arXiv subjects

Yoshio Miura

Publications and source records attributed to Yoshio Miura.

49 records · Page 3Linked to original sources

arXiv2019

Observation of anomalous Ettingshausen effect and large transverse thermoelectric conductivity in permanent magnets

This study focuses on the potential of permanent magnets as thermoelectric converters. It is found that a SmCo$_5$-type magnet exhibits the large anomalous Ettingshausen effect (AEE) at room temperature and that its charge-to-heat current conversion coefficient is more than one order of magnitude greater than that of typical ferromagnetic metals. The large AEE is an exclusive feature of the SmCo$_5$-type magnet among various permanent magnets in practical use, which is independent of the conventional performance of magnets based on static magnetic properties. The experimental results show that the large AEE originates from the intrinsic transverse thermoelectric conductivity of SmCo$_5$. This finding makes a connection between permanent magnets and thermal energy engineering, providing the basis for creating "thermoelectric permanent magnets."

cond-mat.mtrl-sci↗

arXiv2019

Linear-response-based DFT+U method for exploring half-metallic Co-based full Heusler alloys

The density functional theory (DFT)+U method based on the linear response (LR) theory was applied to investigate the electronic structures of Co-based ternary full Heusler alloy Co$_2Y$Si for exploring half-metallic (HM) ferromagnets with a wide HM gap. The LR-based DFT+U calculations tend to obtain a reasonable correlation parameter for $Y$ site, while the correlation of Co site misleads to the unphysical ground state due to the overestimated parameter value that arises from the delocalized electronic structure of Co. Furthermore, we found that the HM gap of Co$_2$MnSi originates from Co $e_u$ orbital in the conduction state and Co-Mn hybridizing $t_{2g}$ orbital in the valence state around the Fermi energy. This means that the HM gap is a tunable property by selecting the $Y$ element and/or mixing several elements into the $Y$ site through $t_{2g}$ atomic-orbital coupling. Our LR-based DFT+U method was extended to other ternary Co$_2Y$Si and quaternary Co$_2$($Y$,Mn)Si. We found that Co$_2$(Ti$_{0.25}$,Mn$_{0.75}$)Si and Co$_2$(Fe$_{0.25}$,Mn$_{0.75}$)Si show HM nature, with the Fermi energy being at almost the center of the minority band gap, which leads to high thermal stability.

cond-mat.mtrl-sci↗

arXiv2019

Strain-Induced Reversible Manipulation of Orbital Magnetic Moments in Ni/Cu Multilayers on Ferroelectric BaTiO3

Controlling magnetic anisotropy by orbital magnetic moments related to interfacial strains has considerable potential for the development of future devices using spins and orbitals. For the fundamental physics, the relationship between strain and orbital magnetic moment is still unknown, because there are few tools to probe changes of orbital magnetic moment. In this study, we developed an electric-field- (E)-induced X-ray magnetic circular dichroism (EXMCD) technique to apply E to a ferroelectric BaTiO3 substrate. We reversibly tuned the interfacial lattice constants of Ni/Cu multilayers on BaTiO3 using this technique. As the domain structures in BaTiO3 are modulated by E, EXMCD measurements reveal that the changes in the magnetic anisotropy of Ni/Cu films are induced through the modulation of orbital magnetic moments in Ni with magneto-elastic contributions. The strained Ni layer that induces the perpendicular magnetic anisotropy without E is released at E = 8 kV/cm, and in-plane magnetization also occurs. We observed that EXMCD measurements clarified the origin of the reversible changes in perpendicular magnetic anisotropy and established the relationship between macroscopic inverse magnetostriction effects and microscopic orbital moment anisotropy.

cond-mat.mtrl-sci↗

arXiv2019

First-principles study of the anisotropic magneto-Peltier effect

We study theoretically the anisotropic magneto-Peltier effect, which was recently demonstrated experimentally. A first-principles-based Boltzmann transport approach including the spin-orbit interaction shows that Ni has a larger anisotropy of the Peltier coefficient ($ΔΠ$) than Fe, consistent with experiments. It is clarified that spin-flip electron transitions due to the spin-orbit interaction are the key in the mechanism of the large anisotropic magneto-Peltier effect. Using our method, we further predict several ferromagnetic metals with much larger $ΔΠ$ than that of Ni.

cond-mat.mtrl-sci↗

arXiv2018

Perpendicular magnetic anisotropy at the Fe/MgAl$_2$O$_4$ interface: Comparative first-principles study with Fe/MgO

We present a theoretical study on interfacial magnetocrystalline anisotropy for Fe/MgAl$_2$O$_4$. This system has a very small lattice mismatch at the interface and therefore is suitable for realizing a fully coherent ferromagnet/oxide interface for magnetic tunnel junctions. On the basis of density functional theory, we calculate the interfacial anisotropy constant $K_{\rm i}$ and show that this system has interfacial perpendicular magnetic anisotropy (PMA) with $K_{\rm i} \approx 1.2\,{\rm mJ/m^2}$, which is a little bit smaller than that of Fe/MgO ($K_{\rm i} \approx$ 1.5--1.7$\,{\rm mJ/m^2}$). Second-order perturbation analysis with respect to the spin-orbit interaction clarifies that the difference in $K_{\rm i}$ between Fe/MgAl$_2$O$_4$ and Fe/MgO originates from the difference in contributions from spin-flip scattering terms at the interface. We propose that the insertion of tungsten layers into the interface of Fe/MgAl$_2$O$_4$ is a promising way to obtain huge interfacial PMA with $K_{\rm i} \gtrsim 3\,{\rm mJ/m^2}$.

cond-mat.mtrl-sci↗

arXiv2018

Anatomy of interfacial spin-orbit coupling in Co/Pd multilayers using X-ray magnetic circular dichroism and first-principles calculations

Element-specific orbital magnetic moments and their anisotropies in perpendicularly magnetised Co/Pd multilayers are investigated using Co L-edge and Pd M-edge angle-dependent x-ray magnetic circular dichroism. We show that the orbital magnetic moments in Co are anisotropic, whereas those in Pd are isotropic. The first-principles density-functional-theory calculations also suggest that the Co/Pd interfacial orbital magnetic moments in Co are anisotropic and contribute to the perpendicular magnetic anisotropy (PMA), and that the isotropic ones in Pd manipulates the Co orbitals at the interface through proximity effects. Orbital-resolved anatomy of Co/Pd interfaces reveals that the orbital moment anisotropy in Co and spin-flipped transition related to the magnetic dipoles in Pd are essential for the appearance of PMA.

cond-mat.mtrl-sci↗

arXiv2018

Theoretical study on magnetic tunneling junctions with semiconductor barriers CuInSe$_2$ and CuGaSe$_2$ including a detailed analysis of band-resolved transmittances

We study spin-dependent transport properties in magnetic tunneling junctions (MTJs) with semiconductor barriers, Fe/CuInSe$_2$/Fe(001) and Fe/CuGaSe$_2$/Fe(001). By analyzing their transmittances at zero bias voltage on the basis of the first-principles calculations, we find that spin-dependent coherent tunneling transport of $Δ_1$ wave functions yields a relatively high magnetoresistance (MR) ratio in both the MTJs. We carry out a detailed analysis of the band-resolved transmittances in both the MTJs and find an absence of the selective transmission of $Δ_1$ wave functions in some energy regions a few eV away from the Fermi level due to small band gaps in CuInSe$_2$ and CuGaSe$_2$.

cond-mat.mtrl-sci↗

arXiv2017

Giant interfacial perpendicular magnetic anisotropy in Fe/CuIn$_{1-x}$Ga$_x$Se$_2$ beyond Fe/MgO

We study interfacial magnetocrystalline anisotropies in various Fe/semiconductor heterostructures by means of first-principles calculations. We find that many of those systems show perpendicular magnetic anisotropy (PMA) with a positive value of the interfacial anisotropy constant $K_{\rm i}$. In particular, the Fe/CuInSe$_2$ interface has a large $K_{\rm i}$ of $\sim 2.3\,{\rm mJ/m^2}$, which is about 1.6 times larger than that of Fe/MgO known as a typical system with relatively large PMA. We also find that the values of $K_{\rm i}$ in almost all the systems studied in this work follow the well-known Bruno's relation, which indicates that minority-spin states around the Fermi level provide dominant contributions to the interfacial magnetocrystalline anisotropies. Detailed analyses of the local density of states and wave-vector-resolved anisotropy energy clarify that the large $K_{\rm i}$ in Fe/CuInSe$_2$ is attributed to the preferable $3d$-orbital configurations around the Fermi level in the minority-spin states of the interfacial Fe atoms. Moreover, we have shown that the locations of interfacial Se atoms are the key for such orbital configurations of the interfacial Fe atoms.

cond-mat.mtrl-sci↗

arXiv2017

Bias voltage effects on tunneling magnetoresistance in Fe/MgAl${}_2$O${}_4$/Fe(001) junctions: Comparative study with Fe/MgO/Fe(001) junctions

We investigate bias voltage effects on the spin-dependent transport properties of Fe/MgAl${}_2$O${}_4$/Fe(001) magnetic tunneling junctions (MTJs) by comparing them with those of Fe/MgO/Fe(001) MTJs. By means of the nonequilibrium Green's function method and the density functional theory, we calculate bias voltage dependences of magnetoresistance (MR) ratios in both the MTJs. We find that in both the MTJs, the MR ratio decreases as the bias voltage increases and finally vanishes at a critical bias voltage $V_{\rm c}$. We also find that the critical bias voltage $V_{\rm c}$ of the MgAl${}_2$O${}_4$-based MTJ is clearly larger than that of the MgO-based MTJ. Since the in-plane lattice constant of the Fe/MgAl${}_2$O${}_4$/Fe(001) supercell is twice that of the Fe/MgO/Fe(001) one, the Fe electrodes in the MgAl${}_2$O${}_4$-based MTJs have an identical band structure to that obtained by folding the Fe band structure of the MgO-based MTJs in the Brillouin zone of the in-plane wave vector. We show that such a difference in the Fe band structure is the origin of the difference in the critical bias voltage $V_{\rm c}$ between the MgAl${}_2$O${}_4$- and MgO-based MTJs.

cond-mat.mtrl-sci↗

arXiv2017

First-principles study on magnetic tunneling junctions with semiconducting CuInSe${}_{2}$ and CuGaSe${}_{2}$ barriers

We theoretically investigate two different magnetic tunneling junctions (MTJs) with semiconductor barriers, CuInSe${}_{2}$ (CIS) and CuGaSe${}_{2}$ (CGS), which are the terminal compounds of recently reported mixed semiconductor barrier, CuIn${}_{1-x}$Ga$_{\it x}$Se${}_{2}$. To discuss the transport properties of these systems, we analyze complex band structures, magnetoresistance (MR) ratios, and resistance-area products ($RA$) by using first-principles based calculations in combination with the Landauer formula. It is found that the $Δ_{1}$ wave functions have dominant contributions to the spin-dependent tunneling transport in both CIS- and CGS-based MTJs. We also find that the CGS-based MTJ has a much larger MR ratio and slightly higher $RA$ than those of the CIS-based MTJ, which indicates that a larger MR ratio is expected for a higher Ga concentration $x$ in the CuIn${}_{1-x}$Ga${}_{x}$Se${}_{2}$-based MTJs. We further study the relationship between the band gaps in the barriers and MR ratios by changing the Coulomb repulsions in the Cu 3$d$ states of the CIS and CGS. It is shown that the barrier with a larger band gap yields a larger MR ratio. The comparison of MR ratios and $RA$ between the CIS-, CGS-, and MgO-based MTJs are also given.

cond-mat.mtrl-sci↗

arXiv2012

Origin of perpendicular magneto-crystalline anisotropy in L10-FeNi under tetragonal distortion

We investigated the origin of perpendicular magneto-crystalline anisotropy (MCA) in L10 ordered FeNi alloy using first-principles density-functional calculations. We found that the perpendicular MCA of L10-FeNi arises predominantly from the constituent Fe atoms, which is consistent with recent measurements of the anisotropy of the Fe orbital magnetic moment of L10-FeNi by x-ray magnetic circular dichroism. Analysis of the second-order perturbation of the spin-orbit interaction indicates that spin-flip excitations between the occupied majority-spin and unoccupied minority-spin bands make a considerable contribution to the perpendicular MCA as does the spin-conservation term in the minority-spin bands. Furthermore, the MCA energy increases as the in-plane lattice parameter decreases (increasing the axial ratio c/a). The increase in the MCA energy can be attributed to further enhancement of the spin-flip term due to modulation of the Fe d(xy) and d(x2-y2) orbital components around the Fermi level under the compressive in-plane distortion.

cond-mat.mtrl-sci↗

arXiv2012

A first-principles study of tunneling magnetoresistance in Fe/MgAl2O4/Fe(001) magnetic tunnel junctions

We investigated the spin-dependent transport properties of Fe/MgAl2O4/Fe(001) magnetic tunneling junctions (MTJs) on the basis of first-principles calculations of the electronic structures and the ballistic conductance. The calculated tunneling magnetoresistance (TMR) ratio of a Fe/MgAl2O4/Fe(001) MTJ was about 160%, which was much smaller than that of a Fe/MgO/Fe(001) MTJ (1600%) for the same barrier thickness. However, there was an evanescent state with delta 1 symmetry in the energy gap around the Fermi level of normal spinel MgAl2O4, indicating the possibility of a large TMR in Fe/MgAl2O4/Fe(001) MTJs. The small TMR ratio of the Fe/MgAl2O4/Fe(001) MTJ was due to new conductive channels in the minority spin states resulting from a band-folding effect in the two-dimensional (2-D) Brillouin zone of the in-plane wave vector (k//) of the Fe electrode. Since the in-plane cell size of MgAl2O4 is twice that of the primitive in-plane cell size of bcc Fe, the bands in the boundary edges are folded, and minority-spin states coupled with the delta 1 evanescent state in the MgAl2O4 barrier appear at k//=0, which reduces the TMR ratio of the MTJs significantly.

cond-mat.mes-hall↗

arXiv2004

H_2 Dissociative Adsorption at the Armchair Edges of Graphite

We investigate and discuss how hydrogen behaves at the edges of a graphite sheet, in particular the armchair edge. Our density functional theory-based calculations results show that, in contrast to the zigzag edge [cf., e-J. Surf. Sci. Nanotech. 2 (2004) 77], regardless of orientation, there is an activation barrier hindering H_2 dissociation at the armchair edges. And once they do get dissociatively adsorbed at the armchair edges, we find that it would be extremely hard to desorb the H from their adsorption sites at the armchair edges. Furthermore, we also found that, consistent with our earlier conclusions [cf., J. Phys. Soc. Jpn. 72 (2003) 1867], it is unlikely that we would find a whole H_2 in between plain graphite sheets.

cond-mat.mtrl-sci↗