source gas composition at 700 degrees C. Moreover, adjusting the deposition time could change 0.16BiCoO(3)-0.84BiFeO(3) film thickness. The crystal symmetry changed from rhombohedral to tetragonal as the film thickness decreased for 0.16BiCoO(3)-0.84BiFeO(3) films grown on both (100)SrTiO3 and (100)(c)SrRuO3 parallel to(100)SrTiO3 substrates, implying that the x value of the crystal symmetry boundaries between the tetragonal and rhombohedral structures changes with film thickness.

DOI： 10.1063/1.3073824

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We fabricated 5 at.% Mn-added polycrystalline BiFeO3 films and investigated the annealing temperature effect on structural, ferroelectric and magnetic properties. In the x-ray diffraction patterns, only the diffraction peaks due to the BiFeO3 structure were observed and no secondary phase could be observed at annealing temperatures between 773 and 923 K. Adding Mn suppressed the leakage current density in the high electric field region when compared to pure BiFeO3 films. The conduction mechanism of the Mn-added BiFeO3 films was dominated by Ohmic conduction. Remanent polarization of the Mn-added polycrystalline BiFeO3 films for an applied electric field of approximately 1.5 mV/cm was 63 mu C/cm(2) for the specimen annealed at 773 K and 46 mu C/cm(2) for the specimen annealed at 923 K, although the remanent polarization still exhibited a tendency to increase with an increase in the electric field. Spontaneous magnetization was obtained at high annealing specimens. This study revealed that the annealing temperature strongly affected the ferroelectric and magnetic properties in Mn-added polycrystalline BiFeO3 films. In addition, by optimizing the annealing temperature, we realized multiferroics coexistent with spontaneous magnetization and spontaneous polarization at room temperature in the Mn-added polycrystalline BiFeO3 film.

DOI： 10.1007/s10832-007-9400-3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

BaPbO3 films were fabricated by a chemical solution deposition on the SiO2/Si(100) and MgO(100) substrates followed by a post-deposition annealing at the temperatures between 673 and 1073 K under oxygen flow. Polycrystalline BaPbO3 films were formed together with secondary phases such as PbO and Pb3O4 onto MgO(100) substrates at around 750 K, and the films were crystallized into single phase of BaPbO3 above 823 K. Endothermic peak in differential thermal analysis due to crystallization of BaPbO3 was observed at 750 K, which is consistent with crystallization temperature of BaPbO3 estimated from X-ray diffraction. The electrical resistivity depended on the annealing temperature even in the single phase BaPbO3 films, the lowest resistivity of 3 x 10(-6) mu I (c)center dot m which was comparable to that of bulk BaPbO3 was achieved at the annealing temperature of 873 K.

DOI： 10.1007/s10832-008-9426-1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：THE PHYSICAL SOCIETY OF JAPAN  

Ceramic composites [<I>x</I>Ni<SUB>0.15</SUB>Cu<SUB>0.30</SUB>Zn<SUB>0.55</SUB>Fe<SUB>2</SUB>O<SUB>4</SUB>–(1−<I>x</I>)BaTiO<SUB>3</SUB>] were successfully prepared by a direct solid-state reaction of raw materials (BaCO<SUB>3</SUB>, CuO, α-Fe<SUB>2</SUB>O<SUB>3</SUB>, NiO, TiO<SUB>2</SUB>, and ZnO). The composites are so homogeneous that the ferrite and BaTiO<SUB>3</SUB> grains do not react with each other. The <I>x</I>-dependent permeability and permittivity are found to obey Maxwell–Garnett effective medium theory, which suggests that the composites consist of ferrite particles with barium titanate medium. This model, however, starts to deviate from the experimental data at <I>x</I>=0.75, and the roles of medium and inclusions seem to be exchanged. It can be qualitatively explained by the fact that geometrical close-packing of spheres is limited up to about 74 vol % (Kepler conjecture).

DOI： 10.1143/jpsj.78.124801

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

The effect of pulse switching on the piezoelectric response of 10-mu m-thick Pb(Zr(0.53)Ti(0.47))O(3) films was investigated. The amount of the longitudinal displacement related with piezoelectric response decreases when the bipolar pulse switching count is greater than 10(7). This degradation tendency is consistent with the "fatigue" profile for remanent polarization of the PZT films. However, longitudinal displacement degradation was not observed for unipolar pulse switching. These experimental results suggest that the unipolar drive of the high electrical field does not affect the displacement endurance property of the PZT thick films for micro-actuator application.

DOI： 10.1080/00150190902987715

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