arXiv subjects

Satoshi Kokado

Publications and source records attributed to Satoshi Kokado.

13 recordsLinked to original sources

arXiv2022

Theoretical Study on Anisotropic Magnetoresistance Effects of Arbitrary Directions of Current and Magnetization for Ferromagnets: Application to Transverse Anisotropic Magnetoresistance Effect

We develop a theory of the anisotropic magnetoresistance (AMR) effects of arbitrary directions of current and magnetization for ferromagnets. Here, we use the electron scattering theory with the $s$--$s$ and $s$--$d$ scattering processes, where $s$ is the conduction electron state and $d$ is the localized d states. The resistivity due to electron scattering is expressed by the probability density of the d states of the current direction. The d states are numerically obtained by applying the exact diagonalization method to the Hamiltonian of the d states with the exchange field, crystal field, and spin--orbit interaction. Using the theory, we investigate the transverse AMR (TAMR) effect for strong ferromagnets with a crystal field of cubic or tetragonal symmetry. The cubic systems exhibit the fourfold symmetric TAMR effect, whereas the tetragonal systems show the twofold and fourfold symmetric TAMR effect. On the basis of the above results, we also comment on the experimental results of the TAMR effect for Fe$_4$N.

cond-mat.mtrl-sci↗

arXiv2019

Theoretical Study on Anisotropic Magnetoresistance Effects of I//[100], I//[110], and I//[001] for Ferromagnets with A Crystal Field of Tetragonal Symmetry

Using the electron scattering theory, we obtain analytic expressions for anisotropic magnetoresistance (AMR) ratios for ferromagnets with a crystal field of tetragonal symmetry. Here, a tetragonal distortion exists in the [001] direction, the magnetization ${\mbox{\boldmath $M$}}$ lies in the (001) plane, and the current ${\mbox{\boldmath $I$}}$ flows in the [100], [010], or [001] direction. When the ${\mbox{\boldmath $I$}}$ direction is denoted by $i$, we obtain the AMR ratio as ${\rm AMR}^i (ϕ_i)= C_0^i + C_2^i \cos 2ϕ_i + C_4^i \cos 4 ϕ_i \ldots = \sum_{j=0,2,4,\ldots} C_j^i \cos jϕ_i$, with $i=[100]$, $[110]$, and $[001]$, $ϕ_{[100]} = ϕ_{[001]}=ϕ$, and $ϕ_{[110]}=ϕ'$. The quantity $ϕ$ ($ϕ'$) is the relative angle between ${\mbox{\boldmath $M$}}$ and the $[100]$ ($[110]$) direction, and $C_j^i$ is a coefficient composed of a spin--orbit coupling constant, an exchange field, the crystal field, and resistivities. We elucidate the origin of $C_j^i \cos jϕ_i$ and the features of $C_j^i$. In addition, we obtain the relation $C_4^{[100]} = -C_4^{[110]}$, which was experimentally observed for Ni, under a certain condition. We also qualitatively explain the experimental results of $C_2^{[100]}$, $C_4^{[100]}$, $C_2^{[110]}$, and $C_4^{[110]}$ at 293 K for Ni.

cond-mat.mtrl-sci↗

arXiv2016

Spin polarization ratios of resistivity and density of states estimated from anisotropic magnetoresistance ratio for nearly half-metallic ferromagnets

We derive a simple relational expression between the spin polarization ratio of resistivity, $P_ρ$, and the anisotropic magnetoresistance ratio $Δρ/ρ$, and that between the spin polarization ratio of the density of states at the Fermi energy, $P_{\rm DOS}$, and $Δρ/ρ$ for nearly half-metallic ferromagnets. We find that $P_ρ$ and $P_{\rm DOS}$ increase with increasing $|Δρ/ρ|$ from 0 to a maximum value. In addition, we roughly estimate $P_ρ$ and $P_{\rm DOS}$ for a Co$_2$FeGa$_{0.5}$Ge$_{0.5}$ Heusler alloy by substituting its experimentally observed $Δρ/ρ$ into the respective expressions.

cond-mat.mtrl-sci↗

arXiv2015

Twofold and Fourfold Symmetric Anisotropic Magnetoresistance Effect in A Model with Crystal Field

We theoretically study the twofold and fourfold symmetric anisotropic magnetoresistance (AMR) effects of ferromagnets. We here use the two-current model for a system consisting of a conduction state and localized d states. The localized d states are obtained from a Hamiltonian with a spin--orbit interaction, an exchange field, and a crystal field. From the model, we first derive general expressions for the coefficient of the twofold symmetric term ($C_2$) and that of the fourfold symmetric term ($C_4$) in the AMR ratio. In the case of a strong ferromagnet, the dominant term in $C_2$ is proportional to the difference in the partial densities of states (PDOSs) at the Fermi energy ($E_{\rm F}$) between the $d\varepsilon$ and $dγ$ states, and that in $C_4$ is proportional to the difference in the PDOSs at $E_{\rm F}$ among the $d\varepsilon$ states. Using the dominant terms, we next analyze the experimental results for Fe$_4$N, in which $|C_2|$ and $|C_4|$ increase with decreasing temperature. The experimental results can be reproduced by assuming that the tetragonal distortion increases with decreasing temperature.

cond-mat.mtrl-sci↗

arXiv2013

Anisotropic Magnetoresistance Effect: General Expression of AMR Ratio and Intuitive Explanation for Sign of AMR Ratio

We derive the general expression of the anisotropic magnetoresistance (AMR) ratio of ferromagnets for a relative angle between the magnetization direction and the current direction. We here use the two-current model for a system consisting of a spin-polarized conduction state (s) and localized d states (d) with spin-orbit interaction. Using the expression, we analyze the AMR ratios of Ni and a half-metallic ferromagnet. These results correspond well to the respective experimental results. In addition, we give an intuitive explanation about a relation between the sign of the AMR ratio and the s-d scattering process.

cond-mat.mtrl-sci↗

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↗

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↗

arXiv2006

A theoretical analysis on highly spin-polarized transport of iron nitride Fe_4N

In order to propose a ferromagnet exhibiting highly spin-polarized transport, we theoretically analyzed the spin polarization ratio of the conductivity of the bulk Fe$_4$N with a perovskite type structure, in which N is located at the body center position of fcc-Fe. The spin polarization ratio is defined by $P = (σ_\uparrow - σ_\downarrow) / (σ_\uparrow + σ_\downarrow )$, with $σ_{\uparrow(\downarrow)}$ being the conductivity at zero temperature of the up spin (down spin). The conductivity is obtained by using the Kubo formula and the Slater-Koster tight binding model, where parameters are determined from the least-square fitting of the dispersion curves by the tight binding model to those by the first principles calculation. In the vicinity of the Fermi energy, $|P|$ takes almost 1.0, indicating perfectly spin-polarized transport. In addition, by comparing Fe$_4$N to fcc-Fe (Fe$_4$N$_0$) in the ferromagnetic state with the equilibrium lattice constant of Fe$_4$N, it is shown that the non-magnetic atom N plays an important role in increasing $|P|$.

cond-mat.mtrl-sci↗

arXiv2004

Spin dependent transport of ``nonmagnetic metal/zigzag nanotube encapsulating magnetic atoms/nonmagnetic metal'' junctions

Towards a novel magnetoresistance (MR) device with a carbon nanotube, we propose ``nonmagnetic metal/zigzag nanotube encapsulating magnetic atoms/nonmagnetic metal'' junctions. We theoretically investigate how spin-polarized edges of the nanotube and the encapsulated magnetic atoms influence on transport. When the on-site Coulomb energy divided by the magnitude of transfer integral, $U/|t|$, is larger than 0.8, large MR effect due to the direction of spins of magnetic atoms, which has the magnitude of the MR ratio of about 100%, appears reflecting such spin-polarized edges.

cond-mat.mes-hall↗

arXiv2004

A Theoretical Study on Spin-Dependent Transport of "Ferromagnet/Carbon Nanotube Encapsulating Magnetic Atoms/Ferromagnet" Junctions with 4-Valued Conductances

As a novel function of ferromagnet (FM)/spacer/FM junctions, we theoretically investigate multiple-valued (or multi-level) cell property, which is in principle realized by sensing conductances of four states recorded with magnetization configurations of two FMs; (up,up), (up,down), (down,up), (down,down). In order to sense all the states, 4-valued conductances corresponding to the respective states are necessary. We previously proposed that 4-valued conductances are obtained in FM1/spin-polarized spacer (SPS)/FM2 junctions, where FM1 and FM2 have different spin polarizations, and the spacer depends on spin [J. Phys.: Condens. Matter 15, 8797 (2003)]. In this paper, an ideal SPS is considered as a single-wall armchair carbon nanotube encapsulating magnetic atoms, where the nanotube shows on-resonance or off-resonance at the Fermi level according to its length. The magnitude of the obtained 4-valued conductances has an opposite order between the on-resonant nanotube and the off-resonant one, and this property can be understood by considering electronic states of the nanotube. Also, the magnetoresistance ratio between (up,up) and (down,down) can be larger than the conventional one between parallel and anti-parallel configurations.

cond-mat.mes-hall↗

arXiv2004

Magnetoresistance Effect in Spin-Polarized Junctions of Ferromagnetically Contacting Multiple Conductive Paths: Applications to Atomic Wires and Carbon Nanotubes

For spin-polarized junctions of ferromagnetically contacting multiple conductive paths, such as ferromagnet (FM)/atomic wires/FM and FM/carbon nanotubes/FM junctions, we theoretically investigate spin-dependent transport to elucidate the intrinsic relation between the number of paths and conduction, and to enhance the magnetoresistance (MR) ratio. When many paths are randomly located between the two FMs, electronic wave interference between the FMs appears, and then the MR ratio increases with increasing number of paths. Furthermore, at each number of paths, the MR ratio for carbon nanotubes becomes larger than that for atomic wires, reflecting the characteristic shape of points in contact with the FM.

cond-mat.mes-hall↗

arXiv2003

A theoretical investigation of ferromagnetic tunnel junctions with 4-valued conductances

In considering a novel function in ferromagnetic tunnel junctions consisting of ferromagnet(FM)/barrier/FM junctions, we theoretically investigate multiple valued (or multi-level) cell property, which is in principle realized by sensing conductances of four states recorded with magnetization configurations of two FMs; that is, (up,up), (up,down), (down,up), (down,down). To obtain such 4-valued conductances, we propose FM1/spin-polarized barrier/FM2 junctions, where the FM1 and FM2 are different ferromagnets, and the barrier has spin dependence. The proposed idea is applied to the case of the barrier having localized spins. Assuming that all the localized spins are pinned parallel to magnetization axes of the FM1 and FM2, 4-valued conductances are explicitly obtained for the case of many localized spins. Furthermore, objectives for an ideal spin-polarized barrier are discussed.

cond-mat.mes-hall↗