arXiv subjects

Yoshishige Suzuki

Publications and source records attributed to Yoshishige Suzuki.

At least 19 recordsLinked to original sources

arXiv2026

Transfer Entropy and Flow of Information in Two-Skyrmion System

We theoretically investigate the flow of information in an interacting two-skyrmion system confined in a box at finite temperature. By numerical simulations based on the Thiele-Langevin equation, we demonstrate that the skyrmion motion cannot be fully described by the master equation, highlighting the nontrivial dynamics. Particularly, due to the chiral motion of skyrmion, we find asymmetric flow of information with violating the detailed balance condition. We analyze this system using information-theoretical quantities including Shannon entropy, mutual information, and transfer entropy. The physical significance of transfer entropy, which has been overlooked in previous studies, is elucidated. Notably, the peak position of the transfer entropy, as a function of time delay, is independent of the interaction range yet dependent on the box size. This peak corresponds to the characteristic time required for changing the skyrmion state. Due to the unusual asymmetric circulation of information, the two-skyrmion system can be a unique device for future applications to the natural computing.

cond-mat.mes-hall↗

arXiv2025

Information dynamics, natural computing and Maxwell's demon in two skyrmions system

The probabilistic information flow and natural computational capability of a system with two magnetic skyrmions at room temperature have been experimentally evaluated. Based on this evaluation, an all-solid-state built-in Maxwell's demon operating at room temperature is also proposed. Probabilistic behavior has gained attention for its potential to enable unconventional computing paradigms. However, information propagation and computation in such systems are more complex than in conventional computers, making their visualization essential. In this study, a two-skyrmion system confined within a square potential well at thermal equilibrium was analyzed using information thermodynamics. Transfer entropy and the time derivative of mutual information were employed to investigate the information propagation speed, the absence of a Maxwell's demon in thermal equilibrium, and the system's non-Markovian properties. Furthermore, it was demonstrated that the system exhibits a small but finite computational capability for the nonlinear XOR operation, potentially linked to hidden information in the non-Markovian system. Based on these experiments and analyses, an all-solid-state built-in Maxwell's demon utilizing the two-skyrmion system and operating at room temperature is proposed.

cond-mat.mes-hall↗

arXiv2022

Mass and generalized Thiele equation of the magnetic skyrmion

An analytical expression is obtained for the mass of an isolated magnetic skyrmion and its linearized equation of motion. The magnetic skyrmion is viewed as a topologically protected spin-wave soliton in the magnetic ultrathin films stabilized by the interfacial-Dzyaloshinskii-Moriya interaction. The equations of motion are derived from the Landau-Lifshitz-Gilbert equation for both the skyrmion charge and magnetization centers. They are generalized Thiele equations, including the gyro-term, dissipation term, external force, acceleration term with the tensorial mass, and time derivatives of the external forces. The equation of motion of the center of the skyrmion charge essentially shows the massless nature of the skyrmion. In contrast, the equation of motion for the magnetization center results in a finite mass that is in the same order as the Doring mass density for the linear domain wall. Furthermore, the time derivative of the external force predominantly contributes to the immediate response of the skyrmion motion, i.e., the mass-less property remains even after the skyrmion acquires its kinetic mass. A micromagnetic simulation based on the LLG equation was performed for various magnetic parameters. Obtained trajectories at 0 K are compared with the theoretical predictions.

cond-mat.mes-hall↗

arXiv2021

Diffusion of a magnetic skyrmion in 2-dimensional space

Two-dimensional magnetic skyrmions are particle-like magnetic domains in magnetic thin films. The kinetic property of the magnetic skyrmions at finite temperature is well described by the Thiele equation, including a stochastic field and a finite mass. In this paper, the validity of the constant-mass approximation is examined by comparing the Fourier spectrum of Brownian motions described by the Thiele equation and the Landau-Lifshitz-Gilbert equation. Then, the 4-dimensional Fokker-Planck equation is derived from the Thiele equation with a mass-term. Consequently, an expression of the diffusion flow and diffusion constant in a tensor form is derived, extending Chandrasekhar's method for Thiele dynamics.

cond-mat.stat-mech↗

arXiv2020

Control of spin-orbit torques by interface engineering in topological insulator heterostructures

(Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ topological insulators (TIs) are gathering increasing attention owing to their large charge-to-spin conversion efficiency and the ensuing spin-orbit torques (SOTs) that can be used to manipulate the magnetization of a ferromagnet (FM). The origin of the torques, however, remains elusive, while the implications of hybridized states and the strong material intermixing at the TI/FM interface are essentially unexplored. By combining interface chemical analysis and spin-transfer ferromagnetic resonance (ST-FMR) measurements, we demonstrate that intermixing plays a critical role in the generation of SOTs. By inserting a suitable normal metal spacer, material intermixing is reduced and the TI properties at the interface are largely improved, resulting in strong variations in the nature of the SOTs. A dramatic enhancement of a field-like torque, opposing and surpassing the Oersted-field torque, is observed, which can be attributed to the non-equilibrium spin density in Rashba-split surface bands and to the suppression of spin memory loss.

cond-mat.mes-hall↗

arXiv2020

Skyrmion Brownian circuit implemented in a continuous ferromagnetic thin film

The fabrication of a skyrmion circuit which stabilizes skyrmions is important to realize micro- to nano-sized skyrmion devices. One example of promising skyrmion-based device is Brownian computers, which have been theoretically proposed, but not realized. It would require a skyrmion circuit in which the skyrmion is stabilized and easily movable. However, the usual skyrmion circuits fabricated by etching of the ferromagnetic film decrease the demagnetization field stabilizing the skyrmions, and thus prevent their formation. In this study, a skyrmion Brownian circuit implemented in a continuous ferromagnetic film with patterned SiO$_2$ capping to stabilize the skyrmion formation. The patterned SiO$_2$ capping controls the saturation field of the ferromagnetic layer and forms a wire-shaped skyrmion potential well, which stabilizes skyrmion formation in the circuit. Moreover, we implement a hub (Y-junction) circuit without pinning sites at the junction by patterned SiO$_2$ capping. This technique enables the efficient control of skyrmion-based memory and logic devices, as well as Brownian computers.

cond-mat.mes-hall↗

arXiv2020

Manipulating 1-dimensinal skyrmion motion by external magnetic field gradient

We have investigated an analytic formula of the 1-dimensional magnetic skyrmion dynamics under external magnetic field gradient. We find excellent agreement between the analytical model and micromagnetic simulation results for various magnetic parameters such as the magnetic field gradient, Gilbert damping constant. We also observe much faster velocity of the chiral domain wall (DW) motion. The chiral DW is exist with smaller interfacial Dzyaloshinskii-Moriya interaction energy density cases. These results provide to develop efficient control of skyrmion for spintronic devices.

cond-mat.mes-hall↗

arXiv2019

Investigation of gating effect in Si spin MOSFET

A gate voltage application in a Si-based spin metal-oxide-semiconductor field-effect transistor (spin MOSFET) modulates spin accumulation voltages, where both electrical conductivity and drift velocity are modified while keeping constant electric current. An unprecedented reduction in the spin accumulation voltages in a Si spin MOSFET under negative gate voltage applications is observed in a high electric bias current regime. To support our claim, the electric bias current dependence of the spin accumulation voltage under the gate voltage applications is investigated in detail and compared to a spin drift diffusion model including the conductance mismatch effect. We proved that the drastic decrease of the mobility and spin lifetime in the Si channel is due to the optical phonon emission at the high electric bias current, which consequently reduced the spin accumulation voltage.

physics.app-ph↗

arXiv2019

Deterministic magnetization switching by voltage-control of magnetic anisotropy and Dzyaloshinskii-Moriya interaction under in-plane magnetic field

Based on the micromagnetic simulations the magnetization switching in a triangle magnetic element by voltage-control of magnetic anisotropy and Dzyaloshinskii-Moriya interaction under in-plane magnetic field is proposed. The proposed switching scheme is not the toggle switching but the deterministic switching where the magnetic state is determined by the polarity of the applied voltage pulse. The mechanism and conditions for the switching are clarified. The results provide a fast and low-power writing method for magnetoresistive random access memories.

cond-mat.mes-hall↗

arXiv2019

Reservoir computing with dipole-coupled nanomagnets

The feasibility of reservoir computing based on dipole-coupled nanomagnets is demonstrated using micro-magnetic simulations. The reservoir consists of an 2x10 array of nanomagnets. The static-magnetization directions of the nanomagnets are used as reservoir states. To update these states, we change the magnetization of one nanomagnet according to a single-bit-sequential signal. We also change the uniaxial anisotropy of the other nanomagnets using a voltage-induced magnetic-anisotropy change to enhance information flow, storage, and linear/nonlinear calculations. Binary tasks with AND, OR, and XOR operations were performed to evaluate the performance of the magnetic-array reservoir. The reservoir-computing output matrix was found to be trainable to perform AND, OR, and XOR operations with an input delay of up to three bits.

cs.ET↗

arXiv2019

Magnetic anisotropy of ferromagnetic metals in low-symmetry systems

We have constructed an analytic formula to treat the perpendicular magnetic anisotropy energy in ferromagnetic metals with low symmetry, such as C4V and C3V. We find that the anisotropy energy is proportional to a part of the expectation values of the orbital angular momentum and magnetic dipole operator. Although the result is similar to the model proposed by Laan [J. Phys.: Condens. Matter 10, 3239 (1998)], we have derived a concrete expression for the spin-flip virtual excitation process term, which can be dominant in atoms with small magnetic moments and/or small exchange splitting. Pt monatomic layer with proximity-induced spin polarization grown on Fe is an example of this. Other multilayer systems such as Co/Pd and Co/Ni and bilayer systems such as Fe(CoB)/MgO can be discussed similarly. Moreover, the relation between perpendicular magnetic anisotropy energy and measurable physical parameters is discussed based on X-ray magnetic circular dichroism spectroscopy.

cond-mat.mtrl-sci↗

arXiv2018

Quantitative and systematic analysis of bias dependence of spin accumulation voltage in a non-degenerate Si spin valve

Spin accumulation voltages in a non-degenerate Si spin valve are discussed quantitatively as a function of electric bias current using systematic experiments and model calculations. As an open question in semiconductor spintronics, the origin of the deviation of spin accumulation voltages measured experimentally in a non-degenerate Si spin valve is clarified from that obtained by model calculation using the spin drift diffusion equation including the effect of the spin-dependent interfacial resistance of tunneling barriers. Unlike the case of metallic spin valves, the bias dependence of the resistance-area product for a ferromagnet/MgO/Si interface, resulting in the reappearance of the conductance mismatch, plays a central role to induce the deviation.

cond-mat.mtrl-sci↗

arXiv2018

Macromagnetic simulation for reservoir computing utilizing spin dynamics in magnetic tunnel junctions

The figures-of-merit for reservoir computing (RC), using spintronics devices called magnetic tunnel junctions (MTJs), are evaluated. RC is a type of recurrent neural network. The input information is stored in certain parts of the reservoir, and computation can be performed by optimizing a linear transform matrix for the output. While all the network characteristics should be controlled in a general recurrent neural network, such optimization is not necessary for RC. The reservoir only has to possess a non-linear response with memory effect. In this paper, macromagnetic simulation is conducted for the spin-dynamics in MTJs, for reservoir computing. It is determined that the MTJ-system possesses the memory effect and non-linearity required for RC. With RC using 5-7 MTJs, high performance can be obtained, similar to an echo-state network with 20-30 nodes, even if there are no magnetic and/or electrical interactions between the magnetizations.

cond-mat.mtrl-sci↗

arXiv2017

Electric-field-induced changes of magnetic moments and magnetocrystalline anisotropy in ultrathin cobalt films

In this study, the microscopic origins of the voltage-controlled magnetic anisotropy (VCMA) in 3d-ferromagnetic metals are revealed. Using in-situ X-ray fluorescence spectroscopy that provides a high quantum efficiency, electric-field-induced changes in orbital magnetic moment and magnetic dipole Tz terms in ultrathin Co films are demonstrated. An orbital magnetic moment difference of 0.013μB. was generated in the presence of electric fields of +(-)0.2 V/nm. The VCMA of Co was properly estimated by the induced change in orbital magnetic moment, according to the perturbation theory model. The induced change in magnetic dipole Tz term only slightly contributed to the VCMA in 3d-ferromagnetic metals.

cond-mat.mtrl-sci↗

arXiv2017

Spincaloritronic signal generation in non-degenerate Si

Spincaloritronic signal generation due to thermal spin injection and spin transport is demonstrated in a non-degenerate Si spin valve. The spin-dependent Seebeck effect is used for the spincaloritronic signal generation, and the thermal gradient of about 200 mK at an interface of Fe and Si enables generating a spin voltage of 8 μV at room temperature. A simple expansion of a conventional spin drift-diffusion model with taking into account the spin-dependent Seebeck effect shows semiconductor materials are quite potential for the spincaloritronic signal generation comparing with metallic materials, which can allow efficient heat recycling in semiconductor spin devices.

physics.app-ph↗

arXiv2017

Investigation of spin scattering mechanism in silicon channels of Fe/MgO/Si lateral spin valves

The temperature evolution of spin relaxation time, τsf, in degenerate silicon (Si)-based lateral spin valves is investigated by means of the Hanle effect measurements. τsf at 300 K is estimated to be 1.68+-0.03 ns and monotonically increased with decreasing temperature down to 100 K. Below 100 K, in contrast, it shows almost a constant value of ca. 5 ns. The temperature dependence of the conductivity of the Si channel shows a similar behavior to that of the τsf, i.e., monotonically increasing with decreasing temperature down to 100 K and a weak temperature dependence below 100 K. The temperature evolution of conductivity reveals that electron scattering due to magnetic impurities is negligible. A comparison between τsf and momentum scattering time reveals that the dominant spin scattering mechanism in the Si is the Elliott-Yafet mechanism, and the ratio of the momentum scattering time to the τsf attributed to nonmagnetic impurities is approximately 3.77*10^-6, which is more than two orders of magnitude smaller than that of copper.

physics.app-ph↗

arXiv2016

Observation of large spin accumulation voltages in non-degenerate Si spin devices due to spin drift effect: Experiments and theory

A large spin-accumulation voltage of more than 1.5 mV at 1 mA, i.e., a magnetoresistance of 1.5 Ω, was measured by means of the local three-terminal magnetoresistance in nondegenerate Si-based lateral spin valves (LSVs) at room temperature. This is the largest spin-accumulation voltage measured in semiconductor-based LSVs. The modified spin drift-diffusion model, which successfully accounts for the spin drift effect, explains the large spin-accumulation voltage and significant bias-current-polarity dependence. The model also shows that the spin drift effect enhances the spin-dependent magnetoresistance in the electric two terminal scheme. This finding provides a useful guiding principle for spin metal-oxide semiconductor field-effect transistor (MOSFET) operations.

cond-mat.mtrl-sci↗

arXiv2015

Room-temperature operation of Si spin MOSFET with high on/off spin signal ratio

We experimentally demonstrate a Si spin metal-oxide-semiconductor field-effect transistor (MOSFET) that exhibits a high on/off ratio of source-drain current and spin signals at room temperature. The spin channel is non-degenerate n-type Si, and an effective application of gate voltage in the back-gated structure allows the spin MOSFET operation. This achievement can pave the way to practical use of the Si spin MOSFET.

cond-mat.mes-hall↗