arXiv subjects

Akimasa Sakuma

Publications and source records attributed to Akimasa Sakuma.

At least 37 records · Page 2Linked to original sources

arXiv2016

Monte Carlo analysis for finite temperature magnetism of Nd$_2$Fe$_{14}$B permanent magnet

We investigate the effects of magnetic inhomogeneities and thermal fluctuations on the magnetic properties of a rare earth intermetallic compound, Nd$_2$Fe$_{14}$B. The constrained Monte Carlo method is applied to a Nd$_2$Fe$_{14}$B bulk system to realize the experimentally observed spin reorientation and magnetic anisotropy constants $K^{\rm A}_m (m=1, 2, 4)$ at finite temperatures. % Subsequently, it is found that the temperature dependence of $K^{\rm A}_1$ deviates from the Callen--Callen law, $K^{\rm A}_1(T) \propto M(T)^3$, even above room temperature, $T_{\rm R}\sim 300\rm\, K$, when the Fe (Nd) anisotropy terms are removed to leave only the Nd (Fe) anisotropy terms. This is because the exchange couplings between Nd moments and Fe spins are much smaller than those between Fe spins. % It is also found that the exponent $n$ in the external magnetic field $H_{\rm ext}$ response of barrier height $\mathcal{F}_{\rm B}=\mathcal{F}_{\rm B}^0(1-H_{\rm ext}/H_0)^n$ is less than $2$ in the low-temperature region below $T_{\rm R}$, whereas $n$ approaches $2$ when $T>T_{\rm R}$, indicating the presence of Stoner--Wohlfarth-type magnetization rotation. This reflects the fact that the magnetic anisotropy is mainly governed by the $K^{\rm A}_1$ term in the $T>T_{\rm R}$ region.

cond-mat.mtrl-sci↗

arXiv2016

Temperature dependence of the threshold magnetic field for nucleation and domain wall propagation in an inhomogeneous structure with grain boundary

In order to study the dependence of the coercive force of sintered magnets on temperature, nucleation and domain wall propagation at the grain boundary are studied as rate-determining processes of the magnetization reversal phenomena in magnets consisting of bulk hard magnetic grains contacting via grain boundaries of a soft magnetic material. These systems have been studied analytically for a continuum model at zero temperature (A. Sakuma, et al. J. Mag. Mag. Mat. {\bf 84} 52 (1990)). In the present study, the temperature dependence is studied by making use of the stochastic Landau-Lifshitz-Gilbert equation at finite temperatures. In particular, the threshold fields for nucleation and domain wall propagation are obtained as functions of ratios of magnetic interactions and anisotropies of the soft and hard magnets for various temperatures. It was found that the threshold field for domain wall propagation is robust against thermal fluctuations, while that for nucleation is fragile. The microscopic mechanisms of the observed temperature dependence are discussed.

cond-mat.mtrl-sci↗

arXiv2016

Effects of spin fluctuation on the magnetic anisotropy constant of itinerant electron magnets

In the disordered local moment picture, we calculated the magnetization (M) and magnetic anisotropy energy (MAE) of FePt, CoPt, and MnAl ordered alloys and body-centered tegragonal FeCo (bct-FeCo) disordered alloy, assuming spatially fluctuated spin configurations at finite temperatures. All alloys exhibit the relation K1(T)/K1(0)=(M(T)/M(0))^n with the exponent (n) around 2. This is consistent with the two-ion anisotropy model, in contrast to the usual single-ion anisotropy model exhibiting n=3. Because these systems have different mechanisms of MAE, we suggest that this relation is a general rule for itinerant electron systems.

cond-mat.mtrl-sci↗

arXiv2015

Effects of trace elements on the crystal field parameters of Nd ions at the surface of Nd$_2$Fe$_{14}$B grains

Using first-principles calculations, we investigate the positional dependence of trace elements such as O and Cu on the crystal field parameter $A_2^0$, proportional to the anisotropy constant $K_u$ of Nd ions placed at the surface of Nd$_2$Fe$_{14}$B grains. The results suggest the possibility that the $A_2^0$ parameter of Nd ions at the (001) surface of Nd$_2$Fe$_{14}$B grains exhibits a negative value when the O or Cu atom is located near the surface, closer than its equilibrium position. At the (110) surface, however, O atoms located at the equilibrium position provide a negative $A_2^0$, while for Cu additions $A_2^0$ remains positive regardless of Cu's position. Thus, Cu atoms are expected to maintain a positive local $K_u$ of surface Nd ions more frequently than O atoms when they approach the grain surfaces in the Nd-Fe-B grains.

cond-mat.mtrl-sci↗

arXiv2015

General Formalism for Magnetic Anisotropy Constants

Direct expressions for the magnetic anisotropy constants are given at a finite temperature from microscopic viewpoints. In the present derivation, it is assumed that the Hamiltonian is a linear function with respect to the magnetization direction. We discuss in detail the first-order constant $K_1$ and show that the results reproduce previous results. We also apply our method to Nd$_2$Fe$_{14}$B compounds and demonstrate that the temperature dependencies of the magnetocrystalline anisotropy constants $K_1$, $K_2$, and $K_3$ are successfully computed.

cond-mat.mtrl-sci↗

arXiv2015

Spin-wave-induced spin torque in Rashba spin-orbit coupling system

We study the effects of Rashba spin-orbit coupling on the spin torque induced by spin waves, which are the plane wave dynamics of magnetization. The spin torque is derived from linear response theory, and we calculate the dynamic spin torque by considering the impurity-ladder-sum vertex corrections. This dynamic spin torque is divided into three terms: a damping term, a $distortion$ term, and a correction term for the equation of motion. The $distorting$ torque describes a phenomenon unique to the Rashba spin-orbit coupling system, where the distorted motion of magnetization precession is subjected to the anisotropic force from the Rashba coupling. The oscillation mode of the precession exhibits an elliptical trajectory, and the ellipticity depends on the strength of the nesting effects, which could be reduced by decreasing the electron lifetime.

cond-mat.mtrl-sci↗

arXiv2015

Theoretical evaluation on the temperature dependence of magnetic anisotropy constants of Nd2Fe14B - Effects of exchange field and crystal field strength -

To identify the possible mechanism of coercivity (Hc) degradation of Nd-Fe-B sintered magnets, we study the roles of the exchange field acting on the 4f electrons in Nd ions and theoretically investigate how the variation of the exchange field affects the values of the magnetic anisotropy constants K1 and K2. We find that, with decreasing exchange field strength, both values decrease as a result of the lower asphericity of the 4f electron cloud, indicating that the local anisotropy constants might become small around the grain boundaries where the exchange fields are decreased owing to the smaller coordination number.

cond-mat.mtrl-sci↗

arXiv2014

Mechanism of uniaxial magnetocrystalline anisotropy in transition metal alloys

Magnetocrystalline anisotropy in transition metal alloys (FePt, CoPt, FePd, MnAl, MnGa, and FeCo) was studied using first-principles calculations to elucidate its specific mechanism. The tight-binding linear muffin-tin orbital method in the local spin-density approximation was employed to calculate the electronic structure of each compound, and the anisotropy energy was evaluated using the magnetic force theorem and the second-order perturbation theory in terms of spin-orbit interactions. We systematically describe the mechanism of uniaxial magnetocrystalline anisotropy in real materials and present the conditions under which the anisotropy energy can be increased. The large magnetocrystalline anisotropy energy in FePt and CoPt arises from the strong spin-orbit interaction of Pt. In contrast, even though the spin-orbit interaction in MnAl, MnGa, and FeCo is weak, the anisotropy energies of these compounds are comparable to that of FePd. We found that MnAl, MnGa, and FeCo have an electronic structure that is efficient in inducing the magnetocrystalline anisotropy in terms of the selection rule of spin-orbit interaction.

cond-mat.mtrl-sci↗

arXiv2013

First Principles Calculation of Magnetocrystalline Anisotropy Energy of MnBi and MnB_{1-x}Sn_x

We calculated the magnetic anisotropy constant Ku of MnBi using a first principles approach, and obtained a negative Ku in agreeable with experimental results. Furthermore, we also found a band filling dependence indicating that a slight decrease in the valence electron number will change Ku from negative to positive. When some of the Bi is replaced with Sn to decrease the valence electron number, the Ku value of MnBi1-xSnx drastically changes to a positive value, Ku~2 MJ/m3, for x > 0.05.

cond-mat.mtrl-sci↗

arXiv2012

Effect of Spin Torque on Magnetization Switching Speed Having Nonuniform Spin Distribution

We study the influence of the spin torque, which depends on the space and time derivative of magnetization, on magnetization reversal time in a ferromagnetic fine particle. The spin torque operates to dissipate the angular momentum of the magnetization precession, and the torque increases in a spin vortex structure. We calculate the magnetization reversal time under a DC magnetic field using the Landau-Lifshitz-Gilbert equation containing a spin torque term. We found that the spin torque changes the magnetization switching speed significantly during the reversal process by maintaining a spin vortex in an intermediate state.

cond-mat.mes-hall↗

arXiv2012

Microscopic Theory of Magnon-Drag Thermoelectric Transport in Ferromagnetic Metals

A theoretical study of the magnon-drag Peltier and Seebeck effects in ferromagnetic metals is presented. A magnon heat current is described perturbatively from the microscopic viewpoint with respect to electron--magnon interactions and the electric field. Then, the magnon-drag Peltier coefficient $Π_\MAG$ is obtained as the ratio between the magnon heat current and the electric charge current. We show that $Π_\MAG=C_\MAG T^{5/2}$ at a low temperature $T$; that the coefficient $C_\MAG$ is proportional to the spin polarization $P$ of the electric conductivity; and that $P>0$ for $C_\MAG<0$, but $P<0$ for $C_\MAG>0$. From experimental results for magnon-drag Peltier effects, we estimate that the strength of the electron--magnon interaction is about 0.3 eV$\cdotÅ^{3/2}$ for permalloy.

cond-mat.mes-hall↗

arXiv2012

Degree of order dependence on magnetocrystalline anisotropy in bct FeCo alloys

We investigate the magnetocrystalline anisotropy (MCA) energy of tetragonal distorted FeCo alloys depending on the degree of order by first-principles electronic structure calculation combined with the coherent potential approximation. The obtained results indicate that the MCA energy of FeCo alloys strongly depends on the degree of order under optimal conditions, where the axial ratio of the bct structure is 1.25 and the composition is Fe0.5Co0.5. We find that the modification of the electronic structure resulting from electron scattering by chemical disorder has a considerable influence on the MCA under these conditions.

cond-mat.mtrl-sci↗

arXiv2012

Charge and Spin Transport in Magnetic Tunnel Junctions: Microscopic Theory

We study the charge and spin currents passing through a magnetic tunnel junction (MTJ) on the basis of a tight-binding model. The currents are evaluated perturbatively with respect to the tunnel Hamiltonian. The charge current has the form $A[\bm M_1(t)\times\dot{\bm M}_1(t)]\cdot\bm M_2+B\dot{\bm M}_1(t)\cdot\bm M_2$, where $\bm M_1(t)$ and $\bm M_2$ denote the directions of the magnetization in the free layer and fixed layer, respectively. The constant $A$ vanishes when one or both layers are insulators, {while the constant $B$ disappears when both layers are insulators or the same ferromagnets.} The first term in the expression for charge current represents dissipation driven by the effective electric field induced by the dynamic magnetization. In addition, from an investigation of the spin current, we obtain the microscopic expression for the enhanced Gilbert damping constant $\varDelta α$. We show that $\varDeltaα$ is proportional to the tunnel conductance and depends on the bias voltage.

cond-mat.mes-hall↗

arXiv2011

Anisotropic Magnetoresistance Effects in Fe, Co, Ni, Fe_4N, and Half-Metallic Ferromagnet: A Systematic Analysis

We theoretically analyze the anisotropic magnetoresistance (AMR) effects of bcc Fe (+), fcc Co (+), fcc Ni (+), Fe$_4$N (-), and a half-metallic ferromagnet (-). The sign in each ( ) represents the sign of the AMR ratio observed experimentally. We here use the two-current model for a system consisting of a spin-polarized conduction state and localized d states with spin--orbit interaction. From the model, we first derive a general expression of the AMR ratio. The expression consists of a resistivity of the conduction state of the $σ$ spin ($σ=\uparrow$ or $\downarrow$), $ρ_{s σ}$, and resistivities due to s--d scattering processes from the conduction state to the localized d states. On the basis of this expression, we next find a relation between the sign of the AMR ratio and the s--d scattering process. In addition, we obtain expressions of the AMR ratios appropriate to the respective materials. Using the expressions, we evaluate their AMR ratios, where the expressions take into account the values of $ρ_{s \downarrow}/ρ_{s \uparrow}$ of the respective materials. The evaluated AMR ratios correspond well to the experimental results.

cond-mat.mtrl-sci↗

arXiv2010

Spin-Atomic Vibration Interaction and Spin-Flip Hamiltonian of a Single Atomic Spin in a Crystal Field

We derive the spin-atomic vibration interaction $V_{\rm SA}$ and the spin-flip Hamiltonian $V_{\rm SF}$ of a single atomic spin in a crystal field. We here apply the perturbation theory to a model with the spin-orbit interaction and the kinetic and potential energies of electrons. The model also takes into account the difference in vibration displacement between an effective nucleus and electrons, $Δ{\boldmath $r$}$. Examining the coefficients of $V_{\rm SA}$ and $V_{\rm SF}$, we first show that $V_{\rm SA}$ appears for $Δ{\boldmath $r$}$$\ne$0, while $V_{\rm SF}$ is present independently of $Δ{\boldmath $r$}$. As an application, we next obtain $V_{\rm SA}$ and $V_{\rm SF}$ of an Fe ion in a crystal field of tetragonal symmetry. It is found that the magnitudes of the coefficients of $V_{\rm SA}$ can be larger than those of the conventional spin-phonon interaction depending on vibration frequency. In addition, transition probabilities per unit time due to $V_{\rm SA}$ and $V_{\rm SF}$ are investigated for the Fe ion with an anisotropy energy of $-|D|S_Z^2$, where $D$ is an anisotropy constant and $S_Z$ is the $Z$ component of a spin operator.

cond-mat.mes-hall↗

arXiv2009

Spin-polarized electronic structures and transport properties of Fe-Co alloys

The electrical resistivities of Fe-Co alloys owing to random alloy disorder are calculated using the Kubo-Greenwood formula. The obtained electrical esistivities agree well with experimental data quantitatively at low temperature. The spin-polarization of Fe50Co50 estimated from the conductivity (86%) has opposite sign to that from the densities of the states at the Fermi level (-73%). It is found that the conductivity is governed mainly by s-electrons, and the s-electrons in the minority spin states are less conductive due to strong scattering by the large densities of the states of d-electrons than the majority spin electrons.

cond-mat.mtrl-sci↗

arXiv2008

Ab-initio Study on the Magnetic Structures in the Ordered Mn3Pt Alloy

We study the electronic states of the magnetically ordered Mn3Pt alloy within the density functional theory. Mn3Pt has been believed that one third of Mn atoms have no magnetic moment in an antiferromagnetic phase (so-called the F-phase) realized in the temperature range of 400 K < T < 475 K. We show that this experimentally suggested spin configuration is energetically so much unfavorable that it would be irrelevant to the F-phase. We discuss the possibility that the spin moments on the one third of Mn atoms are not paramagnetic but thermally fluctuating in the F-phase. The present results have an immediate connection with the recent neutron scattering study [T. Ikeda and Y. Tsunoda, J. Phys. Soc. Jpn., vol. 72, pp. 2614-2621, October. 2003.].

cond-mat.mtrl-sci↗

arXiv2007

Theoretical study of a localized quantum spin reversal by the sequential injection of spins in a spin quantum dot

This is a theoretical study of the reversal of a localized quantum spin induced by sequential injection of spins for a spin quantum dot that has a quantum spin. The system consists of ``electrode/quantum well(QW)/dot/QW/electrode" junctions, in which the left QW has an energy level of conduction electrons with only up-spin. We consider a situation in which up-spin electrons are sequentially injected from the left electrode into the dot through the QW and an exchange interaction acts between the electrons and the localized spin. To describe the sequentially injected electrons, we propose a simple method based on approximate solutions from the time-dependent Schr$\ddot{\rm o}$dinger equation. Using this method, it is shown that the spin reversal occurs when the right QW has energy levels of conduction electrons with only down-spin. In particular, the expression of the reversal time of a localized spin is derived and the upper and lower limits of the time are clearly expressed. This expression is expected to be useful for a rough estimation of the minimum relaxation time of the localized spin to achieve the reversal. We also obtain analytic expressions for the expectation value of the localized spin and the electrical current as a function of time. In addition, we found that a system with the non-magnetic right QW exhibits spin reversal or non-reversal depending on the exchange interaction.

cond-mat.mes-hall↗