YoshiChika OTANI | The University of Tokyo (original) (raw)

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Papers by YoshiChika OTANI

Research paper thumbnail of Tunable acoustic wave rectification in anisotropic magnets

An acoustic wave, a flow of elastic energy, has been widely integrated into modern electronics to... more An acoustic wave, a flow of elastic energy, has been widely integrated into modern electronics to enable various functions, including sensing, communication, signal filtering, etc. In conventional media, acoustic waves follow a reciprocity rule, having an equal possibility for the forward and backward propagation and, thus, allowing for the interference between reflected and original waves in the real application. This interference not only suppresses the transmission efficiency but also risks breaking device structure due to the formation of standing waves. In the end, these features created by reciprocity limit the further development of acoustic waves based applications. A nonreciprocal acoustic waves device is highly desired. To break the symmetry of the propagation, here, we employ magnetorotation coupling [1] to capture the chirality of the acoustic waves in a perpendicularly anisotropic ultra-thin film Ta/CoFeB(1.6 nm)/MgO, achieving a giant nonreciprocal acoustic wave attenu...

Research paper thumbnail of Nonreciprocal surface acoustic wave propagation via magneto-rotation coupling

Science Advances, 2020

Coupling of surface acoustic waves with anisotropic magnets demonstrates 100% nonreciprocity.

Research paper thumbnail of Evaluation of spin diffusion length and spin Hall angle of the antiferromagnetic Weyl semimetal Mn3Sn

Research paper thumbnail of Field-controlled ultrafast magnetization dynamics in two-dimensional nanoscale ferromagnetic antidot arrays

Beilstein Journal of Nanotechnology, 2018

Ferromagnetic antidot arrays have emerged as a system of tremendous interest due to their interes... more Ferromagnetic antidot arrays have emerged as a system of tremendous interest due to their interesting spin configuration and dynamics as well as their potential applications in magnetic storage, memory, logic, communications and sensing devices. Here, we report experimental and numerical investigation of ultrafast magnetization dynamics in a new type of antidot lattice in the form of triangular-shaped Ni80Fe20 antidots arranged in a hexagonal array. Time-resolved magneto-optical Kerr effect and micromagnetic simulations have been exploited to study the magnetization precession and spin-wave modes of the antidot lattice with varying lattice constant and in-plane orientation of the bias-magnetic field. A remarkable variation in the spin-wave modes with the orientation of in-plane bias magnetic field is found to be associated with the conversion of extended spin-wave modes to quantized ones and vice versa. The lattice constant also influences this variation in spin-wave spectra and spi...

Research paper thumbnail of Observation of anisotropic energy transfer in magnetically coupled magnetic vortex pair

Applied Physics Letters, 2016

We have experimentally investigated the energy transfer and storage in the magnetostatically coup... more We have experimentally investigated the energy transfer and storage in the magnetostatically coupled vortices in a pair of disks. By measuring the frequency dependence of the rectified dc voltage, we observed a specific gyrating motion due to anomalous energy storage at the off-resonant frequency for anti-parallel polarities. Micromagnetic simulations based on the Landau-Lifshitz-Gilbert equation qualitatively reproduce the experimental results and reveal that the behavior arises from the anisotropic energy transfer, i.e., the modulation of effective damping constant of the pair disks, originating from the phase difference between coupled vortex cores. These findings can be of use in magnetic vortex based logic operations.

Research paper thumbnail of Effect of excitation power on voltage induced local magnetization dynamics in an ultrathin CoFeB film

Scientific Reports, 2017

Voltage or electric field induced magnetization dynamics promises low power spintronics devices. ... more Voltage or electric field induced magnetization dynamics promises low power spintronics devices. For successful operation of some spintronics devices such as magnetic oscillators and magnetization switching devices a clear understanding of nonlinear magnetization dynamics is required. Here, we report a detailed experimental and micromagnetic simulation study about the effect of excitation power on voltage induced local magnetization dynamics in an ultrathin CoFeB film. Experimental results show that the resonance line-width and frequency remains constant, whereas cone angle of the magnetization precession increases linearly with square-root of excitation power below threshold value, known as linear excitation regime. Above threshold power, the dynamics enters into nonlinear regime where resonance line-width monotonically increases and resonance frequency monotonically decreases with increasing excitation power. Simulation results reveal that a strong nonlinear and incoherent magneti...

Research paper thumbnail of Voltage-induced magnetization dynamics in CoFeB/MgO/CoFeB magnetic tunnel junctions

Scientific Reports, 2017

Recent progress in magnetic tunnel junctions (MTJs) with a perpendicular easy axis consisting of ... more Recent progress in magnetic tunnel junctions (MTJs) with a perpendicular easy axis consisting of CoFeB and MgO stacking structures has shown that magnetization dynamics are induced due to voltage-controlled magnetic anisotropy (VCMA), which will potentially lead to future low-power-consumption information technology. For manipulating magnetizations in MTJs by applying voltage, it is necessary to understand the coupled magnetization motion of two magnetic (recording and reference) layers. In this report, we focus on the magnetization motion of two magnetic layers in MTJs consisting of top layers with an in-plane easy axis and bottom layers with a perpendicular easy axis, both having perpendicular magnetic anisotropy. According to rectified voltage (Vrec) measurements, the amplitude of the magnetization motion depends on the initial angles of the magnetizations with respect to the VCMA direction. Our numerical simulations involving the micromagnetic method based on the Landau-Lifshitz...

Research paper thumbnail of Tuning the spin Hall effect of Pt from the moderately dirty to the superclean regime

Research paper thumbnail of Spin relaxation characteristics in Ag nanowire covered with various oxides

Applied Physics Letters, 2015

We have studied spin relaxation characteristics in a Ag nanowire covered with various oxide layer... more We have studied spin relaxation characteristics in a Ag nanowire covered with various oxide layers of Bi2O3, Al2O3, HfO2, MgO, or AgOx by using non-local spin valve structures. The spin-flip probability, a ratio of momentum relaxation time to spin relaxation time at 10 K, exhibits a gradual increase with an atomic number of the oxide constituent elements, Mg, Al, Ag, and Hf. Surprisingly, the Bi2O3 capping was found to increase the probability by an order of magnitude compared with other oxide layers. This finding suggests the presence of an additional spin relaxation mechanism such as Rashba effect at the Ag/Bi2O3 interface, which cannot be explained by the simple Elliott-Yafet mechanism via phonon, impurity, and surface scatterings. The Ag/Bi2O3 interface may provide functionality as a spin to charge interconversion layer.

Research paper thumbnail of Tunable picosecond spin dynamics in two dimensional ferromagnetic nanodot arrays with varying lattice symmetry

RSC Advances, 2015

Tunable configurational anisotropy in spin-waves with up to 8-fold symmetry in closely packed nan... more Tunable configurational anisotropy in spin-waves with up to 8-fold symmetry in closely packed nanodot lattices with rectangular, honeycomb and octagonal symmetry. The extrinsic nature of the anisotropy is due to angular variation of the magnetostatic field distribution.

Research paper thumbnail of Switching Magnetization of a Nanoscale Ferromagnetic Particle Using Nonlocal Spin Injection

Physical Review Letters, 2006

Research paper thumbnail of Current distribution insidePy∕Culateral spin-valve devices

Research paper thumbnail of Enhancement of spin accumulation in a nonmagnetic layer by reducing junction size

Research paper thumbnail of Indication of intrinsic spin Hall effect in4dand5dtransition metals

Research paper thumbnail of Spin current related phenomena in metallic nano-structures

Physica E: Low-dimensional Systems and Nanostructures, 2011

Research paper thumbnail of Giant spin-accumulation signal and pure spin-current-induced reversible magnetization switching

Research paper thumbnail of Domain wall nucleation assisted by nonlocal spin injection

Journal of Physics D: Applied Physics, 2007

Research paper thumbnail of Spin-polarized current induced in a single ferromagnetic strip due to inhomogeneous spin injection

Journal of Applied Physics, 2005

We report that the inhomogeneous spin accumulation in a nonmagnetic layer induces the spin-polari... more We report that the inhomogeneous spin accumulation in a nonmagnetic layer induces the spin-polarized current flowing in a detector ferromagnetic strip lying underneath the nonmagnetic layer. In Ni-Fe∕Cu planar spin-valve structures, the induced spin-polarized current causes the spin-valve-like signal in the Ni-Fe wire without any boundary resistance in the Cu∕Ni-Fe junction. The relation between the inhomogeneous spin accumulation and the magnitude of the induced spin-valve-like signal is investigated by varying the width of the Cu wire.

Research paper thumbnail of Manipulation of the magnetic state of a small ferromagnetic particle by means of nonlocal spin-injection techniques (invited)

Journal of Applied Physics, 2006

Nonlocal spin-valve configuration is used for spin current injection into a nanoscale ferromagnet... more Nonlocal spin-valve configuration is used for spin current injection into a nanoscale ferromagnetic particle to reverse its magnetization. The nonlocal spin injection aligns the magnetization of the nanoscale particle along the spin injector, whereas the reverse switching is hardly observed up to the available maximum exciting current of about 15 mA. This asymmetric behavior implies the presence of asymmetric barrier formed in the vicinity of the interface. The magnitude of the essential spin current for the reversal is determined to be about 160μA, on the reasonable order of magnitude compared with the experimental values for conventional pillar structures.

Research paper thumbnail of Spin-current-assisted domain-wall depinning in a submicron magnetic wire

Journal of Applied Physics, 2003

We demonstrate experimentally the domain-wall depinning triggered by injecting the polarized spin... more We demonstrate experimentally the domain-wall depinning triggered by injecting the polarized spin current into the domain wall. The domain wall is pinned at the junction of a large pad and a narrow wire prior to the current injection experiment. When the polarized spin current is injected along the direction of the domain-wall propagation, the pinned domain wall is freed and pushed into the wire at the critical current that monotonously decreases by increasing the applied bias magnetic field. These results imply that the injection of the spin-polarized current into the domain wall causes additional magnetic pressure due to the spin-momentum transfer between the spin-polarized current and the localized magnetic moment.

Research paper thumbnail of Tunable acoustic wave rectification in anisotropic magnets

An acoustic wave, a flow of elastic energy, has been widely integrated into modern electronics to... more An acoustic wave, a flow of elastic energy, has been widely integrated into modern electronics to enable various functions, including sensing, communication, signal filtering, etc. In conventional media, acoustic waves follow a reciprocity rule, having an equal possibility for the forward and backward propagation and, thus, allowing for the interference between reflected and original waves in the real application. This interference not only suppresses the transmission efficiency but also risks breaking device structure due to the formation of standing waves. In the end, these features created by reciprocity limit the further development of acoustic waves based applications. A nonreciprocal acoustic waves device is highly desired. To break the symmetry of the propagation, here, we employ magnetorotation coupling [1] to capture the chirality of the acoustic waves in a perpendicularly anisotropic ultra-thin film Ta/CoFeB(1.6 nm)/MgO, achieving a giant nonreciprocal acoustic wave attenu...

Research paper thumbnail of Nonreciprocal surface acoustic wave propagation via magneto-rotation coupling

Science Advances, 2020

Coupling of surface acoustic waves with anisotropic magnets demonstrates 100% nonreciprocity.

Research paper thumbnail of Evaluation of spin diffusion length and spin Hall angle of the antiferromagnetic Weyl semimetal Mn3Sn

Research paper thumbnail of Field-controlled ultrafast magnetization dynamics in two-dimensional nanoscale ferromagnetic antidot arrays

Beilstein Journal of Nanotechnology, 2018

Ferromagnetic antidot arrays have emerged as a system of tremendous interest due to their interes... more Ferromagnetic antidot arrays have emerged as a system of tremendous interest due to their interesting spin configuration and dynamics as well as their potential applications in magnetic storage, memory, logic, communications and sensing devices. Here, we report experimental and numerical investigation of ultrafast magnetization dynamics in a new type of antidot lattice in the form of triangular-shaped Ni80Fe20 antidots arranged in a hexagonal array. Time-resolved magneto-optical Kerr effect and micromagnetic simulations have been exploited to study the magnetization precession and spin-wave modes of the antidot lattice with varying lattice constant and in-plane orientation of the bias-magnetic field. A remarkable variation in the spin-wave modes with the orientation of in-plane bias magnetic field is found to be associated with the conversion of extended spin-wave modes to quantized ones and vice versa. The lattice constant also influences this variation in spin-wave spectra and spi...

Research paper thumbnail of Observation of anisotropic energy transfer in magnetically coupled magnetic vortex pair

Applied Physics Letters, 2016

We have experimentally investigated the energy transfer and storage in the magnetostatically coup... more We have experimentally investigated the energy transfer and storage in the magnetostatically coupled vortices in a pair of disks. By measuring the frequency dependence of the rectified dc voltage, we observed a specific gyrating motion due to anomalous energy storage at the off-resonant frequency for anti-parallel polarities. Micromagnetic simulations based on the Landau-Lifshitz-Gilbert equation qualitatively reproduce the experimental results and reveal that the behavior arises from the anisotropic energy transfer, i.e., the modulation of effective damping constant of the pair disks, originating from the phase difference between coupled vortex cores. These findings can be of use in magnetic vortex based logic operations.

Research paper thumbnail of Effect of excitation power on voltage induced local magnetization dynamics in an ultrathin CoFeB film

Scientific Reports, 2017

Voltage or electric field induced magnetization dynamics promises low power spintronics devices. ... more Voltage or electric field induced magnetization dynamics promises low power spintronics devices. For successful operation of some spintronics devices such as magnetic oscillators and magnetization switching devices a clear understanding of nonlinear magnetization dynamics is required. Here, we report a detailed experimental and micromagnetic simulation study about the effect of excitation power on voltage induced local magnetization dynamics in an ultrathin CoFeB film. Experimental results show that the resonance line-width and frequency remains constant, whereas cone angle of the magnetization precession increases linearly with square-root of excitation power below threshold value, known as linear excitation regime. Above threshold power, the dynamics enters into nonlinear regime where resonance line-width monotonically increases and resonance frequency monotonically decreases with increasing excitation power. Simulation results reveal that a strong nonlinear and incoherent magneti...

Research paper thumbnail of Voltage-induced magnetization dynamics in CoFeB/MgO/CoFeB magnetic tunnel junctions

Scientific Reports, 2017

Recent progress in magnetic tunnel junctions (MTJs) with a perpendicular easy axis consisting of ... more Recent progress in magnetic tunnel junctions (MTJs) with a perpendicular easy axis consisting of CoFeB and MgO stacking structures has shown that magnetization dynamics are induced due to voltage-controlled magnetic anisotropy (VCMA), which will potentially lead to future low-power-consumption information technology. For manipulating magnetizations in MTJs by applying voltage, it is necessary to understand the coupled magnetization motion of two magnetic (recording and reference) layers. In this report, we focus on the magnetization motion of two magnetic layers in MTJs consisting of top layers with an in-plane easy axis and bottom layers with a perpendicular easy axis, both having perpendicular magnetic anisotropy. According to rectified voltage (Vrec) measurements, the amplitude of the magnetization motion depends on the initial angles of the magnetizations with respect to the VCMA direction. Our numerical simulations involving the micromagnetic method based on the Landau-Lifshitz...

Research paper thumbnail of Tuning the spin Hall effect of Pt from the moderately dirty to the superclean regime

Research paper thumbnail of Spin relaxation characteristics in Ag nanowire covered with various oxides

Applied Physics Letters, 2015

We have studied spin relaxation characteristics in a Ag nanowire covered with various oxide layer... more We have studied spin relaxation characteristics in a Ag nanowire covered with various oxide layers of Bi2O3, Al2O3, HfO2, MgO, or AgOx by using non-local spin valve structures. The spin-flip probability, a ratio of momentum relaxation time to spin relaxation time at 10 K, exhibits a gradual increase with an atomic number of the oxide constituent elements, Mg, Al, Ag, and Hf. Surprisingly, the Bi2O3 capping was found to increase the probability by an order of magnitude compared with other oxide layers. This finding suggests the presence of an additional spin relaxation mechanism such as Rashba effect at the Ag/Bi2O3 interface, which cannot be explained by the simple Elliott-Yafet mechanism via phonon, impurity, and surface scatterings. The Ag/Bi2O3 interface may provide functionality as a spin to charge interconversion layer.

Research paper thumbnail of Tunable picosecond spin dynamics in two dimensional ferromagnetic nanodot arrays with varying lattice symmetry

RSC Advances, 2015

Tunable configurational anisotropy in spin-waves with up to 8-fold symmetry in closely packed nan... more Tunable configurational anisotropy in spin-waves with up to 8-fold symmetry in closely packed nanodot lattices with rectangular, honeycomb and octagonal symmetry. The extrinsic nature of the anisotropy is due to angular variation of the magnetostatic field distribution.

Research paper thumbnail of Switching Magnetization of a Nanoscale Ferromagnetic Particle Using Nonlocal Spin Injection

Physical Review Letters, 2006

Research paper thumbnail of Current distribution insidePy∕Culateral spin-valve devices

Research paper thumbnail of Enhancement of spin accumulation in a nonmagnetic layer by reducing junction size

Research paper thumbnail of Indication of intrinsic spin Hall effect in4dand5dtransition metals

Research paper thumbnail of Spin current related phenomena in metallic nano-structures

Physica E: Low-dimensional Systems and Nanostructures, 2011

Research paper thumbnail of Giant spin-accumulation signal and pure spin-current-induced reversible magnetization switching

Research paper thumbnail of Domain wall nucleation assisted by nonlocal spin injection

Journal of Physics D: Applied Physics, 2007

Research paper thumbnail of Spin-polarized current induced in a single ferromagnetic strip due to inhomogeneous spin injection

Journal of Applied Physics, 2005

We report that the inhomogeneous spin accumulation in a nonmagnetic layer induces the spin-polari... more We report that the inhomogeneous spin accumulation in a nonmagnetic layer induces the spin-polarized current flowing in a detector ferromagnetic strip lying underneath the nonmagnetic layer. In Ni-Fe∕Cu planar spin-valve structures, the induced spin-polarized current causes the spin-valve-like signal in the Ni-Fe wire without any boundary resistance in the Cu∕Ni-Fe junction. The relation between the inhomogeneous spin accumulation and the magnitude of the induced spin-valve-like signal is investigated by varying the width of the Cu wire.

Research paper thumbnail of Manipulation of the magnetic state of a small ferromagnetic particle by means of nonlocal spin-injection techniques (invited)

Journal of Applied Physics, 2006

Nonlocal spin-valve configuration is used for spin current injection into a nanoscale ferromagnet... more Nonlocal spin-valve configuration is used for spin current injection into a nanoscale ferromagnetic particle to reverse its magnetization. The nonlocal spin injection aligns the magnetization of the nanoscale particle along the spin injector, whereas the reverse switching is hardly observed up to the available maximum exciting current of about 15 mA. This asymmetric behavior implies the presence of asymmetric barrier formed in the vicinity of the interface. The magnitude of the essential spin current for the reversal is determined to be about 160μA, on the reasonable order of magnitude compared with the experimental values for conventional pillar structures.

Research paper thumbnail of Spin-current-assisted domain-wall depinning in a submicron magnetic wire

Journal of Applied Physics, 2003

We demonstrate experimentally the domain-wall depinning triggered by injecting the polarized spin... more We demonstrate experimentally the domain-wall depinning triggered by injecting the polarized spin current into the domain wall. The domain wall is pinned at the junction of a large pad and a narrow wire prior to the current injection experiment. When the polarized spin current is injected along the direction of the domain-wall propagation, the pinned domain wall is freed and pushed into the wire at the critical current that monotonously decreases by increasing the applied bias magnetic field. These results imply that the injection of the spin-polarized current into the domain wall causes additional magnetic pressure due to the spin-momentum transfer between the spin-polarized current and the localized magnetic moment.