Published papers

Recently, chirality-induced spin selectivity (CISS) has been observed in chiral molecules and has become attractive for applications in magnetoresistance (MR) devices. In this study, we fabricate CISS-based organic spin valves (OSVs) consisting of the chiral molecule 1,1′-Bi-2-naphthol (BINOL), sandwiched between an Au electrode and Co/Pt layers with perpendicular magnetic anisotropy, with and without an AlOx layer. We successfully fabricated Au/R-(+)-BINOL/Co/Pt/Co/Pt/Cr (Au/R-(+)-BINOL/[Co/Pt]2) and Au/S-(−)-BINOL/[Co/Pt]2 OSVs. These devices, which exhibit sharp interfaces, demonstrate clear CISS-based MR effects under a low magnetic field of ∼20 mT at room temperature. The MR curves of both Au/R-(+)-BINOL/[Co/Pt]2 and Au/S-(−)-BINOL/[Co/Pt]2 OSVs correspond well to the perpendicular magnetization curves of the [Co/Pt]2 electrodes. The polarity of the resistance change also exhibits opposite behaviors in the two devices. This indicates that an MR effect based on CISS is successfully observed in the Au/BINOL/[Co/Pt]2 OSVs. In addition, we fabricate Au/BINOL/AlOx/[Co/Pt]2 OSVs, in which an AlOx layer is inserted. We find that inserting an AlOx layer at the molecule/Co interface decreases both the MR and signal-to-noise (S/N) ratios of the MR curves compared with those of devices without an AlOx layer. The results of this study can contribute to elucidating the mechanism of CISS and provide new insights into its effect for spintronic device applications.

Surface acoustic waves (SAWs) excite GHz phonons that resonantly hybridize with magnons in the ferromagnet. Reducing the SAW fundamental frequency compresses the overtone spacing, enabling quasicontinuous and broadband spectral access to magnon-phonon interactions. We observe an enhancement of the magnon-phonon interaction strength at the monostable-bistable transition of magnetization, evaluated from the normalized magnon-induced SAW absorption and attributed to increased magnetic susceptibility. This approach enables systematic mapping of magnon-phonon hybridization over wide frequency and magnetic-field ranges and supports tunable magnonic functionalities.

To develop electric-field control of magnetization dynamics in a magnetic material for magnonic devices with low-energy power consumption operation, an epitaxial half-metallic Co2FeSi/LiNbO3 multiferroic heterostructure is experimentally demonstrated. The epitaxial Co2FeSi/LiNbO3 multiferroic heterostructure shows a low damping constant (α) of ∼0.006 and the value of α is decreased to ∼0.004 by applying an electric field. This means that the magnetization dynamics in an epitaxial half-metallic Co2FeSi layer can be controlled via the piezoelectric strain of LiNbO3 through the magnetoelastic coupling. This study leads to a way toward the realization of magnonic devices with low-energy power consumption operation.

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.

Green materials for efficient charge-to-spin conversion are desired for common spintronic applications. Recent studies have documented the efficient generation of spin torque using spin-orbit interactions (SOIs); however, SOI use relies on the employment of rare metals such as platinum. Here, we demonstrate that a nanometer-thick gradient from silicon to aluminum, which consists of readily available elements from earth resources, can produce a spin torque as large as that of platinum despite the weak SOI of these compositions. The spin torque efficiency can be improved by decreasing the thickness of the gradient, while a sharp interface was not found to increase the spin torque. Moreover, the electric conductivity of the gradient material can be up to twice as large as that of platinum, which provides a way to reduce Joule heating losses in spintronic devices.

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.

We report current-induced spin torques in epitaxial NiMnSb films on a commercially available epiready GaAs substrate. The NiMnSb was grown by cosputtering from three targets using optimized parameters. The films were processed into microscale bars to perform current-induced spin-torque measurements. Magnetic dynamics were excited by microwave currents, and electric voltages along the bars were measured to analyze the symmetry of the current-induced torques. We found that the extracted symmetry of the spin torques matches those expected from spin-orbit interaction in a tetragonally distorted half-Heusler crystal. Both fieldlike and dampinglike torques are observed in all the samples characterized, and the efficiency of the current-induced torques is comparable to that of ferromagnetic metal/heavy-metal bilayers.

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.

Dynamical spin injection based on the ferromagnetic resonance is an attractive and powerful method for generating and manipulating the spin current. To date, in a ferromagnet/nonmagnet bilayer system, the spin-pumping effect from the ferromagnet into the nonmagnet has been considered only as the mechanism of the dynamical spin injection. Here, we examine another method for the dynamical spin injection based on the heating effect due to the ferromagnetic resonance in a (Co,Fe)B/Pt bilayer system. The (Co,Fe)B film heated by the ferromagnetic resonance produces the temperature gradient across the (Co,Fe)B/Pt interface, resulting in the thermal spin injection from the (Co,Fe)B layer into the Pt film. The obtained electrical signal clearly shows the spin Hall signature and exceeds 20 mV Ω-1 m-1, which is relatively large compared to the values reported in similar bilayer structures driven by spin pumping. The structural dependence of the inverse spin Hall signal and its power dependence provide consistent results with the dynamical thermal-spin-injection model due to the ferromagnetic resonance.

Y3Fe5O12 is one of the magnetic insulators that can realize high-speed and low-power-consuming spintronics devices. However, it is hard to prepare a high-quality Y3Fe5O12 film via a conventional sputtering process owing to its low crystalline texture, which leads to a crucial increase in the Gilbert damping constant. Moreover, a single crystalline Gd3Ga5O12 substrate, whose lattice constant is well matched with Y3Fe5O12, is indispensable to improve the crystallinity of the Y3Fe5O12 film. In this article, we demonstrated an epitaxial growth of multiple domains for a 30-nm-thick Y3Fe5O12 film by means of magnetron sputtering on a single crystalline 128° Y-X LiNbO3 substrate , which has been widely utilized in surface acoustic wave devices. From the pole figure of x-ray diffraction, an oblique epitaxial growth of Y3Fe5O12(400) is successfully observed on the 128° Y-X LiNbO3 substrate after a high-temperature post-annealing. The saturation magnetization is equivalent to the value of the epitaxial Y3Fe5O12 film on the Gd3Ga5O12 substrate. The relatively low effective Gilbert damping constant of 0.0039 also supports the high crystalline texture of the Y3Fe5O12 film. The developed growth technique will pave the way for the application of the Y3Fe5O12 film on magneto-acoustic devices.

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.

We report on current-induced ferromagnetic resonance techniques to characterise spin-Hall effect at high temperatures. A microwave current was injected into a patterned CoFeB/Pt bi-layer grown on a glass substrate, simultaneously exerting spin-transfer torques through the spin-Hall effect and also causing Joule heating enabling the control of the device temperature. We measured the device temperature by using the device itself as a local temperature sensor. A clear reduction of CoFeB magnetisation was observed as the device temperature was increased allowing us to estimate the Curie temperature of our CoFeB film to be 920 K. The spin-Hall angle of Pt was quantified as (1.72 ± 0.03) × 10−2 at 300 K and was slightly increased to (1.75 ± 0.02) × 10−2 at 410 K. This simple method can be widely used for quantifying the spin-Hall angle of a large variety of materials at high temperatures.

We investigated the size effect on ferromagnetic resonance (FMR) in a submicron-wide single permalloy bar. The resonant frequency markedly increased with decreasing bar width to less than 1 m, since the demagnetizing field is effectively modified by changing the bar width even in thin films. The resonant frequency difference between 100- and 1000-nm-wide bars was over 4 GHz in the absence of a magnetic field. This characteristic is promising for practical microwave devices because the desired resonant frequency can be obtained simply by varying the width of narrow ferromagnetic bars so that it is not necessary to change the material or magnetic field. © 2013 The Japan Society of Applied Physics.

The ferromagnetic resonance of a single micron-scale dot of permalloy was measured by using a vector network analyzer. The resonant frequency shifted to higher frequency with increasing aspect ratio of the sample. This tendency was remarkable for narrower width samples, especially the 1-μm-width dots. This means that the demagnetization effect of thin films for the ferromagnetic resonance becomes large when the sample width is less than 1 μm. The resonant frequency can be largely controlled by changing the sample width for a thin film with a width of less than 1 μm, which is a favorable characteristics for various microwave applications. © 2013 The Korean Physical Society.

Damping factor of a Permalloy (Py) thin film is estimated by using the magnetostatic spin wave propagation. The attenuation lengths are obtained by the dependence of the transmission intensity on the antenna distance, and decrease with increasing magnetic fields. The relationship between the attenuation length, damping factor, and external magnetic field is derived theoretically, and the damping factor was determined to be 0.0063 by fitting the magnetic field dependence of the attenuation length, using the derived equation. The obtained value is in good agreement with the general value of Py. Thus, this estimation method of the damping factor using spin waves attenuation can be useful tool for ferromagnetic thin films.

The amplitude of the spin wave (magnetostatic surface spin wave) depends on the propagation direction. Nonreciprocity hardly depends on the permalloy (Py) thickness between 100 and 300 nm, and it is determined by the spin wave resonant frequency. The nonreciprocal parameter decreases with increasing frequency, resulting in the large amplitude asymmetry at high frequencies, which is well supported by a result of theoretical analysis. The nonreciprocity of a 25-nm-thick-film, however, is lower than those of thicker films. This is qualitatively explained by a schematic model based on the origin of nonreciprocity.

The heat dissipation due to the resonant precessional motion of the magnetization in a ferromagnetic metal has been investigated. We demonstrated that the temperature during the ferromagnetic resonance can be simply detected by the electrical resistance measurement of the Cu strip line in contact with the ferromagnetic metal. The temperature change of the Cu strip due to the ferromagnetic resonance was found to exceed 10 K, which significantly affects the spin-current transport. The influence of the thermal conductivity of the substrate on the heating was also investigated.

The permalloy (Py) thickness dependence of the magnetostatic spin wave (MSSW) propagation was investigated. Large-group velocity is realized for thick Py films and the MSSW can propagate more than 150 μm. However, attenuation length hardly changes for samples with a Py thickness of more than 100 nm, despite the increasing group velocity with increasing thickness. The eddy current effect decreases the wave channel thickness and it could cause the damping enhancement due to intralayer spin pumping in thick Py films.

We have investigated the stability for the resonant spin precession under the strong microwave magnetic field by a specially developed detection method using the anisotropic magnetoresistance effect. The electrically separated excitation and detection circuits enable us to investigate the influence of the heating effect and the nonuniform spin dynamics independently. The large detecting current is found to induce the field shift of the resonant spectra because of the Joule heating. From the microwave power dependence, we found that the linear response regime for the standing spin wave is larger than that for the ferromagnetic resonance. This robust characteristic of the standing spin wave is an important advantage for the high power operation of the spin-wave device.

Ferromagnetic resonance (FMR) is performed on kagome artificial spin ice (ASI) formed of disconnected Ni80Fe20 nanowires. Here we break the threefold angular symmetry of the kagome lattice by altering the coercive field of each sublattice via shape anisotropy modification. This allows for distinct high-frequency responses when a magnetic field is aligned along each sublattice and additionally enables simultaneous spin-wave resonances to be excited in all nanowire sublattices, unachievable in conventional kagome ASI. The different coercive field of each sublattice allows selective magnetic switching via global field, unlocking novel microstates inaccessible in homogeneous-nanowire ASI. The distinct spin-wave spectra of these states are detected experimentally via FMR and linked to underlying microstates using micromagnetic simulation.