arXiv subjects

Satoshi Iihama

Publications and source records attributed to Satoshi Iihama.

11 recordsLinked to original sources

arXiv2026

Giant staggered Dzyaloshinskii-Moriya vectors emerged in synthetic antiferromagnets with spatially alternating stress

Dzyaloshinskii-Moriya (DM) interaction is a source of chiral magnetic physics, and it manifests microstructural symmetry which could link to exotic magnetic and electronic properties such as altermagnetism. For a new paradigm in magnetism and spintronics, it is crucial to have control on the DM interaction as well as ordering of the DM vectors which characterize the DM interaction. Here, we report on a strong interlayer DM interaction emerged in Co/Ru magnetic superlattices whose structural symmetry is unambiguously broken by alternating mechanical strains developed using the peculiar technique of our stress mechanism. Moreover, theoretical analyses reveal that our magnetic superlattices host a staggered order of the DM vectors. Our results indicate that the spatially varying strains with our breakthrough technique are quite effective in breaking the symmetry of the system and prove that it can control the DM interaction strength as well as the DM vector order.

cond-mat.mtrl-sci↗

arXiv2026

Stability-threshold control of helicity-selective antiferromagnetic resonance

We study helicity-selective antiferromagnetic resonance in an antiferromagnetic-insulator/nonmagnetic-metal junction with sublattice-dependent damping and spin-orbit torque. By formulating the linearized Landau--Lifshitz--Gilbert equation as a 2x2 non-Hermitian eigenvalue problem, we analyze the complex resonance frequencies and the stability threshold, using MnF$_2$ parameters as a representative example of a uniaxial antiferromagnet. We show that the absorption is strongly enhanced near the stability threshold, where one helicity mode becomes weakly damped, leading to linewidth narrowing and pronounced helicity selectivity in the sub-THz regime. The selected helicity can be switched by reversing the current direction, which reverses the spin-orbit torque acting on the interfacial sublattice. These results identify stability-threshold control as a design principle for electrically tunable helicity-selective antiferromagnetic resonance in uniaxial antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2026

Distinct memory properties in spin-wave reservoir computing based on synthetic antiferromagnet

Spin-wave-based physical reservoir computing (RC) is a promising candidate for energy-efficient physical implementations of artificial intelligence because of its potential for nanoscale integration with low power consumption. Most of the previous studies on spin-wave RC have utilized spin waves excited in a single-layer ferromagnet. In this study, we focused on spin waves in a synthetic antiferromagnet (SAF), consisting of two ferromagnetic layers coupled antiferromagnetically, and investigated additional memory properties of spin-wave RC. We theoretically and numerically demonstrate the emergence of two distinct memory properties in the SAF device due to the distinct spin-wave characteristics of the acoustic and optical modes inherent in SAFs.

cond-mat.mtrl-sci↗

arXiv2025

Single-Shot Magnetization Reversal in Ferromagnetic Spin Valves Enabled via Heat Control

We study laser induced ultrafast magnetization reversal in a ferromagnetic spin valve by comparing the effects of direct laser excitation and ultrashort hot electron pulses. A wedged Cu layer is deposited atop the spin valve to tune energy transmission to the magnetic stack for both optical and hot-electron excitation. We demonstrate single shot magnetization reversal of the free layer using hot electron pulses. Moreover, such reversal is achieved even with picosecond laser pulses. The influence of laser fluence, Cu thickness ($t_{\mathrm{Cu}}$), and pulse duration is investigated in detail. Our results indicate that the key factor enabling magnetization reversal is full demagnetization of the free layer, driven by a rapid rise in its electronic temperature achieved via either direct laser or hot electron excitation. This work advances the understanding of ultrafast magnetization reversal via nonlocal heat and spin transport under strongly out of equilibrium conditions.

cond-mat.mtrl-sci↗

arXiv2025

Light-induced torque in ferromagnetic metals via orbital angular momentum generated by photon-helicity

We investigated photon-helicity-induced magnetization precession in Co$_{1-x}$Pt$_{x}$ alloy thin films. In addition to field-like torque, attributable to magnetic field generation owing to {\it the inverse Faraday effect}, we observed non-trivial and large damping-like torque which has never been discussed for single ferromagnetic layer. The composition dependence of those two torques is effectively elucidated by a model that considers mutual coupling via spin-orbit interaction between magnetization and the electronic orbital angular momentum generated by photon-helicity. This work significantly enhances our understanding of the physics relevant to the interplay of photon-helicity and magnetization in magnetic metals.

cond-mat.mtrl-sci↗

arXiv2023

Different spin relaxation property observed in linearly and circularly polarized laser induced terahertz emission from Bi/Co bilayer

Recently, helicity-dependent photocurrent was reported in Bi single thin fi lms. It is proposed that the origin of this photocurrent is the combination of photo-spin conversion and spin-charge conversion effects in Bi and efficient spin conversion in Bi is expected. In this study, we measured two types of terahertz (THz) emissions from Bi/Co bilayer films induced by spin current generation using laser-induced demagnetization of the Co layer and photo-spin conversion effect in the Bi layer to investigate the spin current induced by the two mechanisms simultaneously. We clearly observed diff erent Bi thickness dependence of peak intensity and that of bandwidth for THz spin current in two experiments, i.e., spin current induced by demagnetization of Co and that by photo-spin conversion in Bi. The different Bi thickness dependence of spin current intensity and bandwidth in two experiments is caused by different spin relaxation properties of optically excited spin currents in Bi layers.

cond-mat.mtrl-sci↗

arXiv2023

Universal scaling between wave speed and size enables nanoscale high-performance reservoir computing based on propagating spin-waves

Neuromorphic computing using spin waves is promising for high-speed nanoscale devices, but the realization of high performance has not yet been achieved. Here we show, using micromagnetic simulations and simplified theory with response functions, that spin-wave physical reservoir computing can achieve miniaturization down to nanoscales keeping high computational power comparable with other state-of-art systems. We also show the scaling of system sizes with the propagation speed of spin waves plays a key role to achieve high performance at nanoscales.

physics.app-ph↗

arXiv2022

Enhancement of Spin-Charge Conversion Efficiency for Co$_{3}$Sn$_{2}$S$_{2}$ across Transition from Paramagnetic to Ferromagnetic Phase

Co$_{3}$Sn$_{2}$S$_{2}$ (CSS) is one of the shandite compounds and becomes a magnetic Weyl semimetal candidate below the ferromagnetic phase transition temperature ($\textit{T}_\textrm{C}$). In this paper, we investigate the temperature ($\textit{T}$) dependence of conversion between charge current and spin current for the CSS thin film by measuring the spin-torque ferromagnetic resonance (ST-FMR) for the trilayer consisting of CSS / Cu / CoFeB. Above $\textit{T}_\textrm{C}$ ~ 170 K, the CSS / Cu / CoFeB trilayer exhibits the clear ST-FMR signal coming from the spin Hall effect in the paramagnetic CSS and the anisotropic magnetoresistance (AMR) of CoFeB. Below $\textit{T}_\textrm{C}$, on the other hand, it is found that the ST-FMR signal involves the dc voltages ($\textit{V}_\textrm{dc}$) not only through the AMR but also through the giant magnetoresistance (GMR). Thus, the resistance changes coming from both AMR and GMR should be taken into account to correctly understand the characteristic field angular dependence of $\textit{V}_\textrm{dc}$. The spin Hall torque generated from the ferromagnetic CSS, which possesses the same symmetry as that for spin Hall effect, dominantly acts on the magnetization of CoFeB. A definite increase in the spin-charge conversion efficiency ($ξ$) is observed at $\textit{T}$ < $\textit{T}_\textrm{C}$, indicating that the phase transition to the ferromagnetic CSS promotes the highly efficient spin-charge conversion. In addition, our theoretical calculation shows the increase in spin Hall conductivity with the emergence of magnetic moment at $\textit{T}$ < $\textit{T}_\textrm{C}$, which is consistent with the experimental observation.

cond-mat.mes-hall↗

arXiv2021

Giant effective damping of octupole oscillation in an antiferromagnetic Weyl semimetal

A magnetic Weyl semimetal is a recent focus of extensive research as it may exhibit large and robust transport phenomena associated with topologically protected Weyl points in momentum space. Since a magnetic texture provides a handle for the configuration of the Weyl points and its transport response, understanding of magnetic dynamics should form a basis of future control of a topological magnet. Mn3Sn is an example of an antiferromagnetic Weyl semimetal that exhibits a large response comparable to the one observed in ferromagnets despite a vanishingly small magnetization. The non-collinear spin order in Mn3Sn can be viewed as a ferroic order of cluster magnetic octupole and breaks the time-reversal symmetry, stabilizing Weyl points and the significantly enhanced Berry curvature near the Fermi energy. Here we report our first observation of time-resolved octupole oscillation in Mn3Sn. In particular, we find the giant effective damping of the octupole dynamics, and it is feasible to conduct an ultrafast switching at < 10 ps, a hundred times faster than the case of spin-magnetization in a ferromagnet. Moreover, high domain wall velocity over 10 km/s is theoretically predicted. Our work paves the path towards realizing ultrafast electronic devices using the topological antiferromagnet.

cond-mat.mtrl-sci↗

arXiv2021

Interface-induced field-like optical spin torque in a ferromagnet/heavy metal heterostructure

The manipulation of magnetization in a metallic ferromagnet by using optical helicity has been much attracted attention for future opto-spintronic devices. The optical helicity induced torques on the magnetization, {\it optical spin torque}, have been observed in ferromagnetic thin films recently. However, the interfacial effect of the optical spin torque in ferromagnet/nonmagnetic heavy metal heterostructures have not been addressed so far, which are widely utilized to efficiently control magnetization via electrical means. Here, we studied optical spin torque vectors in the ferromagnet/nonmagnetic heavy metal heterostructures and observed that in-plane field-like optical spin torque was significantly increased with decreasing ferromagnetic layer thicknesses. The interfacial field-like optical spin torque was explained by the optical Rashba-Edelstein effect caused by the structural inversion symmetry breaking. This work will aid in the efficient optical manipulation of thin film nanomagnets using optical helicity.

cond-mat.mtrl-sci↗

arXiv2019

Large Spin Anomalous Hall Effect in L1$\rm_{0}$-FePt: Symmetry and Magnetization Switching

We quantitatively evaluate a spin anomalous Hall effect (SAHE), generating spin angular momentum flow (spin current, $J_{\rm s}$), in an L1$\rm_{0}$-FePt ferromagnet by exploiting giant magnetoresistance devices with L1$\rm_{0}$-FePt / Cu / Ni$\rm_{81}$Fe$\rm_{19}$ . From the ferromagnetic resonance linewidth modulated by the charge current ($J_{\rm c}$) injection, the spin anomalous Hall angle ($ α_{\rm SAH} $) is obtained to be 0.25 $ \pm $ 0.03. The evaluation of $ α_{\rm SAH} $ at different configurations between $J_{\rm c}$ and magnetization enables us to discuss the symmetry of SAHE and gives the unambiguous evidence that SAHE is the source of $J_{\rm s}$. Thanks to the large $ α_{\rm SAH} $, we demonstrate the SAHE-induced magnetization switching.

cond-mat.mes-hall↗