Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PHYSREVB.107.165416

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

The ability to drive a spin system to state far from the equilibrium is indispensable for investigating spin structures of antiferromagnets and their functional nonlinearities for spintronics. While optical methods have been considered for spin excitation, terahertz (THz) pulses appear to be a more convenient means of direct spin excitation without requiring coupling between spins and orbitals or phonons. However, room-temperature responses are usually limited to small deviations from the equilibrium state because of the relatively weak THz magnetic fields in common approaches. Here, we studied the magnetization dynamics in a HoFeO₃ crystal at room temperature. A custom-made spiral-shaped microstructure was used to locally generate a strong multicycle THz magnetic near field perpendicular to the crystal surface; the maximum magnetic field amplitude of about 2 T was achieved. The observed time-resolved change in the Faraday ellipticity clearly showed second- and third-order harmonics of the magnetization oscillation and an asymmetric oscillation behaviour. Not only the ferromagnetic vector M but also the antiferromagnetic vector L plays an important role in the nonlinear dynamics of spin systems far from equilibrium.

DOI： 10.1038/s41467-023-37473-1

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Other Link： https://www.nature.com/articles/s41467-023-37473-1

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

Spin–orbit torque (SOT) switching in chiral antiferromagnets, such as D0₁₉-Mn₃Sn, has opened prospects in antiferromagnetic spintronics. In this paper, we investigated the critical switching voltage as a function of the voltage pulse width in order to explore the thermal stability factor Δ in the D0₁₉-Mn₃Sn. Although the critical switching voltage is found to decrease with increasing the voltage pulse width as expected by the thermally activated switching process with Δ of 131.1 at 300 K, our detailed examinations reveal that the Joule heating due to the voltage pulse could induce the magnetic phase transition in the D0₁₉-Mn₃Sn, which significantly alters the energy landscape relevant to the SOT switching. This assisting mechanism for the SOT switching inhibits an accurate determination of Δ but could deliver a smaller Δ than the actual. Our study provides cautionary note in estimating the thermal stability of the chiral antiferromagnets.

DOI： 10.1063/5.0144602

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

DOI： 10.1103/physrevapplied.19.l031003

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevApplied.19.L031003/fulltext

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

<p>  The superconducting diode effect in which electrical resistance is zero in only one direction has recently been reported in superconductors without inversion symmetry. Previous studies investigated the nonreciprocity of the critical current, but little has been known about the rectification effect when AC currents are applied. Herein, we examined the rectification characteristics of a non-centrosymmetric Nb/V/Ta artificial superlattice under AC currents. The rectification strength can be modulated by an applied magnetic field, and its polarity can be tuned by the magnetic field. Furthermore, we find that the magnetic field dependence of the rectification is different from that of the nonreciprocal critical current.</p>

DOI： 10.3379/msjmag.2309r001

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CiNii Research

Publishing type：Research paper (scientific journal)  

DOI： 10.35848/1882-0786/AC99B9

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Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.35848/1347-4065/AC9025

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Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0099618

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

Abstract

Chiral antiferromagnets are regarded as a new class of antiferromagnets owing to its intriguing spin transport properties such as a sizable anomalous Hall effect (AHE). In this work, we fabricated chiral antiferromagnetic L1₂-Mn₃Ir (111)-oriented films and investigated the anomalous Hall conductivity with respect to the chemical order parameter $S.$ It is found that the anomalous Hall conductivity in (111)-oriented films is much more pronounced compared to (001)-oriented films, which is consistent with the theoretical prediction. We also found that the anomalous Hall conductivity strongly depends on the longitudinal conductivity, which could suggest an extrinsic contribution to the AHE in L1₂-Mn₃Ir.

DOI： 10.35848/1347-4065/ac7625

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Other Link： https://iopscience.iop.org/article/10.35848/1347-4065/ac7625/pdf

Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1038/s41565-022-01159-4

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Other Link： https://www.nature.com/articles/s41565-022-01159-4

Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0087643

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Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0078688

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Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.jmmm.2022.169251

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Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PHYSREVAPPLIED.18.014032

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.jmmm.2022.169416

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Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.35848/1882-0786/ac3575

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Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0070012

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.jmmm.2021.168093

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

DOI： 10.1063/5.0056056

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Publishing type：Research paper (scientific journal)  

Antiferromagnets are one of the few candidate materials that can work at THz frequency and are, therefore, a potential material for THz technology recently attracting interest from high-speed communication and sensing applications. In this work, we investigate an antiferromagnetic resonant frequency (ωr) tunability for cation doped α-Fe2O3. It is found that, with various cation dopants (Al, Ru, Rh, and In), the resonant frequency can be tuned in a range between a millimeter-wave band and a THz band. We also complementally discuss the mechanism of the Morin temperature (TM) shift upon cation doping by the temperature and dopant dependence of ωr. A good frequency tunability shown in this work suggests that cation doped α-Fe2O3 is a useful material for millimeter-wave and THz applications.

DOI： 10.1063/5.0053586

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

We report that the superconducting critical magnetic field can be nonreciprocal under a bias of an electric current in a superconducting [Nb/V/Ta]( n ) superlattice without a center of inversion. The critical magnetic field showed a clear difference between positive and negative magnetic fields. Furthermore, the magnitude relation between the positive and negative critical magnetic fields is reversed when the direction of the electric current is reversed. Our findings indicate that the superconducting gap can be anisotropic by the application of an electric current.

DOI： 10.35848/1882-0786/ac03c0

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Publishing type：Research paper (scientific journal)   Publisher：The Magnetics Society of Japan  

DOI： 10.3379/msjmag.2107l002

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Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.35848/1347-4065/abfdc1

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Other Link： https://iopscience.iop.org/article/10.35848/1347-4065/abfdc1/pdf

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

DOI： 10.1088/1361-648x/abec1a

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Other Link： https://iopscience.iop.org/article/10.1088/1361-648X/abec1a/pdf

Publishing type：Research paper (scientific journal)   Publisher：The Magnetics Society of Japan  

DOI： 10.3379/msjmag.2103r004

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Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.35848/1882-0786/abd86a

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/abd86a/pdf

Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.35848/1347-4065/abd67d

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Other Link： https://iopscience.iop.org/article/10.35848/1347-4065/abd67d/pdf

Publishing type：Research paper (scientific journal)   Publisher：The Magnetics Society of Japan  

DOI： 10.3379/msjmag.2011r002

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Publishing type：Research paper (scientific journal)   Publisher：The Magnetics Society of Japan  

DOI： 10.3379/msjmag.2101l002

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Publishing type：Research paper (scientific journal)  

DOI： 10.1097/ALN.0000000000004016

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

The propagation of coherent spin waves in permalloy microstrips was investigated using a heterodynemagneto-optical Kerr effect technique. The two-dimensional imaging reveals the mode interference with a high-order spin-wave mode along the microstrip width. The spin-wave dispersion relations across the microstrip width were deduced from fast Fourier transformations on the complex spin-wave amplitude of the one-dimensional scan, and multiple spin-wave modes were observed at the edges of the microstrips. These experimental results provide direct evidence for the quantization of spin-wave dispersion due to the finite width of the microstrips and enables us to evaluate the attenuation length for each quantized spin-wave mode.

DOI： 10.1103/PhysRevB.102.214440

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

DOI： 10.1063/5.0020020

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Publishing type：Research paper (scientific journal)   Publisher：The Magnetics Society of Japan  

DOI： 10.3379/msjmag.2011l003

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.jmmm.2020.167253

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media {LLC}  

DOI： 10.1038/s41586-020-2590-4

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Other Link： http://www.nature.com/articles/s41586-020-2590-4

Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.35848/1882-0786/aba0dc

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/aba0dc/pdf

Publishing type：Research paper (scientific journal)  

This paper introduces the concept of spin-orbit-torque-magnetic random access memory (SOT-MRAM) based physical unclonable function (PUF). The secret of the PUF is stored into a random state of a matrix of perpendicular SOT-MRAMs. Here, we show experimentally and with micromagnetic simulations that this random state is driven by the intrinsic nonlinear dynamics of the free layer of the memory excited by the SOT. In detail, a large enough current drives the magnetization along an in-plane direction. Once the current is removed, the in-plane magnetic state becomes unstable evolving toward one of the two perpendicular stable configurations randomly. In addition, we propose a hybrid CMOS/spintronics model to simulate a PUF realized by an array of 16 × 16 SOT-MRAM cells and evaluate the electrical characteristics. Hardware authentication based on this PUF scheme has several characteristics, such as CMOS-compatibility, non-volatility (no power consumption in standby mode), reconfigurability (the secret can be reprogrammed), and scalability, which can move a step forward the design of spintronic devices for application in security.

DOI： 10.1063/5.0013408

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

DOI： 10.1103/PhysRevMaterials.4.074402

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevMaterials.4.074402/fulltext

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

DOI： 10.1103/PhysRevLett.125.017203

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevLett.125.017203/fulltext

Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

Electron and spin transport in chiral antiferromagnets are of great interest in both fundamental and application point of views. In this letter, we explore and compare spin transport properties of a chiral antiferromagnetic L12-ordered Mn3Ir and a disordered Mn3Ir by the spin pumping experiments. Larger Gilbert damping enhancement of the Fe20Ni80 was found in L12-ordered Mn3Ir/Fe20Ni80 and L12-ordered Mn3Ir/Ti/Fe20Ni80 multilayers comparing to those with disordered ones. Our results suggest that the chiral spin structure causes larger spin dissipation than disordered spin structure.

DOI： 10.35848/1882-0786/ab9876

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Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

In this study, we theoretically investigated properties of magnetostatic spin waves in rare-earth and transition-metal ferrimagnets with various angular momentum. The resonant frequency of high-(low-) frequency mode has the lowest (highest) value at angular momentum compensation temperature (T ). It was found that both resonance modes show a maximum group velocity at T . Furthermore, the low-frequency mode shows higher group velocity than the high-frequency mode at any temperature. Our findings are the key to advance both the physics and the technology to develop research field of magnonics based on the antiferromagnetic spin waves. A A

DOI： 10.35848/1882-0786/ab916d

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

We investigated caustic-like spin wave beams radiated from a narrow waveguide to a continuous film using a Permalloy. For this purpose, we developed a spatially resolved optical heterodyne detection of propagating spin waves by using the polar Kerr effect, which allows the measurement of the intensity and the phase information of the spin waves. We characterized magnetostatic surface spin wave modes in one-dimensional (1D) propagation and caustic-like spin wave beams in two-dimensional (2D) propagation as a function of excitation frequency. The fast Fourier transformations of the real space images reconstruct 1D and 2D spin wave dispersion, which agree with the theoretical calculations.

DOI： 10.1063/5.0010410

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

DOI： 10.1063/1.5142495

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Publishing type：Research paper (scientific journal)   Publisher：American Association for the Advancement of Science (AAAS)  

The nonreciprocity of propagating spin waves, i.e., the difference in amplitude and/or frequency depending on the propagation direction, is essential for the realization of spin wave–based logic circuits. However, the nonreciprocal frequency shifts demonstrated so far are not large enough for applications because they originate from interfacial effects. In addition, switching of the spin wave nonreciprocity in the electrical way remains a challenging issue. Here, we show a switchable giant nonreciprocal frequency shift of propagating spin waves in interlayer exchange–coupled synthetic antiferromagnets. The observed frequency shift is attributed to large asymmetric spin wave dispersion caused by a mutual dipolar interaction between two magnetic layers. Furthermore, we find that the sign of the frequency shift depends on relative configuration of two magnetizations, based on which we demonstrate an electrical switching of the nonreciprocity. Our findings provide a route for switchable and highly nonreciprocal spin wave–based applications.

DOI： 10.1126/sciadv.aaz6931

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PubMed

Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevApplied.13.034052

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Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevB.101.060402

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Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevB.101.014448

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Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5128241

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Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5131768

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Publishing type：Research paper (scientific journal)  

We developed ferrimagnetic [Co(0.5)/Gd(1)/Pt(1)]N (unit: nm) multilayers with structural inversion symmetry breaking and investigated the dependence of magnetic properties and magnetic domain structures on the repetition number (N = 1 – 50). The magnetization compensation temperature increases as N increases, and saturates at around 210 K for N ≥ 20. All films with various repetition number possess the out-of-plane magnetic easy axis, and multi domain structure at the remanence state was observed in the film with N = 50. These results show that the magnetic properties of the ferrimagnetic Co/Gd/Pt multilayers can be tuned by the repetition number.

DOI： 10.3379/msjmag.2001R002

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Publisher：Japan Society of Applied Physics  

DOI： 10.7567/1347-4065/ab6505

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

DOI： 10.1103/physrevx.9.041016

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Publishing type：Research paper (scientific journal)  

We provide a macroscopic theory and experimental results for magnetic resonances of antiferromagnetically-coupled ferrimagnets. Our theory, which interpolates the dynamics of antiferromagnets and ferromagnets smoothly, can describe ferrimagnetic resonances across the angular momentum compensation point. We also present experimental results for spin-torque induced ferrimagnetic resonance at several temperatures. The spectral analysis based on our theory reveals that the Gilbert damping parameter, which has been considered to be strongly temperature dependent, is insensitive to temperature. We envision that our work will facilitate further investigation of ferrimagnetic dynamics by providing a theoretical framework suitable for a broad range of temperatures.

DOI： 10.7567/1882-0786/ab33d5

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Publishing type：Research paper (scientific journal)  

Antiferromagnetic materials offer ultrafast spin dynamics and could be used to build devices that are orders of magnitude faster than those based on ferromagnetic materials. Spin-transfer torque is key to the electrical control of spins and has been demonstrated in ferromagnetic spintronics. However, experimental exploration of spin-transfer torque in antiferromagnets remains limited, despite a number of theoretical studies. Here, we report an experimental examination of the effects of spin-transfer torque on the motion of domain walls in antiferromagnetically coupled ferrimagnets. Using a ferrimagnetic gadolinium–iron–cobalt (GdFeCo) alloy in which Gd and FeCo moments are coupled antiferromagnetically, we find that non-adiabatic spin-transfer torque acts like a staggered magnetic field, providing efficient control of the domain walls. We also show that the non-adiabaticity parameter of the spin-transfer torque is significantly larger than the Gilbert damping parameter, in contrast to the case of non-adiabatic spin-transfer torque in ferromagnets.

DOI： 10.1038/s41928-019-0303-5

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

Recent advances in antiferromagnetic spin dynamics using rare-earth (RE) and transition-metal (TM) ferrimagnets have attracted much interest for spintronic devices with a high speed and density. In this study, the spin wave properties in the magnetostatic backward volume mode and surface mode in RE-TM ferrimagnetic GdxCo1-x films with various composition x are investigated using spin wave spectroscopy. The obtained group velocity and attenuation length are well explained by the ferromagnet-based spin wave theory when the composition of GdxCo1-x is far from the compensation point. (C) 2019 The Japan Society of Applied Physics

DOI： 10.7567/1347-4065/ab2f98

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Publishing type：Research paper (scientific journal)  

We studied perpendicular magnetic anisotropy (PMA) and magnetization compensation temperature (T ) in Tb/CoFeB/MgO films. Interlayer antiferromagnetic coupling between Tb and CoFeB layers enables useful antiferromagnetic properties containing a CoFeB/MgO junctions, which are important for the practical applications to obtain giant tunnel magnetoresistance and voltage-control of magnetic anisotropy. While the PMA is mainly attributed to the interface at CoFeB/MgO, T can be controlled by the thickness ratio of Tb and CoFeB layers as the magnetic moments of them are antiferromagnetically coupled. Our results provide many possibilities to add exciting new functions to ferrimagnetic spintronics. M M

DOI： 10.7567/1882-0786/ab2c23

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Publishing type：Research paper (scientific journal)  

Symmetry breaking is a fundamental concept that prevails in many branches of physics . In magnetic materials, broken inversion symmetry induces the Dzyaloshinskii–Moriya interaction (DMI), which results in fascinating physical behaviours with the potential for application in future spintronic devices . Here, we report the observation of a bulk DMI in GdFeCo amorphous ferrimagnets. The DMI is found to increase linearly with an increasing thickness of the ferrimagnetic layer, which is a clear signature of the bulk nature of DMI. We also found that the DMI is independent of the interface between the heavy metal and ferrimagnetic layer. This bulk DMI is attributed to an asymmetric distribution of the elemental content in the GdFeCo layer, with spatial inversion symmetry broken throughout the layer. We expect that our experimental identification of a bulk DMI will open up additional possibilities to exploit this interaction in a wide range of materials. 1–5 6–14 15–17

DOI： 10.1038/s41563-019-0380-x

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PubMed

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

We systematically investigate the perpendicular magnetocrystalline anisotropy (MCA) in Co-Pt/Pd-based multilayers. Our magnetic measurement data show that the asymmetric Co/Pd/Pt multilayer has a significantly larger perpendicular magnetic anisotropy (PMA) energy compared to the symmetric Co/Pt and Co/Pd multilayer samples. We further support this experiment by first-principles calculations on CoPt2, CoPd2, and CoPtPd, which are composite bulk materials that consist of three atomic layers in a unit cell, Pt/Co/Pt, Pd/Co/Pd, and Pt/Co/Pd, respectively. By estimating the contribution of bulk spin-momentum coupling to the MCA energy, we show that the CoPtPd multilayer with symmetry breaking has a significantly larger PMA energy than the other multilayers that are otherwise similar but lack symmetry breaking. This observation thus provides evidence of the PMA enhancement due to the structural inversion symmetry breaking and highlights the asymmetric CoPtPd as an artificial magnetic material with bulk spin-momentum coupling, which opens a pathway toward the design of materials with strong PMA.

DOI： 10.1103/PhysRevB.99.180410

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

In antiferromagnetic spintronics where manipulation of the antiferromagnetic spins is a central technological challenge, it is important to understand the dynamic properties, especially their THz spin dynamics and the magnetic damping. Here, we investigate thoroughly the antiferromagnetic spin dynamics in NiO by broadband THz spectroscopy and find the remarkable difference in the damping constants alpha = 5.0 +/- 0.4 x 10(-4) and 7.4 +/- 0.4 x 10(-4), respectively, for mono- and polycrystalline NiO, indicating the existence of strong extrinsic contributions to the damping, such as two-magnon scattering. Our results shed light on the mechanism of the antiferromagnetic dissipative dynamics and give important insights for developing antiferromagnetic and THz spintronics devices.

DOI： 10.1103/PhysRevMaterials.3.051402

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Publishing type：Research paper (scientific journal)  

We investigate the Gilbert damping parameter α for rare earth (RE)-transition metal (TM) ferrimagnets over a wide temperature range. Extracted from the field-driven magnetic domain-wall mobility, α was as low as the order of 10-3 and was almost constant across the angular momentum compensation temperature TA, starkly contrasting previous predictions that α should diverge at TA due to a vanishing total angular momentum. Thus, magnetic damping of RE-TM ferrimagnets is not related to the total angular momentum but is dominated by electron scattering at the Fermi level where the TM has a dominant damping role. This low value of the Gilbert damping parameter suggests that ferrimagnets can serve as versatile platforms for low-dissipation high-speed magnetic devices.

DOI： 10.1103/PhysRevLett.122.127203

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PubMed

Publishing type：Research paper (scientific journal)  

In the presence of a magnetic field, the flow of charged particles in a conductor is deflected from the direction of the applied force, which gives rise to the ordinary Hall effect. Analogously, moving skyrmions with non-zero topological charges and finite fictitious magnetic fields exhibit the skyrmion Hall effect, which is detrimental for applications such as skyrmion racetrack memory. It was predicted that the skyrmion Hall effect vanishes for antiferromagnetic skyrmions because their fictitious magnetic field, proportional to net spin density, is zero. Here we investigate the current-driven transverse elongation of pinned ferrimagnetic bubbles. We estimate the skyrmion Hall effect from the angle between the current and the bubble elongation directions. The angle and, hence, the skyrmion Hall effect vanishes at the angular momentum compensation temperature where the net spin density vanishes. Furthermore, our study establishes a direct connection between the fictitious magnetic field and the spin density.

DOI： 10.1038/s41565-018-0345-2

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PubMed

Publishing type：Research paper (scientific journal)  

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

Although bulk magnetic properties of various antiferromagnets have been vigorously studied since long ago, their properties in the form of thin films, which are more relevant to antiferromagnetic spintronic devices, have not been investigated as much. In this work, we characterized the Néel temperature of Cr O thin films by investigating the temperature dependence of the spin Hall magnetoresistance in Cr O /Pt bilayers. A precise determination of the Néel temperature was made possible by carefully designing the direction of the magnetic anisotropy in Cr O . The results provide a reliable way to determine the Néel temperature of antiferromagnetic thin films. 2 3 2 3 2 3

DOI： 10.1063/1.5082220

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Publishing type：Research paper (scientific journal)  

We investigate the temperature dependence of the magnetic damping in exchange-biased Pt/Fe50Mn50/Fe20Ni80/SiOx multilayers. In samples having a strong exchange bias, we observed a drastic decrease of the magnetic damping of Fe-Ni with increasing temperature up to the blocking temperature. The results essentially indicate that the nonlocal enhancement of the magnetic damping can be choked by the adjacent antiferromagnet and its temperature-dependent exchange bias. We also point out that such a strong temperature-dependent damping may be very beneficial for spintronic applications.

DOI： 10.1103/PhysRevApplied.11.011001

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Publishing type：Research paper (scientific journal)  

© 2019 The Japan Society of Applied Physics. We investigate temperature, temporal and magnetic-field dependence of unidirectional magnetoresistance (UMR) in metallic bilayers. The UMR is found to decrease rapidly with reducing temperature and converges to a finite value at low-temperature limit. The temporal dependence shows that the UMR emerges in a nanosecond time scale, which depends on the current amplitude. The magnetic-field dependence shows that the UMR is almost constant up to 5 T. These experimental results imply that the high-energy magnons, which are not considered in existing theory, can be involved in the observed UMR. Our results therefore suggest that a more extended theory is required for the complete understanding of UMR.

DOI： 10.7567/1882-0786/ab1b54

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Publisher：Journal of the Magnetics Society of Japan  

Nb, V, and Ta are all well-known superconducting elements in a simple substance. We report the superconductivity of Nb/V/Ta superlattices sputtered onto MgO(100) substrates, in which global inversion symmetry was broken along the stacking direction. It was found that the superlattices had long-range crystalline coherence with the well-defined periodicity of the constituent layers, and they exhibited superconducting transition despite the thicknesses of the layers.

DOI： 10.3379/msjmag.1903L001

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

There is a need to control magnetic properties at a desired location in a magnetic film towards a realization of fundamental devices, such as domain wall logic or magnonic applications. Here, we demonstrate the formation of a magnetic domain structure at a desired location in a Pt/Co film, using electrical gating with a meshed gate electrode and sweeping the applied magnetic field. As the magnetic properties can be changed by modulating the electron density at the surface of the Co layer, this method in principle provides higher speed and power-efficient operation in inducing a nanoscale domain structure or in configuring a volatile magnonic crystal. Published by AIP Publishing.

DOI： 10.1063/1.5078553

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

DOI： 10.1103/physrevb.98.180408

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.98.180408/fulltext

Publishing type：Research paper (scientific journal)  

We report on magnetoresistance measurements in a heavy metal/ Mn3Ir multilayers. After a post annealing process, we observed the magnetoresistance associated with the ordered crystalline structure of the Mn3Ir. The resistance change as a function of the strength as well as the direction of the applied field suggests that the magnetoresistance is partially related to a modification of the Néel order by the magnetic field. Our further detailed investigation revealed that there is an additional component of the resistance change, perhaps due to the non-collinear magnetic structure associated with the L12-ordered Mn3Ir, which cannot be accounted for by any conventional magnetoresistance effects.

DOI： 10.3379/msjmag.1901R001

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

DOI： 10.1103/PhysRevLett.121.167202

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PubMed

Other Link： http://orcid.org/0000-0001-7071-0823

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

Spin current transmission in antiferromagnetic materials is one of the important and intriguing problems in the recently emerged field of antiferromagnetic spintronics. We investigate the spin current transmission in polycrystalline NiO by referencing the effective Gilbert damping constant in NiO/FeNi bilayers. The results are discussed with reference to the spin pumping theory assuming that the NiO layer is spin diffusive. We extracted the spin diffusion length of &gt
22 nm for the NiO and the mixing conductance of &gt
6 nm%2 at the NiO/FeNi interface. The obtained spin diffusion length and the mixing conductance are within reasonable ranges compared with previous reports.

DOI： 10.7567/APEX.11.073003

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

The Curie temperature Tc is one of the most fundamental physical properties of ferromagnetic materials and can be described by the Weiss molecular field theory with the exchange interaction of neighboring atoms. Here, we demonstrate the electrical control of exchange coupling in cobalt films through direct magnetization measurements. We find that the reduction in magnetization with temperature, which is caused by thermal spin wave excitation and scales with Bloch's law, dearly depends on the applied electric field. Furthermore, we confirm that the correlation between the electric-field-induced modulation of Tc and that of exchange coupling follows the Weiss molecular field theory. (C) 2018 The Japan Society of Applied Physics

DOI： 10.7567/APEX.11.073002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

The domain-wall (DW) motion of ferrimagnets is investigated with respect to the angular-momentum-compensation temperature TA for a wide range of magnetic fields μ0Hz, ranging from the depinning to flow regimes. While the DW velocity v exhibits a maximum at TA in the flow regime regardless of μ0Hz, the temperature at which v is maximized depends on μ0Hz because of the temperature dependence of the depinning field μ0Hdep, causing difficulty in determining TA. To solve this problem, we propose a reduced coordinate μ0Hz % μ0Hdep, which reveals that v is maximized at TA even in the depinning regime. Our work suggests that μ0Hdep should be identified prior to determining TA.

DOI： 10.7567/APEX.11.063001

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

Electric-field modulations of magnetic properties have been extensively studied not only for practical applications but also for fundamental interest. In this study, we investigated the electric field effect on the exchange interaction in ultrathin Co films with ionic liquids. The exchange coupling J was characterized from the direct magnetization measurement as a function of temperature using Pt/ultrathin Co/MgO structures. The trend of the electric field effect on J is in good agreement with that of the theoretical prediction, and a large change in J by applying a gate voltage was observed by forming an electric double layer using ionic liquids. (C) 2018 The Japan Society of Applied Physics

DOI： 10.7567/APEX.11.063002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

We report a systematic experimental study on the refraction and reflection of magnetostatic spin-waves at a thickness step between two Permalloy films of different thickness. The transmitted spin-waves for the transition from a thick film to a thin film have a higher wave vector compared to the incoming waves. Consequently, such systems may find use as passive wavelength transformers in magnonic networks. We investigate the spin-wave transmission behavior by studying the influence of the external magnetic field, incident angle, and thickness ratio of the films using time-resolved scanning Kerr microscopy and micro-focused Brillouin light scattering.

DOI： 10.7567/APEX.11.053002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

Ferromagnetic resonance (FMR) is one of the most popular techniques to characterize dynamic properties of ferromagnetic materials. Among various FMR measurement techniques, the homodyne FMR detection has been frequently used to characterize thin-film ferromagnetic multilayers owing to its high sensitivity. However, a drawback of this technique was considered to be the requirement for a structural inversion asymmetry, which makes it unsuitable to characterize a single layer of ferromagnet. In this study, we demonstrate a homodyne FMR detection of the Kittel’s mode FMR dynamics of a single layer of FeNi by creating a non-uniform radio-frequency excitation current.

DOI： 10.7567/APEX.11.053008

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

We propose an alternative method of determining the magnetic anisotropy field μ0HK in ferro-/ferrimagnets. On the basis of the droplet nucleation model, there exists linearity between domain-wall (DW) energy density and in-plane magnetic field. We find that the slope is simply represented by μ0HK and Dzyaloshinskii–Moriya interaction (DMI). By measuring the in-plane magnetic field dependence of the coercivity field, closely corresponding to the DW energy density, a robust value for μ0HK can be quantified. This robust value can be used to determine μ0HK over a wide range of values, overcoming the limitations caused by the small strength of the external magnetic field typically used in experiments.

DOI： 10.7567/JJAP.57.050308

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Chiral spin textures of a ferromagnetic layer in contact to a heavy non-magnetic metal, such as Néel-type domain walls and skyrmions, have been studied intensively because of their potential for future nanomagnetic devices. The Dyzaloshinskii-Moriya interaction (DMI) is an essential phenomenon for the formation of such chiral spin textures. In spite of recent theoretical progress aiming at understanding the microscopic origin of the DMI, an experimental investigation unravelling the physics at stake is still required. Here we experimentally demonstrate the close correlation of the DMI with the anisotropy of the orbital magnetic moment and with the magnetic dipole moment of the ferromagnetic metal in addition to Heisenberg exchange. The density functional theory and the tight-binding model calculations reveal that inversion symmetry breaking with spin-orbit coupling gives rise to the orbital-related correlation. Our study provides the experimental connection between the orbital physics and the spin-orbit-related phenomena, such as DMI.

DOI： 10.1038/s41467-018-04017-x

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

The influence of temperature on the magnetic-field-driven domain wall motion is investigated in GdFeCo ferrimagnets with perpendicular magnetic anisotropy (PMA). We find that the depinning field strongly depends on temperature. Moreover, it is also found that the saturation magnetization exhibits a similar dependence on temperature to that of the depinning field. From the creep-scaling criticality, a simple relation between the depinning field and the properties of PMA is clearly identified theoretically as well as experimentally. Our findings open a way for a better understanding of how the magnetic properties influence the depinning field in the magnetic system and would be valuably extended to depinning studies in other systems.

DOI： 10.1063/1.5022809

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

Electric field effects on magnetism in metals have attracted widespread attention, but the microscopic mechanism is still controversial. We experimentally show the relevancy between the electric field effect on magnetism and on the electronic structure in Pt in a ferromagnetic state using element-specific measurements: x-ray magnetic circular dichroism (XMCD) and x-ray absorption spectroscopy (XAS). Electric fields are applied to the surface of ultrathin metallic Pt, in which a magnetic moment is induced by the ferromagnetic proximity effect resulting from a Co underlayer. XMCD and XAS measurements performed under the application of electric fields reveal that both the spin and orbital magnetic moments of Pt atoms are electrically modulated, which can be explained not only by the electric-field-induced shift of the Fermi level but also by the change in the orbital hybridizations.

DOI： 10.1103/PhysRevLett.120.157203

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

Antiferromagnetic materials could represent the future of spintronic applications thanks to the numerous interesting features they combine: they are robust against perturbation due to magnetic fields, produce no stray fields, display ultrafast dynamics, and are capable of generating large magnetotransport effects. Intense research efforts over the past decade have been invested in unraveling spin transport properties in antiferromagnetic materials. Whether spin transport can be used to drive the antiferromagnetic order and how subsequent variations can be detected are some of the thrilling challenges currently being addressed. Antiferromagnetic spintronics started out with studies on spin transfer and has undergone a definite revival in the last few years with the publication of pioneering articles on the use of spin-orbit interactions in antiferromagnets. This paradigm shift offers possibilities for radically new concepts for spin manipulation in electronics. Central to these endeavors are the need for predictive models, relevant disruptive materials, and new experimental designs. This paper reviews the most prominent spintronic effects described based on theoretical and experimental analysis of antiferromagnetic materials. It also details some of the remaining bottlenecks and suggests possible avenues for future research. This review covers both spin-transfer-related effects, such as spin-transfer torque, spin penetration length, domain-wall motion, and "magnetization" dynamics, and spin-orbit related phenomena, such as (tunnel) anisotropic magnetoresistance, spin Hall, and inverse spin galvanic effects. Effects related to spin caloritronics, such as the spin Seebeck effect, are linked to the transport of magnons in antiferromagnets. The propagation of spin waves and spin superfluids in antiferromagnets is also covered.

DOI： 10.1103/RevModPhys.90.015005

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

Spin injection into 3d transition metals and their alloys has been investigated intensively in recent years. In this study, we focus on the FeRh alloy, which is often used in the context of antiferromagnetic spintronics. We explore the interaction between the spin current and magnetic moments in FeRh by means of spin-torque ferromagnetic resonance. We obtain a spin Hall efficiency of ξSH = 24 ± 3%, which indicates that the spin current from Pt interacts with the magnetic moments in FeRh.

DOI： 10.7567/APEX.11.013008

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DOI： 10.1038/s41598-018-32508-w

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DOI： 10.1103/PhysRevLett.121.137201

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DOI： 10.7567/JJAP.57.080309

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

Spin interaction in antiferromagnetic materials is of central interest in the recently emerging antiferromagnetic spintronics. In this Letter, we explore the spin current interaction in antiferromagnetic FeMn by the spin pumping effect. Exchange biased FeNi/FeMn films, in which the Neel vector can be presumably controlled via the exchange spring effect, are employed to investigate the damping enhancement depending on the relative orientation between the Neel vector and the polarization of the pumped spin current. The correlation between the enhanced damping and the strength of the exchange bias suggests that the twisting of the Neel vector induces an additional spin dissipation, which verifies that the Slonczewski-type spin torque is effective even in antiferromagnetic materials.

DOI： 10.1103/PhysRevLett.119.267204

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

Antiferromagnetic spintronics is an emerging research field which aims to utilize antiferromagnets as core elements in spintronic devices(1,2). A central motivation towards this direction is that antiferromagnetic spin dynamics is expected to be much faster than its ferromagnetic counterpart(3). Recent theories indeed predicted faster dynamics of antiferromagnetic domain walls (DWs) than ferromagnetic DWs(4-6). However, experimental investigations of antiferromagnetic spin dynamics have remained unexplored, mainly because of the magnetic field immunity of antiferromagnets(7). Here we show that fast field-driven antiferromagnetic spin dynamics is realized in ferrimagnets at the angular momentum compensation point TA. Using rare earth-3d-transition metal ferrimagnetic compounds where net magnetic moment is nonzero at TA, the field-driven DW mobility is remarkably enhanced up to 20 km s(-1) T-1. The collective coordinate approach generalized for ferrimagnets(8) and atomistic spin model simulations(6,9) show that this remarkable enhancement is a consequence of antiferromagnetic spin dynamics at TA. Our finding allows us to investigate the physics of antiferromagnetic spin dynamics and highlights the importance of tuning of the angular momentum compensation point of ferrimagnets, which could be a key towards ferrimagnetic spintronics.

DOI： 10.1038/nmat4990

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

We fabricated electric-field-effect transistor structures with a channel layer of an itinerant ferromagnet SrRuO3 (SRO) heterostructured with a cap layer of BaTiO3 and investigated the electric field effects on the anomalous Hall effect (AHE) in SRO. We show that by applying positive and negative gate voltages (V-G), the anomalous Hall conductivity in the heterostructured SRO increases and decreases, respectively, regardless of its sign, while no change in the magnetic easy axis direction is observed. The results indicate that the observed V-G-induced modulations of the AHE do not simply originate from changes in the magnetization and magnetic anisotropy and indicate that V-G-induced changes in an integral of the Berry curvature over the filled electronic states play a key role in the observed electric field effects on the AHE in SRO. We also found that V-G-induced modulations on the AHE are negligibly small for transistor structures with a SRO channel having no BTO cap layer. This implies that the cap-layer-induced modifications in the Ru-O-Ru bond angles in the channel affect the Berry phase, enhancing the electric field effects on the AHE in SRO.

DOI： 10.1103/PhysRevB.96.214422

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The asymmetric magnetoresistance (MR) in Py/Pt bilayers is investigated. It increases linearly with respect to the current density up to a threshold and increases more rapidly above this threshold. To reveal the origin of the threshold behavior, we investigate the magnetic field dependence of the asymmetric MR. It is found that the magnetic field strongly suppresses the asymmetric MR only above the threshold current density. Micromagnetic simulation reveals that the reduction of magnetization due to the spin-torque oscillation can be the origin of the threshold behavior of the asymmetric MR. (C) 2017 The Japan Society of Applied Physics

DOI： 10.7567/APEX.10.073001

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We study the perpendicular magnetic anisotropy (PMA) of the CoFeB/MgO bilayers in contact with W, Ta, IrMn, and Ti, which have been suggested for use as the spin-orbit-torque-related underlayers. The saturation magnetization of CoFeB depends on the underlayer material used owing to the formation of a dead layer that affects the PMA strength of each film. The X-ray magnetic circular dichroism measurement reveals that interfacial intermixing suppresses only the perpendicular orbital moment of Fe, whereas it simultaneously suppresses both the perpendicular and in-plane orbital moments of Co. (C) 2017 The Japan Society of Applied Physics

DOI： 10.7567/APEX.10.073006

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

Harmonic Hall measurements were performed in Pt/GdFeCo bilayers to quantify the effective field resulting from the spin-orbit torque at various temperatures, both above and below the magnetization compensation temperature (T-M). We found that the damping-like torque rapidly increased near the T-M of the GdFeCo, which was attributed to a reduction in the net magnetic moment. Moreover, most importantly, the resulting spin Hall efficiency turned out to be constant across the temperature range. Published by AIP Publishing.

DOI： 10.1063/1.4985436

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

We investigate the effect of the Dzyaloshinskii-Moriya interaction (DMI) on magnetic domain nucleation in a ferromagnetic thin film with perpendicular magnetic anisotropy. We propose an extended droplet model to determine the nucleation field as a function of the in-plane field. The model can explain the experimentally observed nucleation in a Co/Ni microstrip with an interfacial DMI. The results are also reproduced by micromagnetic simulation based on the string model. The electrical measurement method proposed here can be widely used to quantitatively determine the DMI energy density.

DOI： 10.1103/PhysRevB.95.220402

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A creep motion of the magnetic domain wall (DW) in a perpendicularly magnetized Co wire, where the DW energy is artificially varied by applying a sloped electric field, is studied. Under the sloped electric field and a constant external magnetic field, the DW velocity gradually changes according to the position of the wire owing to the spatially varying DW energy. Although the sloped DW energy can be a source to drive a DW, no clear electric-field-induced DW motion is observed, most likely because the effective magnetic field induced by the sloped electric field is very small in the present system. (C) 2017 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.56.050305

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We present an energy-efficient magnetic domain-writing scheme for domain wall (DW) motion-based memory devices. A cross-shaped nanowire is employed to inject a domain into the nanowire through current-induced DW propagation. The energy required for injecting the magnetic domain is more than one order of magnitude lower than that for the conventional field-based writing scheme. The proposed scheme is beneficial for device miniaturization because the threshold current for DW propagation scales with the device size, which cannot be achieved in the conventional field-based technique. (C) 2017 The Japan Society of Applied Physics

DOI： 10.7567/APEX.10.043002

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The electric field control of superparamagnetism is realized using a Cu/Ni system, in which the deposited Ni shows superparamagnetic behavior above the blocking temperature. An electric double-layer capacitor (EDLC) with the Cu/Ni electrode and a nonmagnetic counter electrode is fabricated to examine the electric field effect on magnetism in the magnetic electrode. By changing the voltage applied to the EDLC, the blocking temperature of the system is clearly modulated. (C) 2017 The Japan Society of Applied Physics

DOI： 10.7567/APEX.10.013004

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Publishing type：Research paper (scientific journal)  

DOI： 10.1007/S00540-016-2284-0

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DOI： 10.1016/J.INTERMET.2017.01.004

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

We investigate the temperature dependence of the spin Hall magnetoresistance (SMR) in a W/CoFeB bilayer. The SMR is found to increase with decreasing temperature. An analysis based on the SMR theory suggests that the spin Hall angle of W and/or the spin polarization of CoFeB can be the origin of the temperature dependence of the SMR. We also find that the spin diffusion length and the resistivity of W do not significantly vary with temperature, which indicates the necessity of further study on the electron transport mechanism in W films to reveal the origin of the spin Hall effect in W. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.080308

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The temperature dependence of magnetoresistance is investigated in ferrimagnetic GdFeCo/Pt heterostructures. An anomalous Hall effect (AHE) shows a monotonic behavior in temperature even across the magnetization compensation temperature TM, implying that the FeCo moment is responsible for the magnetotransport properties. An anisotropic magnetoresistance (AMR) exhibits a steep increase at low temperatures, which we ascribe to the contribution of a weak antilocalization in an amorphous GdFeCo layer. A spin Hall magnetoresistance (SMR) is found to exist in ferrimagnet/Pt systems and shows a moderate temperature dependence, in contrast to the SMR in YIG/Pt where a significant temperature dependence was observed. These results provide a basic understanding of the magnetotransport in amorphous ferrimagnets/heavy metal heterostructures. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/APEX.9.073001

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

We report the experimental observation of Snell's law for magnetostatic spin waves in thin ferromagnetic Permalloy films by imaging incident, refracted, and reflected waves. We use a thickness step as the interface between two media with different dispersion relations. Since the dispersion relation for magnetostatic waves in thin ferromagnetic films is anisotropic, deviations from the isotropic Snell's law known in optics are observed for incidence angles larger than 25 degrees with respect to the interface normal between the two magnetic media. Furthermore, we can show that the thickness step modifies the wavelength and the amplitude of the incident waves. Our findings open up a new way of spin wave steering for magnonic applications.

DOI： 10.1103/PhysRevLett.117.037204

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

The electric field (EF) effect on the magnetic domain structure of a Pt/Co system was studied, where an EF was applied to the top surface of the Co layer. The width of the maze domain was significantly modified by the application of the EF at a temperature slightly below the Curie temperature. After a detailed analysis, a change in the microscopic exchange stiffness induced by the EF application was suggested to dominate the modulation of the domain width observed in the experiment. The accumulation of electrons at the surface of the Co layer resulted in an increase in the microscopic exchange stiffness and the Curie temperature. The result was consistent with the recent theoretical prediction. Published by AIP Publishing.

DOI： 10.1063/1.4955265

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Exchange bias is a unidirectional magnetic anisotropy developed in a bilayer of ferromagnetic and antiferromagnetic layers. Its technical importance as a "fix layer" is seen in various spintronic devices. The exchange bias can also be a probe to investigate the antiferromagnetic layer as it partly reflects the magnetic state of the antiferromagnet. In this work, we investigated the modulation of the exchange bias by a flow of electric current in Pt/Fe50Mn50/FeNi and Cu/Fe50Mn50/FeNi. We show that the exchange bias can be modulated just by applying the current due to interplay among the Joule heating, Ampere field, and current-induced effective field. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.070304

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Electric field effect on magnetism is an appealing technique for manipulating magnetization at a low energy cost. Here, we show that the local magnetization of an ultrathin Co film can be switched by simply applying a gate electric field without the assistance of any external magnetic field or current flow. The local magnetization switching is explained by nucleation and annihilation of magnetic domains through domain wall motion induced by the electric field. Our results lead to external-field-free and ultralow-energy spintronic applications. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/APEX.9.063004

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

We report the observation of a magnon Hall-like effect for the scattering of magnons at the sample edge in Y3Fe5O12 crystals. The experiments are performed under an out-of-plane magnetic field at room temperature using an infrared thermometry. When the magnons are incident into the sample edge in the unsaturated magnetic structure, appreciable transverse thermal gradient is detected. This thermal gradient could be attributed to magnon Hall effect for sample-edge scattering. We unexpectedly find that this effect is largely enhanced in the unsaturated regime, compared with the saturated regime where theoretical mechanism of magnon Hall effect due to the Berry curvature has been proposed previously.

DOI： 10.1002/pssb.201552520

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The dynamics of a magnetic domain wall (DW) under a transverse magnetic field H-y are investigated in two-dimensional (2D) Co/Ni microstrips, where an interfacial Dzyaloshinskii-Moriya interaction (DMI) exists with DMI vector D lying in +y direction. The DW velocity exhibits asymmetric behavior for +/- H-y; that is, the DW velocity becomes faster when H-y is applied antiparallel to D. The key experimental results are reproduced in a 2D micromagnetic simulation, which reveals that the interfacial DMI suppresses the periodic change of the average DW angle phi even above the Walker breakdown and that H-y changes phi, resulting in a velocity asymmetry. This suggests that the 2D DW motion, despite its microscopic complexity, simply depends on the average angle of the DW and thus can be described using a one-dimensional soliton model. These findings provide insight into the magnetic DW dynamics in 2D systems, which are important for emerging spin-orbitronic applications. (C) 2016 Author(s).

DOI： 10.1063/1.4944897

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We investigate the noise in single layer graphene devices from equilibrium to far-from equilibrium and found that the 1/f noise shows an anomalous dependence on the source-drain bias voltage (V-SD). While the Hooge's relation is not the case around the charge neutrality point, we found that it is recovered at very low V-SD region. We propose that the depinning of the electron-hole puddles is induced at finite V-SD, which may explain this anomalous noise behavior. (C) 2016 AIP Publishing LLC.

DOI： 10.1063/1.4943642

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

Topological defects such as magnetic solitons, vortices and skyrmions have started to play an important role in modern magnetism because of their extraordinary stability(1), which can be exploited in the production of memory devices. Recently, a type of antisymmetric exchange interaction, namely the Dzyaloshinskii-Moriya interaction (DMI; refs 2,3), has been uncovered and found to influence the formation of topological defects(4-7). Exploring how the DMI affects the dynamics of topological defects is therefore an important task. Here we investigate the dynamics of the magnetic domain wall (DW) under a DMI by developing a real time DW detection scheme. For a weak DMI, the DW velocity increases with the external field and reaches a peak velocity at a threshold field, beyond which it abruptly decreases. For a strong DMI, on the other hand, the velocity reduction is completely suppressed and the peak velocity is maintained constant even far above the threshold field. Such a distinct trend of the velocity can be explained in terms of a magnetic soliton, the topology of which is protected during its motion. Our results therefore shed light on the physics of dynamic topological defects, which paves the way for future work in topology-based memory applications.

DOI： 10.1038/NPHYS3535

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DOI： 10.1016/J.JSS.2015.10.036

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DOI： 10.1186/S40560-016-0164-1

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Coherent gigahertz-frequency surface acoustic waves (SAWs) traveling on the surface of a piezoelectric crystal can, via the magnetoelastic interaction, resonantly excite traveling surface spin waves in an adjacent thin-film ferromagnet. These excited surface spin waves, traveling with a definite in-plane wave-vector q(parallel to) enforced by the SAW, can be detected by measuring changes in the electro-acoustical transmission of a SAW delay line. Here, we provide a demonstration that such measurements constitute a precise and quantitative technique for spin-wave spectroscopy, providing a means to determine both isotropic and anisotropic contributions to the spin-wave dispersion and damping. We demonstrate the effectiveness of this spectroscopic technique by measuring the spin-wave properties of a Ni thin film for a large range of wave vectors, vertical bar q(parallel to)vertical bar = 2.5 x 10(4)-8 x 10(4) cm(-1), over which anisotropic dipolar interactions vary from being negligible to quite significant. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4938390

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Spin motive force (SMF) is induced by a time and spatial derivative of magnetizations and is dependent on spin polarization. We compare the SMF in FeNi with positive spin polarization with that in a magnetite (Fe3O4) with negative spin polarization. We observe the SMF induced by a nonuniform ferromagnetic resonance in Fe3O4 and find that the SMF in Fe3O4 is opposite to that in FeNi. This result originates from the negative spin polarization of Fe3O4. Our clear observation of the SMF depending on the sign of the spin polarization agrees well with the framework of the SMF theory. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/APEX.8.123001

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

We investigate the influence of the interfacial Dzyaloshinskii-Moriya interaction (DMI) on the propagation of Damon-Eshbach spin waves in micrometer-sized Pt(2 nm)/Co(0.4 nm)/Py(5 nm)/MgO(5 nm) stripes. We use time-resolved scanning Kerr microscopy to image the spin waves excited by a microwave antenna and to directly access their dispersion. The presence of an interfacial DMI manifests itself in an asymmetry in the dispersion for counterpropagating spin waves which reverses sign upon reversal of the direction of the externally applied magnetic field. From this asymmetry we deduce the strength of the interfacial DMI. Micromagnetic simulations confirm that the observed difference in the wave numbers and the signature of the asymmetry are characteristic for the occurrence of an interfacial DMI at the Pt/Co interface and cannot be explained by the uniaxial perpendicular magnetic anisotropy field originating from the same interface.

DOI： 10.1103/PhysRevB.92.220413

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B2-ordered FeRh has been known to exhibit antiferromagnetic-ferromagnetic (AF-F) phase transitions in the vicinity of room temperature. Manipulation of the Neel order via AF-F phase transition and recent experimental observation of the anisotropic magnetoresistance in antiferromagnetic FeRh has proven that FeRh is a promising candidate for antiferromagnetic memory material. In this work, we demonstrate sequential write and read operations in antiferromagnetic memory resistors made of B2-orderd FeRh thin films by a magnetic field and electric current only, which open a realistic pathway towards operational antiferromagnetic memory devices. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4931567

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

Massless Dirac electron systems such as graphene exhibit a distinct half-integer quantum Hall effect, and in the bipolar transport regime co-propagating edge states along the p-n junction are realized. Additionally, these edge states are uniformly mixed at the junction, which makes it a unique structure to partition electrons in these edge states. Although many experimental works have addressed this issue, the microscopic dynamics of electron partition in this peculiar structure remains unclear. Here we performed shot-noise measurements on the junction in the quantum Hall regime as well as at zero magnetic field. We found that, in sharp contrast with the zero-field case, the shot noise in the quantum Hall regime is finite in the bipolar regime, but is strongly suppressed in the unipolar regime. Our observation is consistent with the theoretical prediction and gives microscopic evidence that the edge states are uniquely mixed along the p-n junction.

DOI： 10.1038/ncomms9066

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

Magnetic domain wall (DW) motion induced by nanosecond current pulses is investigated in Co/Ni nanowires where the DW motion is driven by either adiabatic spin transfer torque (STT) or spin Hall torque (SHT). The DW displacement, including transient displacement and steady-state displacement, is found to be linearly related to the current pulse duration for both current-induced torques. The transient displacement is found to be less than 10% of the DW displacement. This result implies that the DW position can be controlled by tuning the duration of the current pulse, which enables a robust operation of racetrack memory. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/APEX.8.073008

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

We show that our ferromagnetic resonance (FMR) measurement based on the rectification effect is sufficiently sensitive for characterizing various static and dynamic magnetic properties of a sub-nanometer ferromagnetic film where the interfacial effects dominate. The extracted properties, such as the Lande g-factor, the effective demagnetizing field, and the Gilbert damping parameter, are reasonably well scaled with the film thickness, indicating that our measurements clearly capture the interfacial properties of the sub-nanometer-thick film. In particular, the capability of the g-factor extraction in the ultrathin film will be very helpful for characterizing the various interfacial effects involved with interfacial orbit moments and spin-orbit interactions. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/APEX.8.073003

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

We investigated the antiferromagnetic-ferromagnetic phase transition of FeRh nanowires induced by an electric current. The critical current I-cr for the phase transition decreases with increasing the sample temperature. It is found that the RcrIcr2, where R-cr is the resistance at I-cr, linearly decreases as the sample temperature increases, suggesting that the temperature raised by the Joule heating is responsible for the transition. We also studied for various wire widths to characterize the heat dissipation of the nanowires. Our results provide a way to explore the Joule heating in high density nanosized electrical devices where significant electric current flow is involved. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.54.073002

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

We studied the magnetic domain wall (DW) creep motion in two driving forces: external magnetic field and spin Hall torque (SHT). The dimensional crossover from two to one dimension is identified by looking at the distribution width of the DW velocity depending on the wire width. The distribution widths are found to be similar regardless of the driving forces in narrow wires, whereas they show nonnegligible differences depending on the driving force in wide wires. Therefore, the dimensional crossover implies that the effective magnetic field induced by the SHT becomes equivalent to the external magnetic field only when the wire is considered as one-dimensional. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/APEX.8.073004

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

Field-driven magnetic domain wall (DW) motion was investigated in ferrimagnetic GdFeCo wires with perpendicular magnetic anisotropy. The DW velocity as a function of the out-of-plane magnetic field exhibited the Walker breakdown phenomenon. The DW velocity was found to be significantly affected by the in-plane bias field: a minimum DW velocity was observed at a finite in-plane magnetic field (H-m), and the sign of H-m was found to reverse when the DW structure was changed from up-down-to down-up. These results imply that chiral DWs are formed in ferrimagnetic GdFeCo wires, despite their bulk and amorphous phase. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/APEX.8.073001

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

We develop a theory for spin transported by coherent Neel dynamics through an antiferromagnetic insulator coupled to a ferromagnetic insulator on one side and a current-carrying normal metal with strong spin-orbit coupling on the other. The ferromagnet is considered within the monodomain limit and we assume its coupling to the local antiferromagnet Neel order at the ferromagnet vertical bar antiferromagnet interface through exchange coupling. Coupling between the charge current and the local Neel order at the other interface is described using spin Hall phenomenology. Spin transport through the antiferromagnet, assumed to possess an easy-axis magnetic anisotropy, is solved within the adiabatic approximation and the effect of spin current flowing into the ferromagnet on its resonance linewidth is evaluated. Onsager reciprocity is used to evaluate the inverse spin Hall voltage generated across the metal by a dynamic ferromagnet as a function the antiferromagnet thickness.

DOI： 10.1103/PhysRevB.92.020409

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

Parity effect, which means that even-odd property of an integer physical parameter results in an essential difference, ubiquitously appears and enables us to grasp its physical essence as the microscopic mechanism is less significant in coarse graining. Here we report a new parity effect of quantum Hall edge transport in graphene antidot devices with pn junctions (PNJs). We found and experimentally verified that the bipolar quantum Hall edge transport is drastically affected by the parity of the number of PNJs. This parity effect is universal in bipolar quantum Hall edge transport of not only graphene but also massless Dirac electron systems. These results offer a promising way to design electron interferometers in graphene.

DOI： 10.1038/srep11723

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PubMed

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

We have developed a novel ferromagnetic resonance (FMR) measurement technique using the magneto-optical Kerr effect. The measurement technique uses microwave-frequency, intensitymodulated light to stroboscopically measure the Kerr angle due to the magnetization precession. We demonstrate that this stroboscopic magneto-optical Kerr effect provides a frequency domain and phase sensitive FMR measurement. The measurement is sensitive enough to detect the precessing magnetization with the precession cone angle below 1 degrees . (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4922126

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

We employed micro-focused Brillouin light scattering to study the amplification of the thermal spin wave eigenmodes by means of a pure spin current, generated by the spin-Hall effect, in a transversely magnetized Pt(4 nm)/NiFe(4 nm)/SiO2(5 nm) layered nanowire with lateral dimensions 500 x 2750 nm(2). The frequency and the cross section of both the center (fundamental) and the edge spin wave modes have been measured as a function of the intensity of the injected dc electric current. The frequency of both modes exhibits a clear redshift while their cross section is greatly enhanced on increasing the intensity of the injected dc. A threshold-like behavior is observed for a value of the injected dc of 2.8 mA. Interestingly, an additional mode, localized in the central part of the nanowire, appears at higher frequency on increasing the intensity of the injected dc above the threshold value. Micromagnetic simulations were used to quantitatively reproduce the experimental results and to investigate the complex non-linear dynamics induced by the spin-Hall effect, including the modification of the spatial profile of the spin wave modes and the appearance of the extra mode above the threshold. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4907612

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

We report the relation between current induced effective fields and ferromagnetic layer thickness. Hall measurements with rotating magnetic field show that the transverse and perpendicular effective fields have linear relations to the inverse of the magnetic moment m per area S. The results imply that both of these effective fields may originate from spin angular momentum transferring. However, the non-zero intercept of the transverse field at m/S = 0 implies that magnetization independent effects, such as Rashba effect, may contribute to transverse field. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4914897

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

We prepare the high quality epitaxial MgO(001)[100]/Pt(001)[100]/NiO(001)[100]/FeNi/SiO2 films to investigate the spin transport in the NiO antiferromagnetic insulator. The ferromagnetic resonance measurements of the FeNi under a spin current injection from the Pt by the spin Hall effect revealed the change of the ferromagnetic resonance linewidth depending on the amount of the spin current injection. The results can be interpreted that there is an angular momentum transfer through the NiO. A high efficient angular momentum transfer we observed in the epitaxial NiO can be attributed to the well-defined orientation of the antiferromagnetic moments and the spin quantization axis of the injected spin current. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4918990

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

We have investigated the temperature dependence of current-induced magnetic domain wall (DW) motion in a Co/Ni nanowire, where the spin Hall torque is responsible for the DW motion. The threshold current density (J(th)) for DW motion is found to be dependent on the temperature T and the depinning magnetic field H-dep, which is different from the symmetric Co/Ni system where J(th) is insensitive to T and H-dep. This result indicates that an extrinsic pinning governs the DW motion when it is driven by the spin Hall torque. Our work therefore suggests that reducing the extrinsic pinning is a key for achieving a low-power-consumption device. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.54.038004

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

We present evidence of a localized magnetic domain-wall (DW) oscillator in Co/Ni nanowires. It is found that a DW is localized by the simultaneous application of a high magnetic field larger than the depinning field and a dc current smaller than the threshold current. A one-dimensional model and micromagnetic simulation reveal that the localized DW is in a precessional mode. Our results suggest that a localized magnetic DW oscillator can be realized by appropriately adjusting the magnetic field and dc current. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/APEX.8.023003

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

We investigate the in-plane field dependence of current-induced magnetic domain wall motion in asymmetric Co/Ninano wires. The application of an in-plane field changed the direction of domain wall motion. In a systematic study, by varying the magnetic layer thickness, we show that the in-plane field induces a crossover from spin Hall torque to adiabatic spin-transfer torque for the magnetic domain wall motion. Furthermore, the dependence of the threshold current density on the in-plane longitudinal field provides a way for determining the strength of the Dzyaloshinskii-Moriya interaction.

DOI： 10.1103/PhysRevB.91.060405

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Publishing type：Research paper (scientific journal)  

DOI： 10.1038/SREP12591

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PubMed

Language：Japanese   Publisher：The Magnetics Society of Japan  

We have shown by micromagnetic simulation that the wavelength of the spin wave (SW) is modulated when the SW propagates across a step between two ferromagnetic films with different thicknesses. Since the wavelength of the SW depends on the film thickness, the thickness difference works as the different medium in Snell's law. The wavelength modulation obtained by the simulation showed a good agreement with Snell's law.

DOI： 10.3379/msjmag.1506R004

CiNii Books

Other Link： https://jlc.jst.go.jp/DN/JLC/20012933769?from=CiNii

Publishing type：Research paper (scientific journal)  

DOI： 10.1007/S00540-015-1991-2

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PubMed

Publishing type：Research paper (scientific journal)  

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

We show that the magnitude of the Dzyaloshinsky-Moriya interaction (DMI) can be determined by a field-driven domain-walls (DWs) annihilation measurement. It is predicted that the DMI induces antiparallel Neel DWs which form a metastable 360 degrees DW when they approach to each other. We find that an annihilation field, over which two DWs collide and vanish, is proportional to the magnitude of the DMI. This result suggests that the DMI can be quantified by a simple measurement of the annihilation field. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.53.108001

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

We investigate the magnetization dynamics in synthetic antiferromagnetic (SyAFM) Fe/[Co/Cu](10)/Co/Pt multilayers using the microwave rectifying effect. We observe two distinct resonant modes in the dynamics. Numerical simulations reveal that the resonant modes can be attributed to acoustic and optical modes, which are the characteristic resonant modes of antiferromagnetically coupled materials. It is found that the'linewidth of the optical mode is larger than that of the acoustic mode by a factor of 5. This broadening of the linewidth in the optical modes can be explained by the mutual spin pumping effect between the magnetizations precessing out of phase. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/APEX.7.063010

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

We report a systematic study of the DW motion in perpendicularly magnetized Co/Ni nanowires having different thicknesses with structure inversion asymmetry. The transition from bulk to interfacial effects is confirmed as a change in the DW motion direction at a reduced layer thickness. The bias field dependence reveals that the adiabatic spin transfer torque (STT) dominates the DW motion in the thick regime, whereas the interfacial torque originating from the spin Hall effect (SHE) is responsible for the DW motion with a fixed DW chirality induced by the Dzyaloshinskii-Moriya interaction (DMI) in the thin regime. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/APEX.7.053006

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

We investigate the wire-width dependence of the critical current density for the motion of a magnetic domain wall (DW) in asymmetric Co/Ni nanowires, where spin Hall torque dominates the current-induced DW motion. It is found that both the critical current density for DW motion and the pinning field of a DW increase as the wire width decreases and that the critical current density for DW motion is proportional to the pinning field of the DW; this finding suggests a tradeoff between low-power operation and thermal stability in magnetic domain-wall-motion devices driven by spin Hall torque. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/APEX.7.053003

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

We report the real-time observation of spin-wave propagation across a step inserted between two ferromagnetic films with different thicknesses. Because the dispersion relation of the spin wave depends on the thickness of the film, the step works as a junction to affect the spin wave propagation. When the spin wave transmits through the junction, the wavenumber undergoes modulation as per Snell's law, which states that the refraction index is proportional to the wavenumber. From the viewpoint of magnonics, the present achievement opens up new possibilities of controlling the wavenumber of spin waves. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/APEX.7.053001

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

Magnetic domain wall (DW) creep motions driven by a magnetic field and by the spin Hall (SH) torque are investigated in perpendicularly magnetized Co/Ni nanowires with structural inversion asymmetry. It is found that the DW arrival time distribution becomes wider when a DW is driven by the SH torque compared with that driven by a magnetic field. Our results indicate that the SH torque and a magnetic field drive a DW in different ways in the DW creep regime, although the SH torque is generally considered to induce an effective field. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/APEX.7.053005

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

Spin-wave (SW)-induced domain wall (DW) motion is investigated in a perpendicularly magnetized system by micromagnetic simulation. We particularly assume simulation parameters for Co/NI nanowires widely used in the experiment for spin-torque-driven DW motion. It is found that the domain wall moves toward the SW source in a broad frequency range, while it resonantly moves away from the SW source at certain frequencies. Our results indicate that the direction of the DW motion can be controlled by tuning the SW excitation frequency. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/APEX.7.033001

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

Two important mechanisms for current-induced domain wall (DW) dynamics, namely, precessional DW motion driven by the adiabatic spin transfer torque and steady DW motion induced by the spin Hall torque, have been proposed and experimentally confirmed. However, the effect of the spin Hall torque on precessional DW motion has not been reported yet. Here, we show that the spin Hall torque affects the precessional DW motion when the in-plane field is applied. It is found that the in-plane field induces a half rotation time difference during DW precession, which gives rise to the nonvanishing spin Hall torque on the precessional DW motion. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/APEX.7.033005

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

We have been investigating the spin motive force induced by magnetization dynamics in ferromagnetic materials. Ferrimagnetic materials are expected to exhibit even larger spin motive force due to their spatially staggered magnetic structure and high frequency spin wave modes. We believe that magnetite (Fe3O4) is one of the candidates for this purpose. Here, we report magnetic resonance in Fe3O4 thin films epitaxially grown on a MgO (001) substrate. The gyromagnetic ratio and the effective saturation magnetization are estimated to be 1.71 x 10(11) s(-1)T(-1) and 364 emu/cm(3), respectively. The Gilbert damping constant of 0.02 is obtained from the magnetic field dependence of the linewidth. We expect to investigate the spin motive force induced from this magnetic resonance.

DOI： 10.1109/TMAG.2013.2274270

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

We show that the angle-dependent Hall measurement is an effective method to determine the current-induced effective fields by investigating MgO/Fe/Ta and MgO/Fe/Pt/Ta multilayer structures. The experimentally obtained Hall resistance under a relatively large dc electrical current is well described by considering two components of current-induced effective fields, which may be related to the spin Hall effect and the Rashba effect. The directions of the effective fields are consistent with and their magnitudes are comparable to those reported previously for similar multilayer structures. (C) 2013 The Japan Society of Applied Physics

DOI： 10.7567/APEX.6.113002

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

This paper predicts the possibility to achieve synchronization (via injection locking to a microwave current) of spin-transfer torque oscillators based on hybrid spin-valves composed by two free layers and two perpendicular polarizers at zero bias field. The locking regions are attained for microwave frequency near 0.5f0, f0, and 2f0, where f 0 is the input oscillator frequency. Those properties make this system promising for applications, such as high-speed frequency dividers and multipliers, and phase-locked-loop demodulators. © 2013 American Institute of Physics.

DOI： 10.1063/1.4795597

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

The authors have investigated the current-induced magnetic domain wall (DW) motion in perpendicularly magnetized Co/Ni nanowire with structural inversion asymmetry (SIA). In this system, DW motion to the direction of electric current flow, not electron flow, and high DW velocity up to 110 m/s were confirmed, which have never been observed in Co/Ni systems without SIA. In addition, we found that the DW velocity showed a strong dependence on the perpendicular magnetic field in the range of +/- 100 Oe. These results suggest that DW in the Co/Ni nanowire with SIA moves in the steady mode, not in the precessional mode. (C) 2013 The Japan Society of Applied Physics

DOI： 10.7567/APEX.6.033001

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

We present the results of a perpendicular magnetic tunnel junction (MTJ) that displays simultaneously low critical switching current and voltage, as well as high thermal stability factor. These results were achieved using a free layer of the MgO/CoFeB/MgO structure by increasing the spin torque efficiency to an average of 3.0 k(B) T/mu A for 37-nm-diameter junctions, about three times that of a MgO/CoFeB/Ta free layer, which makes it the highest value reported to date. By comparing two films with different RA, hence different switching voltage and power, we explore the contributions of heating and voltage-modulated anisotropy change to the switching properties. (C) 2012 The Japan Society of Applied Physics

DOI： 10.1143/APEX.5.093008

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

We report measurements of spin-torque-driven oscillations in magnetic multilayer devices containing two in-plane-oriented free layers designed to have significant coupling between them. They are driven to oscillate by spin-transfer torque from two perpendicularly oriented polarizers. For both measured devices and micromagnetic simulations, we find that the oscillations in the two free layers are phase locked, resulting in a frequency doubling and large output signals. The simulations suggest that the oscillations are due to spatially nonuniform dynamics characterized by coupled large-amplitude motion of the two free layers.

DOI： 10.1103/PhysRevB.86.060411

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

We discuss recent highlights from research at Cornell University, Ithaca, New York, regarding the use of spin-transfer torques to control magnetic moments in nanoscale ferromagnetic devices. We highlight progress on reducing the critical currents necessary to produce spin-torque-driven magnetic switching, quantitative measurements of the magnitude and direction of the spin torque in magnetic tunnel junctions, and single-shot measurements of the magnetic dynamics generated during thermally assisted spin-torque switching.

DOI： 10.1098/rsta.2011.0169

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

An inductive-based detection method of ferromagnetic resonance spectrum is demonstrated. For a single microscopic (similar to 40 nm x 40 mu m x 60 mu m) magnetic dot, which is much smaller than the microwave wavelength, the change in the microwave transmission signal is as large as 15 dB when its magnetic permeability changes. This detection is several orders more sensitive than the conventional transmission line method. Further understanding and development of this concept may lead to a microscopic microwave permeability sensor.

DOI： 10.1109/TMAG.2010.2087376

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

We demonstrate that the spin Hall effect in a thin film with strong spin-orbit scattering can excite magnetic precession in an adjacent ferromagnetic film. The flow of alternating current through a Pt/NiFe bilayer generates an oscillating transverse spin current in the Pt, and the resultant transfer of spin angular momentum to the NiFe induces ferromagnetic resonance dynamics. The Oersted field from the current also generates a ferromagnetic resonance signal but with a different symmetry. The ratio of these two signals allows a quantitative determination of the spin current and the spin Hall angle.

DOI： 10.1103/PhysRevLett.106.036601

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Publishing type：Research paper (scientific journal)  

DOI： 10.1053/J.JVCA.2010.03.013

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PubMed

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

We have fabricated MgO-barrier magnetic tunnel junctions with a Co/Ni switching layer to reduce the demagnetizing field via interface anisotropy. With a fcc-(111) oriented Co/Ni multilayer combined with an FeCoB insertion layer, the demagnetizing field is 2 kOe and the tunnel magnetoresistance can be as high as 106%. Room-temperature measurements of spin-torque switching are in good agreement with predictions for a reduced critical current associated with the small demagnetization for antiparallel-to-parallel switching. For parallel-to-antiparallel switching the small demagnetization field causes spatially nonuniform reversal nucleated at the sample ends, with a low energy barrier but a higher switching current. (C) 2010 American Institute of Physics. [doi:10.1063/1.3481798]

DOI： 10.1063/1.3481798

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

In the first part of this paper, we report a systematic study on the structural evolution under rapid thermal annealing and the corresponding transport properties in magnetic tunnel junctions (MTJs) with a crystalline MgO barrier. The results clearly indicate that high tunneling magnetic resistance can be achieved by annealing MTJs at a very short time, and it is directly related to the formation of (001) crystalline structures. In the second part, we report the spin dynamics in tunneling structure through direct electrical detection. A surprisingly large voltage generation F/I/N and F/I/F in and junctions was observed, which is contradictory to the prediction from the standard spin-pumping theory. We proposed a theoretical formalism to study spin-pumping effects in ferromagnetic multilayer structures. The formalism can yield a remarkably clean physical picture of the spin and charge pumping in tunneling structures. The calculated values are consistent with experimental results.

DOI： 10.1109/TMAG.2009.2024126

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

We investigated the tunneling magnetoresistance change in magnetic tunnel junctions in the presence of external microwaves. The changing relative angle between the free layer and the pinned layer results in a rectification of the average resistance change. Due to its miniature size and its sensitivity to the microwave magnetic field, the magnetic tunnel junction could be utilized as a microwave power sensor with the ability to detect microwave frequencies. Studying microwave power and bias current dependencies reveals desired sensor features with linear responses and enhanced signal levels. (C) 2009 American Institute of Physics. [doi:10.1063/1.3231874]

DOI： 10.1063/1.3231874

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

We have developed an experimental technique to electrically characterize ferromagnetic resonance (FMR) with a large precession cone angle by using magnetic tunnel junctions on a coplanar waveguide. We observed FMR of a Ni(80)Fe(20) layer as the tunneling resistance change capturing the angle between the Ni(80)Fe(20) precession magnetization and a fixed magnetization in a reference electrode. The resonant frequency shifts with increasing microwave power as a result of the large precession cone angle that reduces the effective anisotropy field. We confirmed the validity of our results by the numerical simulations based on the Landau-Lifshitz-Gilbert equation.

DOI： 10.1109/TMAG.2008.2002572

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

In this study, the effective spin-mixing conductance in Ni80Fe20/Pt was investigated by measuring the interface-induced enhancement of the Gilbert damping constant. Ferromagnetic resonance spectra were measured in coplanar waveguide geometry with different incident microwave powers. The nonlinear behavior of normal Gilbert damping G(0) and effective spin-mixing conductance g.. have been observed when the incident microwave power is above a critical ac field h(rf) of 1.6 Oe. Both phenomena are explained by considering the coupling between spin coherent precession and spin wave modes. This work demonstrates the nonlinear behavior of the effective spin-mixing conductance g(up down arrow). It suggests that the nonlinear spin wave modes excited at high incident microwave power are detrimental to the spin pumping effect and should be avoided in future spin battery designs. The capability of tuning G0 and g(up down arrow) through the microwave power is also useful for the fundamental study on the damping mechanism. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3072443]

DOI： 10.1063/1.3072443

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DOI： 10.1016/j.neulet.2009.01.048

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PubMed

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

We significantly reduce the critical current I(c0) for the onset of spin torque switching of the free layer in nanometer-scale in-plane magnetized spin valves by partially cancelling its intrinsic demagnetization field through the utilization of Co/Ni multilayer free layers. The out-of-plane magnetic anisotropy arising from the Co/Ni interfaces reduces the effective demagnetization field (H(eff)) while not significantly affecting the thermal stability of the free layer. A zero-thermal-fluctuation critical current density J(c0)similar to 2x10(6) A/cm(2) is determined through both current ramp rate and nanosecond pulse measurements, and comparisons with large H(eff) control samples confirm that this strategy is efficient in substantially decreasing I(c0).

DOI： 10.1063/1.3107262

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

DOI： 10.3969/j.issn.1001-0548.2009.05.004

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

We theoretically study the recently observed tunnel-barrier-enhanced dc voltage signals generated by magnetization precession in magnetic tunnel junctions. While the spin pumping is suppressed by the high tunneling impedance, two complementary processes are predicted to result in a sizable voltage generation in ferromagnet (F)vertical bar insulator (I)vertical bar normal metal (N) and F vertical bar I vertical bar F junctions with one ferromagnet being resonantly excited. Magnetic dynamics in F vertical bar I vertical bar F systems induce a robust charge pumping, translating into voltage in open circuits. In addition, dynamics in a single ferromagnetic layer develop longitudinal spin accumulation inside the ferromagnet. A tunnel barrier then acts as a nonintrusive probe that converts the spin accumulation into a measurable voltage. Neither of the proposed mechanisms suffers from spin relaxation, which is typically fast on the scale of the exponentially slow tunneling rates. The longitudinal spin-accumulation buildup, however, is very sensitive to the phenomenological ingredients of the spin-relaxation picture.

DOI： 10.1103/PhysRevB.78.020401

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

We report the study of the real-time evolution of tunneling magnetoresistance (TMR) in CoFeB/MgO/CoFeB junctions during annealing at 380 degrees C. The TMR quickly developed at the early stage of the annealing, with 200% magnetoresistance observed in less than 10 min, followed by a slow approach to saturation. This evolution of TMR was correlated with the structural changes, including crystallization of amorphous CoFeB electrodes and improvement of barrier quality during the annealing. (c) 2008 American Institute of Physics.

DOI： 10.1063/1.2903147

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Scopus

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

Microwave assisted magnetization switching was investigated using Fe(30)Co(70)/Al(2)O(3)/Ni(80)Fe(20) magnetic tunnel junctions incorporated with a coplanar waveguide. Coercivity reduction (or switching field) of Ni(80)Fe(20) layer was reduced with a microwave excitation. It was found that the coercivity reduction depends on both frequency and power of the microwave. The numerical simulation based on the Landau-Lifshitz-Gilbert equation reproduced the trend of the experimental data. The results indicate that microwave can be an efficient mean to switch the magnetization of a thin film.

DOI： 10.1063/1.2838290

Web of Science

Scopus

J-GLOBAL

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

Chromium doped zinc telluride thin films with various doping concentrations are fabricated by magnetron sputtering. These films are ferromagnetic and the Curie temperature increases with Cr concentration. X-ray diffraction, transmission electron microscopy, and magnetic circular dichroism characterizations show that the films are free of Cr1-xTex impurities and the ferromagnetism is intrinsic. The transport study shows that the resistivity and magnetoresistance are governed by variable range hopping at low temperatures. A negative magnetoresistance as large as -57% is observed at 5 K. The magnetoresistance and its temperature dependence can be well explained by a model involving an increase in the localization length of carriers with a magnetic field in the hopping region. An anomalous Hall effect is also observed and a possible origin of the sign difference between the anomalous Hall resistance and ordinary Hall resistance is discussed.

DOI： 10.1103/PhysRevB.77.155207

Web of Science

Scopus

J-GLOBAL

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

We report the electrical detection of magnetization dynamics in an Al/AlO(x)/Ni(80)Fe(20)/Cu tunnel junction, where a Ni(80)Fe(20) ferromagnetic layer is brought into precession under ferromagnetic resonance conditions. The dc voltage generated across the junction by the precessing ferromagnet is enhanced about an order of magnitude compared to the voltage signal observed when the contacts in this type of multilayered structure are Ohmic. We discuss the relation of this phenomenon to magnetic spin pumping and speculate on other possible underlying mechanisms responsible for the enhanced electrical signal.

DOI： 10.1103/PhysRevLett.100.067602

Web of Science

Scopus

PubMed

J-GLOBAL

Publishing type：Research paper (scientific journal)  

DOI： 10.1007/S00540-008-0651-1

Web of Science

PubMed

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

Microwave-assisted magnetization switching was investigated using Fe30Co70/AlOx/Ni80Fe20 magnetic tunnel junctions incorporated with a coplanar waveguide. Coercivity field of Ni80Fe20 layer was dramatically reduced in a small amplitude microwave. The authors eliminated the thermal effect in coercivity reduction by comparing two types of measurements which are with and without spin precession in the presence of microwave. It was found that the coercivity reduction depends on both frequency and power of the microwave. The numerical simulation based on Landau-Lifshitz-Gilbert equation reproduced the trend of the experimental data. The results indicate that microwave can be an efficient means to switch the magnetization of a thin film. (c) 2007 American Institute of Physics.

DOI： 10.1063/1.2720746

Web of Science

Scopus

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

Off-axis electron holography was used to investigate the barrier profile of the Py/AlOx/ZrOy/Py magnetic tunnel junctions with different ZrOy thicknesses. The tunneling magnetoresistance (TMR) has a strong dependence on bias voltage and the bias voltage for maximum TMR is shifted from zero. This shift increases with ZrOy barrier thickness due to the increasing barrier asymmetry in the junctions. The evolution of barrier asymmetry was directly observed by the phase change of the off-axis electron holography, which unambiguously shows the barrier profile changes from triangular to trapezoidal shape as increasing of ZrOy thickness.

DOI： 10.1103/PhysRevB.75.134420

Web of Science

Scopus

J-GLOBAL

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

Magnetic nanoparticles coated with a biocompatible polymer are potential candidates for biomedical applications associated with the detection and treatment of cancer. By varying the length of a reaction tube, the nanoparticle size increases from 10 to 30 nm with a 1-3 nm carboxyl terminated polyethylene glycol (cPEG) coating. The saturation magnetization for all samples is greater than 100 emu/g. By increasing the length of the reaction tube there is an incorporation of an amorphous phase. Subjecting samples to a 4 Oe magnetic field oscillating at 500 kHz there are two characteristic temperatures depending on particle size. The temperature measured for larger particles saturates with time to 35 degrees C. Below a critical nanoparticle size the temperature reached exceeds 50 degrees C within the same timeframe.

DOI： 10.1109/TMAG.2006.879075

Web of Science

Scopus

J-GLOBAL

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

Magnetic tunneling junctions (MTJs) with MgO tunnel barrier have been fabricated on both oriented and nonoriented buffer layers on Si(001) substrate by magnetron sputtering. FeCo/MgO/FeCo MTJs fabricated on oriented buffer layers show larger tunneling magnetoresistance (TMR) value up to 84% without high temperature postannealing, whereas those MTJs on nonoriented buffer layers show 45% of TMR. The high-resolution transmission electron microscopy images reveal an excellent morphology and very coherent crystal structure with FeCo(001)[110]/MgO(001)x[100]/FeCo(001)[110] orientation. The results indicate that high TMR can be achieved without high temperature postannealing by sputtering deposition on appropriate oriented buffer layers. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2207835

Web of Science

Scopus

J-GLOBAL

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

Tunnel magnetoresistance (TMR) of 21% is observed at low temperature in hybrid magnetic tunnel junctions (MTJs) composed of a magnetic semiconductor (Zn,Cr)Te and Co electrodes separated by an alumina barrier. The TMR is observed up to 250 K, which is a considerable improvement over previous work on MTJs with semiconductor electrodes. The temperature and bias dependence of the TMR are consistent with a transport model involving spin-polarized tunneling and spin-independent hopping through impurity states.

DOI： 10.1063/1.2205177

Web of Science

Scopus

J-GLOBAL

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

Magnetic tunnel junctions (MTJs) with L1(0)-ordered FePt alloy electrodes were prepared on MgO(110) substrates, and structure and magnetotransport properties of the MTJs have been investigated. Tunnel magnetoresistance (TMR) of 34% is observed at 77 K for a junction with the FePt alloy electrode in which the degree of chemical order S is estimated to be similar to0.7. From S dependence of TMR in the MTJs, it is suggested that the spin polarization of FePt alloy electrodes increases with chemical ordering, which is consistent with first principles band structure calculations. (C) 2004 American Institute of Physics.

DOI： 10.1063/1.1669212

Web of Science

Scopus

J-GLOBAL

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

Fe/MgO/FeCo epitaxial magnetic tunnel junctions (MTJs) were prepared on MgO(100) single crystal substrates by using in situ plasma oxidation for the formation of MgO barriers. The epitaxial relationship of Fe(001)/MgO(001)/FeCo(001) and Fe[100]//MgO[110]//FeCo[100] in the junctions was observed by reflection high-energy electron diffraction. Tunneling transport was clearly observed at low temperatures below about 150 K, and the barrier height of MgO is estimated to be 0.9 eV, which is smaller than the value expected from half of the band gap of bulk MgO. Tunnel magnetoresistance of 23% and 20% was observed at 4.2 and 77 K, respectively. The results suggest that plasma oxidation is useful for fabricating epitaxial magnetic tunnel junctions. (C) 2003 American Institute of Physics.

DOI： 10.1063/1.1557338

Web of Science

Scopus

J-GLOBAL

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

Fe/MgF2/Co magnetic tunnel junctions (MTJs) with an oxide seed layer (MgO, Fe-O) inserted between an Fe bottom electrode and a MgF2 barrier layer have been prepared through the use of molecular beam epitaxy, and the effects of the seed layers on structure of subsequently deposited MgF2 barrier layers and tunnel magnetoresistance (TMR) have been investigated. The crystallographic orientation of MgF2 layers depends significantly on the seed layers, and furthermore, it has been found that the surface roughness of MgF2 layer is reduced by inserting a MgO seed layer. In Fe/MgF2 (3-12 nm)/Co MTJs without any seed layer, top Fe and bottom Co electrodes contact each other through pin holes in MgF2 layers. On the other hand, however, Fe and Co electrodes are separated both magnetically and electrically in Fe/MgO (0.3-0.5 nm)/MgF2 (2-5 nm)/Co MTJs, i.e., MgF2-based MTJs have successfully been prepared by inserting a thin MgO seed layer, resulting in TMR of about 10% observed at 4.2 K. (C) 2002 American Institute of Physics.

DOI： 10.1063/1.1452200

Web of Science

Scopus

J-GLOBAL

Language：English   Publisher：The Magnetics Society of Japan  

Ferromagnetic tunnel junctions (MTJs) in which a thin MgO seed layer was inserted between Fe bottom electrode and MgF₂ insulating barrier were fabricated through the use of molecular beam epitaxy. The structure and tunnel magnetoresistance (TMR) were investigated for Fe (200Å)/MgO (5Å)/MgF₂ (20Å)/Co (200Å) junctions. RHEED observation suggested that an MgF₂ layer deposited at room temperature was in amorphous or nanocrystalline state. The TMR ratio observed for a typical MTJ with barrier height of 1.6 eV was 1.8 % at 4.2 K and decreased with increasing temperature, down to 0.7 % at 293 K. It was found that the bias dependence of TMR shows a strong asymmetric feature, in contrast to conventional MTJs with amorphous alumina barriers.

DOI： 10.3379/jmsjmag.26.405

Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.71.2.0_1218

Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Event date： 2022.9

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Language：Japanese   Presentation type：Oral presentation (invited, special)  

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Venue：東京理科大学、東京  

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Venue：東京農工大、東京  

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Venue：梅田、大阪  

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Venue：中央大学、東京  

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Venue：立命館大学、京都  

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Venue：広島大学、広島  

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Venue：名古屋、愛知  

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Venue：御茶ノ水、東京  

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Venue：中央大学、東京  

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Venue：Pittsburgh; PA; USA  

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Venue：Kyusyu University; Fukuoka  

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Venue：Gaithersburg; MA; USA  

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Venue：金属材料研究所、仙台  

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Venue：Univ. of Tokyo; Tokyo  

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Venue：秋田大学、秋田  

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