arXiv subjects

Daisuke Miura

Publications and source records attributed to Daisuke Miura.

15 recordsLinked to original sources

arXiv2024

Combination of crystal growth with optical floating zone and evaluation of Nd3+:LaAlO3 crystals with the dynamic nuclear polarization of 139La and 27Al

Producing a polarized lanthanum (La) target with high polarization and long relaxation time is crucial for realizing time-reversal violation experiments using polarized neutron beams. We use a LaAlO3 crystal doped with a small amount of Nd3+ ions for the polarized lanthanum target. Optimizing the amount of Nd3+ ions is considerably important because the achievable polarization and relaxation time strongly depend on this amount. We established a fundamental method to grow single crystals of Nd3+:LaAlO3 using an optical floating zone method that employs halogen lamps and evaluated the crystals with the dynamic nuclear polarization (DNP) method for polarizing nuclear spins. Two crystal samples were grown by ourselves and evaluated with the DNP at 1.3 K and 2.3 T for the first time except for the target materials of protons. The enhancement of NMR signals for 139La and 27Al was successfully observed, and the enhancement factors were eventually 3.5+-0.3 and 13+-3 for the samples with Nd3+ ions of 0.05 and 0.01 mol%, respectively. These enhancement factors correspond to absolute vector polarizations of 0.27+-0.02% (Nd 0.05 mol%) and 1.4+-0.3% (Nd 0.01 mol%). Although the obtained polarizations are still low, they are acceptable as a first step. The combination scheme of the crystal growth and evaluation of the crystals is found to be effectively applicable for optimizing the amount of Nd3+ ions for improving the performance of the polarized target.

physics.ins-det↗

arXiv2022

Perpendicularly Polarized Spin Hall Effects Induced by Spin-Dependent Scattering in Ferromagnetic Metals

Spin currents in ferromagnets afford diverse functionalities. We evaluate the extrinsic spin Hall effects of magnetic impurity scattering in ferromagnetic metals. We show that spin-dependent scattering can provide a high spin current polarized perpendicularly to the magnetization direction and is a dominant mechanism in the moderate-conductivity regime. We find that the superposition of the spin-conserve and spin-flip channels causes the spin currents. These findings suggest that optimizing alloy composition is an effective strategy to control the spin Hall effect.

cond-mat.mtrl-sci↗

arXiv2022

Temperature Dependence of Magnetocrystalline Anisotropy in Itinerant Ferromagnets

We theoretically investigated magnetocrystalline anisotropy (MA) at a finite temperature $T$ in ferromagnetic metals. Assuming a Rashba-type ferromagnet with uniaxial MA, we defined the MA constants $K_\mathrm{u}(T)$ derived from several different concepts. Our purpose was to examine the equality between them and to confirm a power law between $K_\mathrm{u}(T)$ and magnetization $M(T)$ in the form of $K_\mathrm{u}(T)/K_\mathrm{u}(0)=[M(T)/M(0)]^α$. We demonstrate that $α$ equals 2 in the itinerant-electron limit and increases with the localized feature of electrons passing through $α=3$, predicted for the single-ion MA in spin models.

cond-mat.str-el↗

arXiv2021

Theoretical Study of Temperature Dependence of Spin Susceptibility in Anisotropic Itinerant Ferromagnets

We developed a framework for directly calculating the spin susceptibility of anisotropic itinerant ferromagnets in the full temperature range within the coherent potential approximation in the disordered local moment picture. As a test of our formulation, we demonstrate the computation for the temperature ($T$) dependence of the spin susceptibility for Rashba-type ferromagnets with the Curie temperature $T_\mathrm{C}$. In a certain parameter, we found that the inverse transverse susceptibility $1/χ_\perp\simeq\mathrm{const.}$ for $T T_\mathrm{C}$, which reflects characteristics of itinerant ferromagnets including a spin-orbit interaction.

cond-mat.str-el↗

arXiv2021

Theoretical Study of Extrinsic Spin-current Generation in Ferromagnets Induced by Anisotropic Spin-flip Scattering

The spin Hall effect (SHE) and the magnetic spin Hall effect (MSHE) are responsible for electrical spin current generation, which is a key concept of modern spintronics. We theoretically investigated the spin conductivity induced by spin-dependent s-d scattering in a ferromagnetic 3d alloy model by employing microscopic transport theory based on the Kubo formula. We derived a novel extrinsic mechanism that contributes to both the SHE and MSHE. This mechanism can be understood as the contribution from anisotropic (spatial-dependent) spin-flip scattering due to the combination of the orbital-dependent anisotropic shape of s-d hybridization and spin flipping, with the orbital shift caused by spin-orbit interaction with the d-orbitals. We also show that this mechanism is valid under crystal-field splitting among the d-orbitals in either the cubic or tetragonal symmetry.

cond-mat.mtrl-sci↗

arXiv2019

Voltage-Controlled Magnonic Spin Tunneling Junction

We theoretically investigate the effective exchange interaction, $J_\mathrm{eff}$, mediated by conductive electrons within a nonmagnetic metal spacer, in the presence of a bias voltage, sandwiched by two ferromagnetic insulators. On the basis of the tight-binding model, we show the voltage and spacer thickness dependences of $J_\mathrm{eff}$, and its contorollability is demonstrated. We also propose a new magnonic device with the functions of both field effect transistor and non-volatile memory.

cond-mat.mes-hall↗

arXiv2019

Theoretical Study on Four-fold Symmetric Anisotropic Magnetoresistance Effect in Cubic Single-crystal Ferromagnetic Model

In this study, we present a theoretical interpretation of the experimental results that the anisotropic magnetoresistance (AMR) effect has a four-fold symmetric component, $c_4$, in cubic ferromagnetic metals. The theoretical model that we employ is based on the Anderson impurity model that includes a four-fold symmetric crystalline electric field, and we assume that the impurities have 3d electron orbitals and spin--orbit interaction (SOI). We describe the DC conductivity on the basis of the Kubo formula, and we investigate $c_4$ by analyzing the magnetization direction dependence of the resultant AMR ratio. Analytical and numerical calculations are performed; the analytical calculation reveals that $c_4$ arises from the fourth-order contribution of the SOI, and the numerical calculation provides the parameter dependencies of $c_{4}$ in our model. From the calculation results, we observe that the splitting of impurity 3d levels due to SOI is responsible for the existence of $c_{4}$ in cubic ferromagnetic metals.

physics.app-ph↗

arXiv2019

Analytic Expression for Magnetic Activation Energy

We theoretically investigate the magnetic activation energy of permanent magnets. Practically, it is widely used in a phenomenological form as $\mathcal{F}_\mathrm{B}(H_\mathrm{ext})=\mathcal{F}_\mathrm{B}^0\left(1-H_\mathrm{ext}/H_0\right)^n,$ where $\mathcal{F}_\mathrm{B}^0$ is the activation energy in the absence of an external magnetic field $H_\mathrm{ext}$, $n$ is a real parameter, and $H_0$ is defined by the equation $\mathcal{F}_\mathrm{B}(H_0)=0$. We derive the general and direct expressions for these phenomenological parameters under the restriction of uniform rotation of magnetization and on the basis of the perturbative theory with respect to $H_\mathrm{ext}$. Further,we apply our results to Nd$_2$Fe$_{14}$B magnets and confirm the validity of the proposed method by comparing with the Monte Carlo calculations.

cond-mat.mtrl-sci↗

arXiv2019

Noncollinearity effects on magnetocrystalline anisotropy for $R_2$Fe$_{14}$B magnets

We present a theoretical investigation of the magnetocrystalline anisotropy (MA) in $R_2$Fe$_{14}$B ($R$ is a rare-earth element) magnets in consideration of the non-collinearity effect (NCE) between the $R$ and Fe magnetization directions. In particular, the temperature dependence of the MA of Dy$_2$Fe$_{14}$B magnets is detailed in terms of the $n$th-order MA constant (MAC) $K_n(T)$ at a temperature $T$. The features of this constant are as follows: $K_1(T)$ has a broad plateau in the low-temperature range and $K_2(T)$ persistently survives in the high-temperature range. The present theory explains these features in terms of the NCE on the MA by using numerical calculations for the entire temperature range, and further, by using a high-temperature expansion. The high-temperature expansion for $K_n(T)$ is expressed in the form of $K_n(T)=κ_1(T)\left[1+δ(T)\right][-δ(T)]^{n-1}$, where $κ_1(T)$ is the part without the NCE and $δ(T)$ is a correction factor for the NCE introduced in this study. We also provide a convenient expression to evaluate $K_n(T)$, which can be determined only by a second-order crystalline electric field coefficient and an effective exchange field.

cond-mat.mtrl-sci↗

arXiv2018

Power law analysis for temperature dependence of magnetocrystalline anisotropy constants of Nd$_2$Fe$_{14}$B magnets

Phenomenological analysis for the temperature dependence of the magnetocrystalline anisotropy (MA) in rare earth magnets is presented. We define phenomenological power laws applicable to compound magnets using the Zener theory, apply these laws to the magnetocrystalline anisotropy constants (MACs) of Nd$_2$Fe$_{14}$B magnets. The results indicate that the MACs obey the power law well, and a general understanding for the temperature-dependent MA in rare earth magnets is obtained through the analysis. Furthermore, to examine the validity of the power law, we discuss the temperature dependence of the MACs in Dy$_2$Fe$_{14}$B and Y$_2$Fe$_{14}$B magnets as examples wherein it is difficult to interpret the MA using the power law.

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↗

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

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↗