Published papers

Ferromagnetic resonance properties in nanostructured multilayers are numerically investigated for the application of spintronics devices. The resonance frequency can be shifted by switching the magnetization state between a parallel and an antiparallel configuration. These remanent states are selectively initialized by choosing the saturation direction of magnetization. In this paper, the signal independency between the two resonant states, the amplitude of the magnetization rotation at the resonance frequency, and the response time to reach equilibrium resonance are systematically studied as a function of the geometrical and magnetic parameters. (C) 2006 American Institute of Physics.

We demonstrate the generation of alternating spin current (SC) via spin-rotation coupling (SRC) using a surface acoustic wave (SAW) in a Cu film. Ferromagnetic resonance caused by injecting SAWs was observed in a Ni-Fe film attached to a Cu film, with the resonance further found to be suppressed through the insertion of a SiO2 film into the interface. The intensity of the resonance depended on the angle between the wave vector of the SAW and the magnetization of the Ni-Fe film. This angular dependence is explicable in terms of the presence of spin transfer torque from a SC generated via SRC.

We demonstrate the time-resolved real-space observation of vortex gyration in a magnetic circular disc by X-ray magnetic circular dichroism photoemission microscopy at the SPring-8 beamline BL25SU, both with and without an additional structure called a pair tag, which serves to control the magnetic curling direction. By comparing the gyration orbits of the vortices, we found that the gyration properties are dependent on the presence of the additional structure in spite of the discs having the same diameter. Furthermore, we reproduced the vortex gyration by micromagnetic simulation. The confinement potential of the vortex can be controlled by the additional structure.

© 2019 American Physical Society. We experimentally demonstrate the nonreciprocal generation of spin current (Js) in a surface-oxidized copper film. The efficiency of conversion is at least 320 times larger than the inverse conversion. This nonreciprocity is due to a novel type of Js generation, which relies on the transfer of angular momentum from the velocity field of free electrons. A gradient in the electrical mobility in the film produces vorticity in the in-plane drift velocity of the free electrons. The inverse process can hardly occur when Js is collinear with the gradient in the electrical mobility.

© 2020 American Physical Society. This Letter demonstrates spin wave resonance (SWR) owing to the gyromagnetic effect by propagating a Rayleigh-type surface acoustic wave (R-SAW) through ferromagnetic thin films. The SWR amplitude in a NiFe film shows a higher-order frequency variation than in a magnetoelastic Ni film. This frequency dependence is well understood in terms of the presence of a gyromagnetic field attributable to the local lattice rotation in the R-SAW. From the frequency dependence of the SWR amplitude, the gyromagnetic SWR could be separated from another SWR caused by a magnetoelastic effect of the ferromagnet.

Surface acoustic waves (SAWs) coupled to magnons have attracted much attention because they allow for the long-range transport of magnetic information that cannot be achieved by magnon alone. We employed pulsed laser interferometry to visualize the entire spatiotemporal dynamics of a SAW that travels on a nickel (Ni) thin film and is coupled to magnons. It was possible to trace the coupling-induced amplitude reduction and phase shift that occurs as the SAW propagates over a distance of 0.4 mm. The observed changes are consistent with results obtained from conventional radio-frequency transmission measurements, which probe the total SAW absorption due to magnon-phonon coupling. This result verifies that our method can accurately capture the spatiotemporal dynamics of a SAW coupled to magnons across the entire length of the sample. Additionally, we validated our time-resolved profiles by comparing them with theoretical results that take the echo wave due to reflection into account. The impact of the echo wave is significant even when it has propagated over a distance of the order of millimeters. Our imaging results highlight the visualization of the long-range propagation of the SAW coupled to magnons and offer more information about the surface vibration profiles in such devices.

Understanding of magnetization process in patterned thin films is important for their applications in downsizing various magnetic devices. This paper deals with an experimental technique which enables us to characterize magnetization process in the ends of patterned ultra-thin films. In this study, only the top layer of magnetic sandwich structures were patterned into an array of fine magnetic particles using electron beam lithography and Ar ion milling to form relief structures in the films. The magnetization process of the arrays was inferred from magnetoresistance (MR) measurements of the films. MR behaviors of particle array samples of substrate/FM1/Cu/FM2 (FM1, FM2 = Co, NiFe, CoPt) in different pattern widths of omega = 0.2 and 0.6 mum, lengths L ranging from 0.2 to 280 mum and thicknesses of t = 6 and 18 mn were studied. From longitudinal MR measurements, a distinct decrease in remanent magnetization was found for sub-mum particle arrays, which may be associated with the formation of edge domains in the particles.

The margin for selective write-operation in a current coincident scheme has been numerically evaluated by considering a magnetostatic interaction in a magnetic random access memory cell array. For a conventional method, the margin over 20% cannot be achieved as the cell size is smaller than 0.2 x 0.4 mu m(2). This is mainly attributed to the degradation of field localization created by a conductor current. The minimum cell size ensuring the practical margin can be decreased to 0.16 x 0.24 mu m(2) by using an opposing current flowing through neighboring conductors. The margin is found to be remarkably decreased as a current pulse width becomes less than 0.4 ns because of a gyromagnetic effect. (c) 2005 American Institute of Physics.

Abstract While compositional gradient‐induced spin‐current generation is explored, its microscopic mechanisms remain poorly understood. Here, the contribution of polarity of compositional gradient on spin‐current generation is explored. A nanoscale compositional gradient, formed by in situ atomic diffusion of ultrathin Ti and W layers, is introduced between 10‐nm‐thick W and Ti layers. Spin‐torque ferromagnetic resonance in ferromagnetic Ni95Cu5 deposited on this gradient reveals that a moderate compositional gradient suppresses negative spin torque from the spin Hall effect in W. In contrast, reversing the Ti/W stacking order, which inverts the gradient, suppresses positive spin torque from the orbital Hall effect in Ti. These findings suggest that the sign of spin torque is governed by the polarity of compositional gradient, providing a novel strategy for efficient spin‐torque generation without relying on materials with strong spin or orbital Hall effect.

Cooperative nucleation of sub-mum(2) domains has been performed for a spin valve strip (0.4 mum width) of NiFe/Co/Cu/Co overlaid with orthogonal two-layer conductors (write and assist conductors). The current induced MR change due to the write current I-w markedly increases with the additional assist current I-a. Threshold value of I-w, required for domain nucleation was decreased from 15 mA/mum to 9 mA/mum by the application of I-a with an amplitude of 10 mA/mum, which confirms selective writing with the current coincident scheme. The cooperative effect disappears, when the phase difference between I-w and I-a becomes larger than the pulse width of 30 ns, Which can be associated with the fast relaxation time of the magnetization process in the sub-mum(2) region.

Write operation in magnetic random access memory (MRAM) with bit density of Gbit/cm2 order has been numerically simulated for low submicron scale magnetic cells. The amplitude of both the switching current I-w and the energy barrier DeltaE of the magnetic cell show strong dependence on the width w and the thickness t of the cell. The calculated results show that the amplitude of I-w is proportional to t/w, while that of DeltaE is proportional to wt(2). To obtain the sufficient energy barrier against a thermal stability (DeltaE>80k(B)T, at 300 K), the thicker magnetic cell is required as the cell is downsized for high-density memory applications, although the thinner one is preferred to lower the switching current. From the calculated results, it appears that the margin of the switching current for selective write operation is decreased with decreasing the lateral aspect ratio of the cell. The addressability for the write operation is also degraded by the formation of structural defect in the cell. In this article, optimum configurations of the magnetic cell applicable for Gbit MRAM are discussed. (C) 2003 American Institute of Physics.

Micromagnetic structures of the 180 degrees domain wall in submicron-size ferromagnetic Co wires were investigated by means of magnetic force microscopy. The domain waif width delta (similar to mu m) observed in the Co wire is about 10 times larger than that observed in Co films of the same thickness. From micromagnetic calculations using the Landau-Lifshitz-Gilbert (LLG) equation, the magnetic structure of the 180 degrees domain wall is expected to be a large vortex accompanied by a magnetic ripple structure. This complicated 180 degrees domain wall is formed to reduce the demagnetizing energy due to a head-on-head configuration of the magnetization, which results in a micron-scale transition area of the domain wall. The wire width dependence of the 180 degrees domain wall width obtained by micromagnetic calculations is almost consistent with that measured from the magnetic force micrograph.

Magnetic and electronic properties of novel semiconductive-oxide/ferromagnet multilayers of Sn-oxide and ferromagnetic metals (Fe, Ni, Co), deposited with a multi-target rf magnetron sputtering system, were studied. Among the studied material systems, Co/Sn-oxide sustains well-defined layer structure and ferromagnetic property for the thinnest thickness of 1 nm, A thermionic CPP transport was observed in [Co(2 nm) /Sn-oxide (4 nm)](10), which activation energy was evaluated as 4.4x10(-3) erg from the thermal dependence of conductivity. Negative magnetoresistance was observed in two orthogonal directions of in-plane external fields, which confirms spin dependent transport in Sn-oxide thin film, The measured MR change is 0.6 % (Delta R = 0.8 Omega, R-s=140 Omega) at room temperature.

The magnetic domain structure of lithographic Co line (1μm×280μm×70 nm) arrays was investigated in terms of a shape effect. The strong shape anisotropy along the Co line plays an important role in the formation of the magnetic domain structure. The magnetic stray field distribution above the Co line array was measured by using magnetic force microscope (MFM). As the magnetization of the Co line was saturated along the line, the stripe domain structure was observed. The direction of the magnetic moment was periodically modulated along the mean magnetization direction. The periodicity (λ= 0.8μm) is much longer than that of the magnetic ripple structure (∼10 nm) which is generally observed in the polycrystalline ferromagnetic Co thin film. The stripe domain structure is caused by the competition between the crystalline anisotropy perpendicular to the film plane and the shape anisotropy of the line.

The magnetization reversal process in a lithographic Co line array was investigated. The coercive force parallel to the Co line increases with the spacing between the Co lines. (C) 1998 Elsevier Science B.V. All rights reserved.

Magnetostatic effect of the micron-size ferromagnetic particle array on a two-dimensional superconductor was investigated. In the bare Nb film, a sharp drop in magnetization due to the flux rearrangement was observed, whereas this behavior was not observed for the Nb film with the ferromagnetic particles arranged on top. The difference in the magnetization curves may be attributed to the regulated flux penetration process by the ferromagnetic particle array. (C) 1996 American Institute of Physics.

The effect of a micron size Sm33Co67 or Fe-Ni ferromagnetic particles array fabricated on a 2D superconducting niobium film was investigated by means of a magnetoresistivity measurement and rotating SQUID magnetometry. The magnetic particles seem to act as flux accumulators or flux guides through which flux lines penetrate inio the superconducting Nb film.

Magnetic properties of Ni fine particles (0.7 mu m in diameter) coated with Pd (5-30 nm thick) were studied before and after hydrogenation. The system offers the possibility of studying hydrogenation under isovolumetric constraint, but this seems to produce little change in the saturation loading. The Pd shell appears to turn weakly ferromagnetic below about 20 K under high magnetic fields, but this magnetic polarization is suppressed by hydrogenation. The lowering of the magnetization of Ni by Pd coating may be related to a change in the surface anisotropy of Ni.