Published papers

© 2020 Spin-torque ferromagnetic resonance (ST-FMR) is a powerful tool to evaluate spin-torque efficiency (ξFMR), the efficiency of current-induced torque in metallic bilayers consisting of ferromagnetic and nonmagnetic materials. In general, ξFMR can be evaluated from the amplitude ratio between symmetric and anti-symmetric components of the ST-FMR spectrum. However, the ratio is also affected by an unnecessary magnetic field owing to the asymmetric alternating current flow used to excite the ST-FMR. In this article, we demonstrate that ξFMR can be precisely evaluated from the full-angular dependence of the ST-FMR spectrum even though the distribution of alternating current flow is highly asymmetric.

© 2020 American Physical Society. We demonstrate a largely enhanced nonreciprocal propagation of spin waves (SWs) magnetoelastically excited in Ni/Si bilayer films. The nonreciprocity is attributed to the shear component of the strain tensor, which can be enlarged by increasing the thickness of the Si layer. By evaporating a 400-nm-thick Si layer on a Ni film, we enhanced the nonreciprocal ratio by up to 1200%. This finding paves the way for rectification of SWs in magnonic devices.

Epitaxial thin films of fully nonequilibrium hcp-Ru50Mo50(0001) nanoalloys were prepared as a chemically disordered alloy, in which the intrinsic spin Hall effect is expected to be negligible. Structural analyses confirmed the epitaxial growth and atomic scale alloying of the films. In contrast to a tiny torque efficiency (ξDL) of ∼0.4% for Ru50Mo50/CoFeB, the ξDL for the Ru50Mo50/Ni heterostructure reached ∼30% with a long-range relaxation length. The apparent dependence of ξDL on the ferromagnetic layer can be attributed to the orbital Hall effect (OHE). Interestingly, a smaller ξDL was observed for Ru/Ni, suggesting that the nonequilibrium Ru50Mo50 enhances its OHE. Furthermore, the enhanced ξDL is maintained by inserting a Ru layer between the Ru50Mo50 and Ni layers, showing orbital transport through Ru. This finding illustrates potential applications of nonequilibrium nanoalloy films in spin orbitronics and contributes to getting insights into the understanding of the interrelationships between nanostructures and orbital transport properties.

The unidirectional spin Hall magnetoresistance (USMR) effect is useful to detect the direction of magnetization in a ferromagnetic metal/nonmagnetic metal bilayer, which is the typical geometry used in a spin-orbit torque switching device. In this Letter, we demonstrate the enhancement of the USMR effect by reducing the Curie temperature TC of the ferromagnetic (FM) layer in a FM/platinum bilayer. The USMR ratio was maximized when the thickness of each FM layer was consistent with the spin diffusion length. We found that the maximum USMR ratio can be doubled by replacing Ni81Fe19 (TC=854 K) with Ni85Cu15 (TC=527 K) as the FM. This enhancement of the USMR effect is attributed to an increase of electron-magnon scattering in accordance with Bloch law. We also found that the use of ferromagnets with face-centered-cubic structures, such as Ni, Ni81Fe19, and Ni85Cu15, increased the USMR effect relative to that of body-centered-cubic Fe.

Piezoelectric LiNbO3 has been widely utilized to generate and detect spin waves in surface acoustic wave devices. Here we demonstrate an epitaxial Co-based Heusler alloy on a LiNbO3 128° Y-cut (LN-128Y) substrate by molecular beam epitaxy. Although there is no matching between the lattice length and symmetry, the insertion of a bcc metal buffer layer enables to grow L21 - ordered epitaxial Co2FeSi films on the LN-128Y substrate. The epitaxial Co2FeSi/LN- 128Y multiferroic heterostructures show a high saturation magnetization of ~1235 eμcc and a low damping constant of ~6 × 10-3. This study will open a way for an efficient electric-field control of the spin waves in surface acoustic wave devices.

We have developed an experimental technique to investigate a nonlinear magnetization dynamics resonantly excited by strong microwave fields. In this technique, a magnetization switching was observed when a sub-nanosecond wide pulsed field was applied to the ferromagnetic resonance-excited ferromagnets. Such a cooperative switching with simultaneous application of microwave- and pulsed-fields can evaluate an effective energy barrier for magnetization switching in various nonlinear magnetization dynamics. In this review, we show the experimental results on (1) the temporal evolution of the energy barrier in the transient of magnetization precession growth and (2) the observation of saddle-node bifurcation in the microwave-excited nonlinear magnetization dynamics.

We present a novel method for generating spin currents using the gyromagnetic effect, a phenomenon discovered over a century ago. This effect, crucial for understanding the origins of magnetism, enables the coupling between various macroscopic rotational motions and electron spins. While higher rotational speeds intensify the effect, conventional mechanical rotations, typically, below 10,000 RPM, produce negligible results comparable to geomagnetic fluctuations, limiting applied research. Our studies demonstrate that spin current generation comparable to that of rare metals can be achieved through atomic rotations induced by GHz-range surface acoustic waves and the rotational motion of conduction electrons in metallic thin films with nanoscale gradient modulation of electrical conductivity. These effects, termed the acoustic gyromagnetic effect and the current-vorticity gyromagnetic effect, are significant in different contexts. The acoustic gyromagnetic effect is notable in high-conductivity materials like aluminum and copper, which are more abundant than conventional spintronics materials with strong spin–orbit interactions (SOIs). Conversely, the current-vorticity gyromagnetic effect requires a large conductivity gradient to produce current vorticity efficiently. This is achieved by using composition gradient structures from highly conductive metals to poorly conductive oxides or semiconductors. Consequently, unlike traditional strong-SOI materials, we can create highly efficient spin current generators with low energy dissipation due to reduced Joule loss.

The magnetization process in an array of 2 mu m x 2 mu m x 0.04 mu m permalloy particles fabricated on a 100 nm carbon membrane was investigated by means of high-resolution Lorentz microscopy and electron holography in magnetic fields parallel and perpendicular to the film surface. For perpendicular fields, the magnetization reversal was found to begin with the nucleation of a vortex which gradually transformed into a closure domain configuration. At the remanent state, the flux closure was achieved with the generation of a new type of doubled domain wall. (C) 1996 American Institute of Physics.

Effects of material parameters on the magnetization process in a sub-micron strip pattern have been studied by micromagnetic simulations. A grain structure in the thin film is modeled by a discretized square element array. A structural parameter is introduced considering discontinuous film structure. The exchange stiffness is also varied as parameters. Simulated transverse susceptibility chi and the longitudinal magnetization reversal field H(rev) in continuous film structure are somewhat smaller than the analytical predictions based on the coherent rotation model. The values of chi and H(rev) significantly decrease with the decrease of inter-grain coupling due to discontinuous structure. Numerical analysis for transients of the internal energy during the magnetization process reveals that the demagnetizing term dominates the transverse magnetization process. While, the demagnetizing and exchange energy terms comparatively contribute to the potential barrier for the longitudinal magnetization reversal. Various types of micromagnetic configurations of edge-curl, buckling and vortex were observed in the magnetization reversal depending on the material parameters. (C) 1999 Elsevier Science B.V. All rights reserved.

Transport properties of sputtered Fe3O4 thin films have been studied with the current perpendicular to plane (CPP) configuration. Fabricated polycrystalline Fe3O4 films exhibit superior CPP conductivity of 12 k Omega(-1) center dot m(-1). In addition to the Verwey transition (VT), observed at 114 K, another type of drastic conductivity change [field induced transition (FT)] was observed at lower temperature (T < 70 K) and higher bias voltage (V-b > 0.18 V) regime. The conductivity change at FT is discontinuous and several orders in magnitude. The transition temperature for FT presents considerable Vb dependence, contrarily no distinguished dependence for VT. (c) 2005 American Institute of Physics.

Electric transport properties have been studied for Fe_3O_4 thin films with current perpendicular to plane geometry, aiming at development of oxide spintronics devices. Excellent conductivity comparable to a bulk single crystal was attained by optimizing an under layer material and film deposition condition. A novel conductivity switching, up to three orders of magnitude, was observed. The threshold value of electric field for switching and its temperature dependence were investigated.

High-quality Fe3O4 films are fabricated on glass substrates with metallic underlayers of Cr/Cu or Al by RF magnetron sputtering at relatively low deposition temperature of 300 degrees C. The measured values of saturation magnetization M-s are 462 emu/cm(3) for Al(50 nm)/ Fe3O4(200 nm) and 422 emu/cm(3) for Cr(45 nm)/ Cu(300 nm)/ Fe3O4(200 nm), which are drastically enhanced compared with that for the reference sample deposited directly on a glass substrate, and practically comparable to the bulk value of 477 emu/ cm(3). (C) 2003 Elsevier B. V. All rights reserved.

We have numerically investigated the co-operative magnetization reversal of magnetic particle arrays with fringe fields coupling. Demagnetization processes of one-dimensional particle arrays are studied taking the inter-particle separation S and uniaxial anisotropy K-u as parameters. Particles are found to rotate toward same or opposite directions depending on the values of S and K-u: (C) 2003 Elsevier B.V. All rights reserved.

Reversal of spin orientation using current-carrying conductors are studied for a micro-structured pseudo-spin valve material of Co79Fe21(3 nm)/Cu(4 nm)/Co73Fe27(4 nm) by magnetoresistive measurements. Selective spin reorientation in one of the two magnetic layers are successfully performed with practical current amplitude smaller than 10 mA/mum. Compositionally differentiated coercive field in the CoFe films would be a dominant cause for the selectivity of the spin reorientation. (C) 2004 Elsevier B.V. All rights reserved.

The switching dynamics of nickel iron wires (length, 100 mu m; width, 2 mu m) were investigated by magnetization reversal under a combined 20-ns-wide microwave impulse and a 2-ns-wide pulsed field. The switching field was clearly reduced by cooperative switching and was independent of the pulsed field width (for widths between 2 and 10 ns). This result suggests that magnetization assisted by the combined microwave field occurs within 2 ns. (C) 2015 The Japan Society of Applied Physics

We found the characteristic dependence of magnetization switching assisted by spin wave excitation on the magnetic field angle (theta) for exchange-coupled L1(0)-FePt/Permalloy bilayers, where theta was the angle from the easy magnetization axis of L1(0)-FePt. Without the spin wave being excited, an incoherent switching occurred through the domain wall motion. The frequency for the switching field (H-sw) reduction under the spin wave excitation did not depend on theta. This characteristic dependence is attributable to less variation in the resonance frequencies of the spin waves against h. We also increased the efficiency of the H-sw reduction further with theta. (C) 2013 AIP Publishing LLC.

Spin-polarized radio frequency (RF) currents and RF-Oersted fields resonantly excite a magnetic vortex core confined in a micron-scale soft magnetic disk. In this study, we measured the rectifying voltage spectra caused by the anisotropic magnetoresistance oscillation due to the gyration of the vortex with different polarity and chirality. The measured spectra are presented such that we can determine the vortex properties and strength of the spin torques and Oersted field accurately and directly through analytical calculation.