Showing 1–43 of 43 results for author: Sakuma, A

First-principles study of exchange stiffness constant of half-metallic Heusler alloys Co2MnZ (Z= Si, Al) at finite temperatures: Spin fluctuation-induced effective half metallicity

Authors: Shogo Yamashita, Akimasa Sakuma, Mikihiko Oogane

Abstract: We performed first-principles calculations at finite temperatures to investigate the temperature dependence of the magnetic properties, such as exchange stiffness constants and Curie temperatures, of Co2MnZ (Z= Si, Al) assuming L21 and B2 structures. In L21 structures, we confirmed a relatively high Curie temperature for Co2MnAl, compatible with that of Co2MnSi; however, its exchange stiffness con… ▽ More We performed first-principles calculations at finite temperatures to investigate the temperature dependence of the magnetic properties, such as exchange stiffness constants and Curie temperatures, of Co2MnZ (Z= Si, Al) assuming L21 and B2 structures. In L21 structures, we confirmed a relatively high Curie temperature for Co2MnAl, compatible with that of Co2MnSi; however, its exchange stiffness constant and single site magnetic excitation energy at zero temperature are much smaller than those of Co2MnSi. This might indicate that the Curie temperature of itinerant magnets cannot be determined by the exchange interaction at zero temperature. We also investigated the temperature dependence of the exchange stiffness constants of both alloys, and we found robustness in the temperature dependence of the exchange stiffness constant for Co2MnAl, assuming an L21 structure. This might lead to a high Curie temperature, contrary to the small exchange stiffness constant. Finally, we examined the temperature dependence of the electronic structure to investigate the origin of the behavior of the exchange stiffness constant at finite temperatures. We confirmed that the spin polarization at chemical potential effectively increases with an increasing temperature due to the altered electronic structure induced by the spin disorder. This might contribute to the robustness of the exchange stiffness constant at finite temperatures. Our results might indicate that renomarization of the electronic structure due to spin disorder at finite temperature influences the exchange interactions of Co2MnAl. △ Less

Submitted 23 September, 2024; originally announced September 2024.

Comments: 10 pages, 6 figures

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

Authors: Koki Nukui, Satoshi Iihama, Kazuaki Ishibashi, Shogo Yamashita, Akimasa Sakuma, Philippe Scheid, Grégory Malinowski, Michel Hehn, Stéphane Mangin, Shigemi Mizukami

Abstract: 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 effectiv… ▽ More 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. △ Less

Submitted 15 February, 2025; v1 submitted 12 May, 2024; originally announced May 2024.

Comments: 7 pages, 4 figures

Magnetization and exchange-stiffness constants of Fe-Al-Si alloys at finite-temperatures: A first-principles study

Authors: Shogo Yamashita, Akimasa Sakuma

Abstract: We investigated the magnetic properties of Sendust (Fe-Al-Si) alloys not only at 0 K but also at finite-temperatures by means of the first-principles calculations assuming A2, B2, and DO3 structures. We confirmed that the itinerant characteristics of 3d electrons of Fe are not negligible for A2 and B2 structures and a significantly small exchange stiffness constant exists at zero-temperature in a… ▽ More We investigated the magnetic properties of Sendust (Fe-Al-Si) alloys not only at 0 K but also at finite-temperatures by means of the first-principles calculations assuming A2, B2, and DO3 structures. We confirmed that the itinerant characteristics of 3d electrons of Fe are not negligible for A2 and B2 structures and a significantly small exchange stiffness constant exists at zero-temperature in a B2 structure. However, the calculated Curie temperatures are in the same order for all structures; this indicates that the Curie temperature cannot be determined only by the exchange interactions at zero-temperature in itinerant electron systems. Temperature dependence of the exchange interaction, namely spin configuration dependence, also might be important for determining it. In addition, this property might also be related to the unique behavior of the temperature dependence of the exchange stiffness constant for the B2 structure, which does not decrease monotonically as temperatures increase, contrary to the behavior expected from the Heisenberg model. In addition, we investigated composition dependence on the exchange stiffness constant at zero-temperature and confirmed that the substitution of Si with Al could improve the amplitude of the exchange stiffness constant at zero-temperature for all structures. △ Less

Submitted 20 March, 2024; originally announced March 2024.

Finite-temperature second-order perturbation analysis of magnetocrystalline anisotropy energy of L10-type ordered alloys

Authors: Shogo Yamashita, Akimasa Sakuma

Abstract: We present a novel finite-temperature second-order perturbation method incorporating spin-orbit coupling to investigate the temperature-dependent site-resolved contributions to the magnetocrystalline anisotropy energy (MAE), specifically K1(T), in FePt, MnAl, and FeNi alloys. Our developed method successfully reproduces the results obtained using the force theorem from our previous work. By employ… ▽ More We present a novel finite-temperature second-order perturbation method incorporating spin-orbit coupling to investigate the temperature-dependent site-resolved contributions to the magnetocrystalline anisotropy energy (MAE), specifically K1(T), in FePt, MnAl, and FeNi alloys. Our developed method successfully reproduces the results obtained using the force theorem from our previous work. By employing this method, we identify the key sites responsible for the distinctive behaviors of MAE in these alloys, shedding light on the inadequacy of the spin model in capturing the temperature dependence of MAE in itinerant magnets. Moreover, we explore the lattice expansion effect on the temperature dependence of on-site contributions to K1(T) in FeNi. Our results not only provide insights into the limitations of the spin model in explaining the temperature dependence of MAE in itinerant ferromagnets but also highlight the need for further investigations. These findings contribute to a deeper understanding of the complex nature of MAE in itinerant magnetic systems. △ Less

Submitted 9 June, 2023; originally announced June 2023.

Comments: 7 pages, 3 figures

Journal ref: Phys. Rev. B 108, 054411 (2023)

Temperature Dependence of Magnetocrystalline Anisotropy in Itinerant Ferromagnets

Authors: Daisuke Miura, Akimasa Sakuma

Abstract: 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… ▽ More 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. △ Less

Submitted 26 January, 2022; originally announced January 2022.

Journal ref: J. Phys. Soc. Jpn. 91, 023706 (2022)

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

Authors: Yuta Yahagi, Daisuke Miura, Akimasa Sakuma

Abstract: 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-c… ▽ More 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. △ Less

Submitted 9 July, 2022; v1 submitted 17 January, 2022; originally announced January 2022.

Comments: 12 pages, 3 figures, 1 table

Journal ref: J. Phys. Soc. Jpn. 91, 083701 (2022)

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

Authors: Daisuke Miura, Akimasa Sakuma

Abstract: 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 tempe… ▽ More 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 $T<T_\mathrm{C}$ and $1/χ_\perp\propto T-T_\mathrm{C}$ for $T>T_\mathrm{C}$, which reflects characteristics of itinerant ferromagnets including a spin-orbit interaction. △ Less

Submitted 12 October, 2021; originally announced October 2021.

Journal ref: J. Phys. Soc. Jpn. 90, 113601 (2021)

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

Authors: Yuta Yahagi, Jakub Zelezny, Daisuke Miura, Akimasa Sakuma

Abstract: 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 mecha… ▽ More 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. △ Less

Submitted 13 April, 2021; v1 submitted 7 March, 2021; originally announced March 2021.

Comments: 34 pages, 15 figures

Journal ref: Phys. Rev. B 104, 094417 (2021)

Detecting quadrupole: a hidden source of magnetic anisotropy for Manganese alloys

Authors: Jun Okabayashi, Yoshio Miura, Yohei Kota, Kazuya Suzuki, Akimasa Sakuma, Shigemi Mizukami

Abstract: Mn-based alloys exhibit unique properties in the spintronics materials possessing perpendicular magnetic anisotropy (PMA) beyond the Fe and Co-based alloys. It is desired to figure out the quantum physics of PMA inherent to Mn-based alloys, which have never been reported. Here, the origin of PMA in ferrimagnetic Mn$_{3-δ}$Ga ordered alloys is investigated to resolve antiparallel-coupled Mn sites u… ▽ More Mn-based alloys exhibit unique properties in the spintronics materials possessing perpendicular magnetic anisotropy (PMA) beyond the Fe and Co-based alloys. It is desired to figure out the quantum physics of PMA inherent to Mn-based alloys, which have never been reported. Here, the origin of PMA in ferrimagnetic Mn$_{3-δ}$Ga ordered alloys is investigated to resolve antiparallel-coupled Mn sites using x-ray magnetic circular and linear dichroism (XMCD/XMLD) and a first-principles calculation. We found that the contribution of orbital magnetic moments in PMA is small from XMCD and that the finite quadrupole-like orbital distortion through spin-flipped electron hopping is dominant from XMLD and theoretical calculations. These findings suggest that the spin-flipped orbital quadrupole formations originate from the PMA in Mn$_{3-δ}$Ga and bring the paradigm shift in the researches of PMA materials using x-ray magnetic spectroscopies. △ Less

Submitted 7 May, 2020; originally announced May 2020.

Comments: 20 pages including 4 figures

Journal ref: Scientific Reports 10, 8744 (2020)

Fermi level tuning and atomic ordering induced giant anomalous Nernst effect in Co2MnAl1-xSix Heusler alloy

Authors: Y. Sakuraba, K. Hyodo, A. Sakuma, S. Mitani

Abstract: Co2MnAl has been predicted to have Weyl points near Fermi level which is expected to give rise to exotic transverse transport properties such as large anomalous Hall(AHE) and Nernst effects(ANE) due to large Berry curvature. In this study, the effect of Fermi level position and atomic ordering on AHE and ANE in Co2MnAl1-xSix were studied systematically. The Co2MnAl film keeps B2-disordred structur… ▽ More Co2MnAl has been predicted to have Weyl points near Fermi level which is expected to give rise to exotic transverse transport properties such as large anomalous Hall(AHE) and Nernst effects(ANE) due to large Berry curvature. In this study, the effect of Fermi level position and atomic ordering on AHE and ANE in Co2MnAl1-xSix were studied systematically. The Co2MnAl film keeps B2-disordred structure regardless of annealing temperature, which results in much smaller anomalous Hall conductivity sigma_xy and transverse Peltier coefficient sigma_xy than those calculated for L21-ordered Co2MnAl. Our newly performed calculation of sigma_xy with taking B2 disordering into account well reproduces experimental result, thus it was concluded that Berry curvature originating from Weyl points is largely reduced by B2 disordering. It was also revealed Al substitution with Si shifts the position of Fermi level and improves the L21-atomic ordering largely, leading to strong enhancement of sigma_xy, which also agreed with our theoretical calculation. The highest thermopower of ANE of 6.1uV, which is comparable to the recent reports for Co2MnGa, was observed for Co2MnAl0.63Si0.37 because of dominant contribution of sigma_xy. This study clearly shows the importance of both Fermi level tuning and high atomic ordering for obtaining the effect of topological feature in Co-based Heusler alloys on transverse transport properties. △ Less

Submitted 14 November, 2019; originally announced November 2019.

Comments: 5 figures. arXiv admin note: text overlap with arXiv:1807.02209

Report number: Phys. Rev. B 101, 134407 (2020)

Voltage-Controlled Magnonic Spin Tunneling Junction

Authors: Kohei Ohgane, Yuta Yahagi, Daisuke Miura, Akimasa Sakuma

Abstract: 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 propo… ▽ More 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. △ Less

Submitted 28 November, 2019; v1 submitted 25 October, 2019; originally announced October 2019.

Journal ref: J. Phys. Soc. Jpn. 89, 013601 (2020)

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

Authors: Yuta Yahagi, Daisuke Miura, Akimasa Sakuma

Abstract: 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 electro… ▽ More 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. △ Less

Submitted 22 July, 2019; v1 submitted 3 May, 2019; originally announced May 2019.

Comments: 15 pages, 7 figures

Journal ref: J. Phys. Soc. Jpn., Vol.89, No.4 (2020)

Analytic Expression for Magnetic Activation Energy

Authors: Daisuke Miura, Akimasa Sakuma

Abstract: 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… ▽ More 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. △ Less

Submitted 15 April, 2019; v1 submitted 13 December, 2018; originally announced December 2018.

Comments: 8 pages, 2 figures

Journal ref: Jpn. J. Appl. Phys. 58, 058002 (2019)

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

Authors: Daisuke Miura, Akimasa Sakuma

Abstract: 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 tempera… ▽ More 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. △ Less

Submitted 1 April, 2019; v1 submitted 9 December, 2018; originally announced December 2018.

Comments: 8 figures

Journal ref: J. Phys. Soc. Jpn. 88, 044804 (2019)

Tunable Spin Seebeck Diode with Magnonic Spin Tunneling Junction

Authors: Diasuke Miura, Akimasa Sakuma

Abstract: We theoretically investigate the spin--wave spin current induced by the spin Seebeck effect in magnonic spin tunneling junctions (MSTJs) for arbitrary magnetization directions. We show that the MSTJ functions as a \textit{tunable} spin Seebeck diode in which the tunneling spin current can be turned on and off with high efficiency by controlling the magnetization direction. We theoretically investigate the spin--wave spin current induced by the spin Seebeck effect in magnonic spin tunneling junctions (MSTJs) for arbitrary magnetization directions. We show that the MSTJ functions as a \textit{tunable} spin Seebeck diode in which the tunneling spin current can be turned on and off with high efficiency by controlling the magnetization direction. △ Less

Submitted 2 November, 2018; originally announced November 2018.

Comments: 3 pages, 1 figure

Journal ref: J. Phys. Soc. Jpn. 87, 125001 (2018)

Strategic enhancement of anomalous Nernst effect in Co2MnAl1-xSix Heusler compounds

Authors: Y. Sakuraba, K. Hyodo, A. Sakuma, S. Mitani

Abstract: The anomalous Nernst effect (ANE), a thermoelectric phenomenon in magnetic materials, has potential for novel energy harvesting applications because its orthogonal relationship between the temperature gradient and the electric field enables us to utilize the large area of non-flat heat sources. In this study, the required thermopower of ANE for practical energy harvesting applications is evaluated… ▽ More The anomalous Nernst effect (ANE), a thermoelectric phenomenon in magnetic materials, has potential for novel energy harvesting applications because its orthogonal relationship between the temperature gradient and the electric field enables us to utilize the large area of non-flat heat sources. In this study, the required thermopower of ANE for practical energy harvesting applications is evaluated in simulations of electric power obtainable from ANE using a low-temperature heat source. A strategy for finding magnetic materials having large ANEs is proposed, which suggests that a large ANE originates from the constructive relationship between large Seebeck, anomalous Hall, and transverse Peltier effects. This strategy leads us to investigate the electric and thermoelectric properties in Co2MnAl1-xSix. As a result, it is found that Co2MnAl0.63Si0.37 has the largest ANE thermopower ever reported, 6.2 uV/K. A first principles calculation of the anomalous Hall conductivity and transverse Peltier coefficient in B2 and L21-ordered Co2MnAl0.63Si0.37 shows close agreement with the experimental results, indicating that the observed large ANE arises from the intrinsic Berry phase curvature of Co2MnAl1-xSix. This study can be a guide for developing materials for new thermoelectric applications using ANE. △ Less

Submitted 14 November, 2019; v1 submitted 5 July, 2018; originally announced July 2018.

Comments: We found a error in this paper, thus we should withdraw it

Anisotropy of exchange stiffness based on atomic-scale magnetic properties in rare-earth permanent magnet Nd$_2$Fe$_{14}$B

Authors: Yuta Toga, Masamichi Nishino, Seiji Miyashita, Takashi Miyake, Akimasa Sakuma

Abstract: We examine the anisotropic properties of the exchange stiffness constant, $\mathcal{A}$, for rare-earth permanent magnet, Nd$_2$Fe$_{14}$B, by connecting analyses with two different scales of length, i.e., Monte Carlo (MC) method with an atomistic spin model and Landau-Lifshitz-Gilbert (LLG) equation with a continuous magnetic model. The atomistic MC simulations are performed on the spin model of… ▽ More We examine the anisotropic properties of the exchange stiffness constant, $\mathcal{A}$, for rare-earth permanent magnet, Nd$_2$Fe$_{14}$B, by connecting analyses with two different scales of length, i.e., Monte Carlo (MC) method with an atomistic spin model and Landau-Lifshitz-Gilbert (LLG) equation with a continuous magnetic model. The atomistic MC simulations are performed on the spin model of Nd$_2$Fe$_{14}$B constructed from ab-initio calculations, and the LLG micromagnetics simulations are performed with the parameters obtained by the MC simulations. We clarify that the amplitude and the thermal property of $\mathcal{A}$ depend on the orientation in the crystal, which are attributed to the layered structure of Nd atoms and weak exchange couplings between Nd and Fe atoms. We also confirm that the anisotropy of $\mathcal{A}$ significantly affects the threshold field for the magnetization reversal (coercivity) given by the depinning process. △ Less

Submitted 24 July, 2018; v1 submitted 16 April, 2018; originally announced April 2018.

Comments: 11 pages, 10 figures

Journal ref: Phys. Rev. B 98, 054418 (2018)

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

Authors: Daisuke Miura, Akimasa Sakuma

Abstract: 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 genera… ▽ More 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. △ Less

Submitted 14 July, 2018; v1 submitted 22 February, 2018; originally announced February 2018.

Comments: 5pages, 6 figures

Journal ref: AIP Advances 8, 075114 (2018)

Temperature- dependence of anomalous Hall conductivity in Rashba-type ferromagnets

Authors: Akimasa Sakuma

Abstract: The applicability and usefulness of Rashba model have been extended by recent observations in the field of spintronics, such as the spin-orbit torque at the junction interfaces between ferromagnetic (FM) metals and non-magnetic (NM) metals and the perpendicular anomalous magnetoresistance (AMR) in heterostructures such as FI/NM or FM/NI (I denotes an insulator). In particular, the observations of… ▽ More The applicability and usefulness of Rashba model have been extended by recent observations in the field of spintronics, such as the spin-orbit torque at the junction interfaces between ferromagnetic (FM) metals and non-magnetic (NM) metals and the perpendicular anomalous magnetoresistance (AMR) in heterostructures such as FI/NM or FM/NI (I denotes an insulator). In particular, the observations of the perpendicular AMR effect stimulate further interest in the Rashba-type spin-orbit interaction (SOI) at interfaces. Thus, the Rashba model with exchange splitting (EXS) is considered not only to play as an effective model for the physical understanding but also to reflect actual bi-layer systems in current spintronics devices. In the present work, we have first investigated the temperature dependence of anomalous Hall conductivity (AHC) of Rashba-type ferromagnets considered effects of spin fluctuations within the disordered local moment (DLM) scheme. The most distinctive feature that we observed is that intrinsic AHC increases with increasing temperature. This can be understood from the aspect of spin chirality, which indicates that the AHC increases with decreasing EXS when the SOI is much smaller than the EXS. The extrinsic part of the Fermi surface term also increases with increasing temperature and has a large contribution, comparable to that of the intrinsic part. Although, such a behaviour has not yet been observed experimentally, we suggest that the physical picture found in this work might lurk in an anomalous Hall effects in Rashbe-type ferromagnets. △ Less

Submitted 3 October, 2017; originally announced October 2017.

First-principles study on the magnetic properties of ordered Nd$_{6}$(Fe,Ga)$_{14}$ alloys

Authors: Kazushige Hyodo, Yuta Toga, Akimasa Sakuma

Abstract: We studied the stable magnetic structure of ordered Nd$_{6}$Fe$_{14-x}$Ga$_x$ ($x = 0, 1)$ alloys, which appears in the grain-boundary (GB) phase of Nd-Fe-B permanent magnets, using first-principles techniques. Slight Ga doping ($x = 1$) was shown to contribute to the stabilization of an anti-ferromagnetic (AF) state, whereas the non-doped case ($x = 0$) was revealed to favor ferromagnetic state r… ▽ More We studied the stable magnetic structure of ordered Nd$_{6}$Fe$_{14-x}$Ga$_x$ ($x = 0, 1)$ alloys, which appears in the grain-boundary (GB) phase of Nd-Fe-B permanent magnets, using first-principles techniques. Slight Ga doping ($x = 1$) was shown to contribute to the stabilization of an anti-ferromagnetic (AF) state, whereas the non-doped case ($x = 0$) was revealed to favor ferromagnetic state rather than AF state with a slight energy difference. △ Less

Submitted 5 December, 2017; v1 submitted 8 December, 2016; originally announced December 2016.

Journal ref: Journal of Applied Physics 122, 243904 (2017)

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

Authors: Yuta Toga, Munehisa Matsumoto, Seiji Miyashita, Hisazumi Akai, Shotaro Doi, Takashi Miyake, Akimasa Sakuma

Abstract: 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. % Subsequentl… ▽ More 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. △ Less

Submitted 5 October, 2016; v1 submitted 1 June, 2016; originally announced June 2016.

Comments: 10 pages, 14 figures

Journal ref: Phys. Rev. B 94, 174433 (2016)

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

Authors: Naofumi Kobayashi, Kazushige Hyodo, Akimasa Sakuma

Abstract: 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 wi… ▽ More 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. △ Less

Submitted 17 May, 2016; originally announced May 2016.

Comments: 11 pages, 5 figures

First-principles evaluation of intrinsic, side-jump, and skew-scattering parts of anomalous Hall conductivities in disordered alloys

Authors: K. Hyodo, Y. Kota, A. Sakuma

Abstract: We develop a first-principles procedure for the individual evaluation of the intrinsic, side-jump, and skew-scattering contributions to the anomalous Hall conductivity $σ_{xy}$. This method is based on the different microscopic conductive processes of each origin of $σ_{xy}$ in the Kubo--Bastin formula. We also present an approach for implementing this scheme in the tight-binding linear muffin-tin… ▽ More We develop a first-principles procedure for the individual evaluation of the intrinsic, side-jump, and skew-scattering contributions to the anomalous Hall conductivity $σ_{xy}$. This method is based on the different microscopic conductive processes of each origin of $σ_{xy}$ in the Kubo--Bastin formula. We also present an approach for implementing this scheme in the tight-binding linear muffin-tin orbital (TB-LMTO) method with the coherent potential approximation (CPA). The validity of this calculation method is demonstrated for disordered FePt and FePd alloys. We find that the estimated value of each origin of $σ_{xy}$ exhibits reasonable dependencies on the electron scattering in these disordered alloys. △ Less

Submitted 21 August, 2016; v1 submitted 5 May, 2016; originally announced May 2016.

Comments: 11 pages, 5 figures

Journal ref: Phys. Rev. B 94, 104404 (2016)

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

Authors: Sasmita Mohakud, Sergio Andraus, Masamichi Nishino, Akimasa Sakuma, Seiji Miyashita

Abstract: 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 con… ▽ More 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. △ Less

Submitted 15 August, 2016; v1 submitted 10 February, 2016; originally announced February 2016.

Comments: RevTeX4-1, 30 pages, 15 figures, corrected for submission to Phys. Rev. B

Journal ref: Phys. Rev. B 94, 054430 (2016)

First-principles study on interface magnetic structure in Nd${}_2$Fe${}_{14}$B/(Fe,Co) exchange spring magnets

Authors: Nobuyuki Umetsu, Yuta Toga, Akimassa Sakuma

Abstract: The magnetic properties of Nd${}_2$Fe${}_{14}$B (NFB)/transition metal (TM = Fe, Co) multilayer systems are studied on the basis of first-principles density functional calculations. We optimize the model structure under a variety of crystallographic alignments of the NFB layer, and analyze the mechanism of interface magnetic coupling. Improvements in remanent magnetization compared to that of sing… ▽ More The magnetic properties of Nd${}_2$Fe${}_{14}$B (NFB)/transition metal (TM = Fe, Co) multilayer systems are studied on the basis of first-principles density functional calculations. We optimize the model structure under a variety of crystallographic alignments of the NFB layer, and analyze the mechanism of interface magnetic coupling. Improvements in remanent magnetization compared to that of single NFB are observed in NFB(001)/Fe, NFB(110)/Fe, and NFB(100)/Co. On the other hand, in NFB(100)/Fe, remanence degradation due to the anti-parallel magnetization alignment between NFB and Fe layers is observed. In this system, which has the shortest optimized interlayer distance among all considered systems, an itinerant electron magnetism is required around the interface to lower the total energy, and accordingly, anti-ferromagnetic coupling is preferred. The significant difference in property between NFB(100)/Fe and NFB(100)/Co is attributed to the difference between their interface structures, optimized interlayer distances, and magnetic stiffness of TM layers. △ Less

Submitted 17 December, 2015; v1 submitted 20 November, 2015; originally announced November 2015.

Comments: accepted in Phys. Rev. B

Journal ref: Phys. Rev. B 93, 014408 (2016)

General Formalism for Magnetic Anisotropy Constants

Authors: Daisuke Miura, Ryo Sasaki, Akimasa Sakuma

Abstract: 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 c… ▽ More 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. △ Less

Submitted 21 May, 2015; originally announced May 2015.

Comments: 11pages, 2 figures

Journal ref: Appl. Phys. Express 8, 113003 (2015)

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

Authors: Nobuyuki Umetsu, Daisuke Miura, Akimasa Sakuma

Abstract: 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 co… ▽ More 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. △ Less

Submitted 24 March, 2015; v1 submitted 11 March, 2015; originally announced March 2015.

Comments: 16 pages, 7 figures

Journal ref: Phys. Rev. B 91, 174440 (2015)

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

Authors: Yuta Toga, Tsuneaki Suzuki, Akimasa Sakuma

Abstract: 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 negativ… ▽ More 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. △ Less

Submitted 9 June, 2015; v1 submitted 27 February, 2015; originally announced February 2015.

Comments: 6 pages, 6 figures

Journal ref: J. Appl. Phys. 117, 223905 (2015)

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

Authors: Ryo Sasaki, Daisuke Miura, Akimasa Sakuma

Abstract: 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 o… ▽ More 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. △ Less

Submitted 8 January, 2015; originally announced January 2015.

Comments: 7 pages, 4 figures

Mechanism of uniaxial magnetocrystalline anisotropy in transition metal alloys

Authors: Yohei Kota, Akimasa Sakuma

Abstract: 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 magneti… ▽ More 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. △ Less

Submitted 3 March, 2014; v1 submitted 23 July, 2013; originally announced July 2013.

Comments: 11 pages, 8 figures, 2 tables

Journal ref: J. Phys. Soc. Jpn. 83, 034715 (2014)

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

Authors: Akimasa Sakuma, Yuki Manabe, Yohei Kota

Abstract: 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,… ▽ More 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. △ Less

Submitted 19 April, 2013; originally announced April 2013.

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

Authors: Kazushige Hyodo, Chiharu Mitsumata, Akimasa Sakuma

Abstract: 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 L… ▽ More 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. △ Less

Submitted 1 November, 2012; originally announced November 2012.

Comments: 4 pages, 4 figures

Journal ref: IEEE Trans.Magn.48,3951(2012)

Degree of order dependence on magnetocrystalline anisotropy in bct FeCo alloys

Authors: Yohei Kota, Akimasa Sakuma

Abstract: 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… ▽ More 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. △ Less

Submitted 16 October, 2012; originally announced October 2012.

Comments: 9 pages, 3 figures, to appear in Applied Physics Express

Journal ref: Applied Physics Express 5 (2012) 113002

Microscopic Theory of Magnon-Drag Thermoelectric Transport in Ferromagnetic Metals

Authors: Daisuke Miura, Akimasa Sakuma

Abstract: 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… ▽ More 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. △ Less

Submitted 29 October, 2012; v1 submitted 4 September, 2012; originally announced September 2012.

Comments: 3 pages, 2 figures, accepted for publication in Journal of the Physical Society of Japan

Journal ref: J. Phys. Soc. Jpn., 81, 113602 (2012)

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

Authors: Satoshi Kokado, Masakiyo Tsunoda, Kikuo Harigaya, Akimasa Sakuma

Abstract: 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 th… ▽ More 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. △ Less

Submitted 21 December, 2011; v1 submitted 21 November, 2011; originally announced November 2011.

Comments: 17 pages, 12 figures, to be published in J. Phys. Soc. Jpn, minor mistakes corrected, final version

Journal ref: J. Phys. Soc. Jpn. 81 (2012) 024705 [open access]

Charge and Spin Transport in Magnetic Tunnel Junctions: Microscopic Theory

Authors: Daisuke Miura, Akimasa Sakuma

Abstract: 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… ▽ More 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. △ Less

Submitted 8 June, 2012; v1 submitted 18 November, 2011; originally announced November 2011.

Comments: 13 pages, 1 figure, accepted for publication in Journal of the Physical Society of Japan

Journal ref: J. Phys. Soc. Jpn., 81, 054709 (2012)

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

Authors: Satoshi Kokado, Kikuo Harigaya, Akimasa Sakuma

Abstract: 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,… ▽ More 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. △ Less

Submitted 10 November, 2010; v1 submitted 19 September, 2010; originally announced September 2010.

Comments: 55 pages, 17 figures, to be published in J. Phys. Soc. Jpn. 79 (2010) No. 11, typos corrected

Journal ref: J. Phys. Soc. Jpn. 79 (2010) 114721-1 - 114721-24

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

Authors: Yohei Kota, Tomohiro Takahashi, Hiroki Tsuchiura, Akimasa Sakuma

Abstract: 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 fou… ▽ More 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. △ Less

Submitted 23 March, 2009; originally announced March 2009.

Comments: 3 pages, 4 figures

Journal ref: J. Appl. Phys. 105, 07B716 (2009)

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

Authors: Yohei Kota, Hiroki Tsuchiura, Akimasa Sakuma

Abstract: 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… ▽ More 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.]. △ Less

Submitted 23 June, 2008; originally announced June 2008.

Comments: 4 pages, 4 figures

Journal ref: IEEE TRANSACTIONS ON MAGNETICS, Volume 44, Issue 11, Part 1, Nov. 2008 Page(s):3131 - 3133

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

Authors: Satoshi Kokado, Kazumasa Ueda, Kikuo Harigaya, Akimasa Sakuma

Abstract: 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 inje… ▽ More 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. △ Less

Submitted 30 August, 2007; v1 submitted 10 August, 2007; originally announced August 2007.

Comments: 12 pages, 12 figures, to be published in Phys. Rev. B, typos corrected

Journal ref: Phys. Rev. B 76 (2007), 054451-1 - 054451-11

Decoherence processes of a quantum two-level system coupled to a fermionic environment

Authors: Naoyoshi Yamada, Akimasa Sakuma, Hiroki Tsuchiura

Abstract: We study decoherence processes of an S = 1/2 localized spin coupled to conduction band electrons in a metal or a semiconductor via an Ising-like interaction. We derive master equations for the density matrix of the localized spin, by tracing out all degrees of freedom in the conduction electron system based on the linked-cluster-expansion technique. It is found that the decoherence occurs more r… ▽ More We study decoherence processes of an S = 1/2 localized spin coupled to conduction band electrons in a metal or a semiconductor via an Ising-like interaction. We derive master equations for the density matrix of the localized spin, by tracing out all degrees of freedom in the conduction electron system based on the linked-cluster-expansion technique. It is found that the decoherence occurs more rapidly for the metallic case than for semiconducting case. △ Less

Submitted 13 February, 2007; originally announced February 2007.

Comments: to appear in J. Appl. Phys

Journal ref: JOURNAL OF APPLIED PHYSICS 101, 09C110 (2007)

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

Authors: Satoshi Kokado, Nobuhisa Fujima, Kikuo Harigaya, Hisashi Shimizu, Akimasa Sakuma

Abstract: 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… ▽ More 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 $|P|$. △ Less

Submitted 28 April, 2006; originally announced May 2006.

Comments: 4 pages, 2 figures, accepted for publication in Phys. Rev. B

Journal ref: Phys. Rev. B 73, 172410 (2006)

Microscopic description of Landau-Lifshitz-Gilbert type equation based on the s-d model

Authors: Akimasa Sakuma

Abstract: A Landau-Lifshitz-Gilbert type equation has been derived by using s-d model in which the s-electron system is regarded as an environment coupled weakly with the localized spins. Based on the irreducible linear response theory, we show that the relaxation function of the s-electron spin leads to the Gilbert type damping term which corresponds to the retarded resistance function in the generalized… ▽ More A Landau-Lifshitz-Gilbert type equation has been derived by using s-d model in which the s-electron system is regarded as an environment coupled weakly with the localized spins. Based on the irreducible linear response theory, we show that the relaxation function of the s-electron spin leads to the Gilbert type damping term which corresponds to the retarded resistance function in the generalized Langevin equation. The Ohmic form of the Gilbert term stems from the fact that the imaginary part of the response function (spin susceptibility) of the itinerant electron system is proportional to the frequency (omega) in the low omega region. It is confirmed that the Caldeira-Leggett theory based on the path-integral approach gives the same result. △ Less

Submitted 6 February, 2006; v1 submitted 3 February, 2006; originally announced February 2006.

Comments: 18 pages