Yoshio Miura

First principles studies for the dissociative adsorption of H2 on graphene

We investigate and discuss the interaction of H2 with graphene based on density functional (DFT) theory. We calculate the potential energy surfaces for the dissociative adsorption of H2 on highly symmetric sites on graphene. Our calculation results show that reconstructions of the carbon atoms play an important role in the H2 -graphene interactions. Activation barrier for H2 dissociation on an unrelaxed graphene is considerably high, ∼4.3 eV for a T–H–T geometry and ∼4.7 eV for a T–B–T geometry. The T–H–T(T–B–T) geometry means that the center of mass position of H2 is at the hollow(bridge) site, and the two H atoms are directed towards the top sites on the graphene. On the other hand, when the carbon atoms are allowed to relax, the activation barrier decreases, and becoming 3.3 eV for the T–H–T geometry and 3.9 eV for the T–B–T geometry. In this case, the two carbon atoms near the hydrogen atoms move 0.33 A towards the gas phase for the T–H–T geometry and 0.26 A for the T–B–T geometry.

Low damping constant for Co2FeAl Heusler alloy films and its correlation with density of states

Gilbert damping for the epitaxial Co2FeAl Heusler alloy films was investigated. Gilbert damping constant for the films was evaluated by analyzing the data of ferromagnetic resonance measured at the frequency of 2–20 GHz. Gilbert damping constant for the film without annealing was rather large, while it decreased remarkably with postannealing. Gilbert damping constant for the film annealed at 600 °C was ≃0.001. These behavior of Gilbert damping constant can be well explained by the fact that the density of states calculated from first principles decreases with increasing the degree of B2 order.

Extensive study of giant magnetoresistance properties in half-metallic Co2(Fe,Mn)Si-based devices

Fully epitaxial Co2FexMn1−xSi(CFMS)/Ag/Co2FexMn1−xSi current-perpendicular-to-plane giant magnetoresistive devices with various Fe/Mn ratios x and top CFMS layer thicknesses tCFMS were prepared. The highest magnetoresistance (MR) ratios, 58% at room temperature and 184% at 30 K, were observed in the sample with x = 0.4 and tCFMS = 3 nm. Enhancement of interface spin-asymmetry was suggested for x = 0.4 compared with that at x = 0. A MR ratio of 58% was also observed even in a very thin trilayer structure, CFMS(4 nm)/Ag(3 nm)/CFMS(2 nm), which is promising for a next-generation magnetic read sensor for high-density hard disk drives.

Experimental and computational studies on structural transitions in the LiBH4–LiI pseudobinary system

Structural transition properties of the LiBH4+xLiI (x=0–1.00) pseudobinary system were examined by powder x-ray diffraction and differential scanning calorimetry combined with periodic density functional theory calculations. We experimentally and computationally confirmed the stabilization of the high-temperature [hexagonal, lithium super(fast-)ionic conduction] phase of LiBH4 with x=0.33 and 1.00, and the results also imply the existence of intermediate phases with x=0.07–0.20. The studies are of importance for further development of LiBH4 and the derived hydrides as solid-state electrolytes.

Effective Pathway for Hydrogen Atom Adsorption on Graphene

We investigate and discuss the interaction of a hydrogen atom (H) with graphene based on the density functional theory (DFT). Our calculation results show that reconstructions of carbon atoms play an important role in the H adsorption on graphene. When constituent carbon atoms are held rigid, endothermic H adsorption is about 0.2 eV, and the activation barrier is 0.3 eV for H adsorption, due to the strong π-bonding network of the hexagonal carbon. On the other hand, when carbon atoms are allowed to relax, the carbon atom directly below the H atom moves 0.33 A upward towards the gas phase, and an s p 3 -like geometry is formed between the H and carbon atoms of graphene. This relaxation stabilizes the hydrogen–carbon interaction, and the exothermic hydrogen adsorption on the graphene has a binding energy of 0.67 eV. We also show that the effective pathway for H adsorption on graphene, which gives an activation barrier for the H adsorption on graphene of 0.18 eV.

First-principles study on half-metallicity of disordered Co2(Cr1−xFex)Al

We have investigated theoretically the effects of atomic disorder on the electronic and magnetic structures of the full-Heusler alloy Co2(Cr1−xFex)Al using the first-principles density functional calculation with the Korringa–Kohn–Rostoker coherent-potential approximation. It was found that Co2(Cr0.6Fe0.4)Al preserves the high spin polarization even in the disordered B2 structure, where (Cr,Fe) and Al randomly occupy octahedral sites of the alloy. On the other hand, the disorder between Co and (Cr,Fe) considerably reduces the spin polarization. Furthermore, the total magnetic moment of Co2(Cr0.6Fe0.4)Al decreases with increasing disorder between Co and (Cr,Fe) due to the antiferromagnetic coupling of the antisite Cr with the ordinary site Cr. These results indicate that control of the disorder between Co and (Cr,Fe) is crucial in order to obtain highly spin polarized full-Heusler alloys Co2(Cr1−xFex)Al.

Ab initio study on stability of half-metallic Co-based full-Heusler alloys

We perform the first-principles density functional calculations of Co-based full-Heusler alloys, exploring 25 possible combinations of Co2YZ (Y=Ti, V, Cr, Mn, and Fe; Z=Al, Ga, Si, Ge, and Sn) in order to clarify the stability for the Co-related atomic disorder, because this type of disorder is considered to degrade the spin polarization of Co2YZ. We found that the disorder between Co and Y atoms correlates with the total valence electron charges around Y atom, because a difference in valence electron charges between Co and Y atoms leads to a different shape of the local potential at each site. This means that Ti-based alloys are better than Cr-, Mn-, and Fe-based alloys in preventing the atomic disorder between Co and Y atoms. From our results, we propose stable Co-based full-Heusler alloys with excellent prospects for half-metallic ferromagnets.

Coherent tunnelling conductance in magnetic tunnel junctions of half-metallic full Heusler alloys with MgO barriers.

We have carried out electronic structure and transport calculations for magnetic tunnel junctions (MTJ) composed of MgO and a half-metallic full Heusler alloy Co(2)MnSi on the basis of the density functional theory and the Landauer formula. We find that the density of states of Co atoms at the Co(2)MnSi/MgO(001) interface shifts toward the higher energy side due to the reduced symmetry, leading to a reduction of the spin polarization at the interface. Furthermore, we show that the majority-spin transmittance as a function of the in-plane wavevector [Formula: see text] has a broad peak centred at [Formula: see text] due to the tunnelling from the Δ(1) channel of Co(2)MnSi, while the transmittance from the Δ(5) channel is three orders of magnitude smaller than that of the Δ(1) channel. These results indicate that coherent tunnelling through the Δ(1) band is dominant also in an MTJ with Co(2)MnSi and an MgO barrier, like in Fe/MgO/Fe(001) MTJ and related systems.

The origin of perpendicular magneto-crystalline anisotropy in L10?FeNi under tetragonal distortion

We investigated the origin of perpendicular magneto-crystalline anisotropy (MCA) in L1(0)-ordered FeNi alloy using first-principles density-functional calculations. We found that the perpendicular MCA of L1(0)-FeNi arises predominantly from the constituent Fe atoms, which is consistent with recent measurements of the anisotropy of the Fe orbital magnetic moment of L1(0)-FeNi by means of 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 for 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(x(2) - y(2)) orbital components around the Fermi level under compressive in-plane distortion.

Magnetoresistance Effect in Tunnel Junctions with Perpendicularly Magnetized D022-Mn3-δGa Electrode and MgO Barrier

The tunnel magnetoresistance (TMR) effect with a perpendicularly magnetized D022-Mn3-δGa (δ= 0.6) electrode was investigated in epitaxially grown D022-Mn3-δGa (30)/Mg (dMg)/MgO (2)/CoFe (2.5) (nm) magnetic tunnel junctions (MTJs). The maximum TMR ratio of 9.8% (22.1%) was achieved at 300 K (10 K) with dMg = 0.4 nm. The bias voltage dependence of differential conductance spectra suggests the existence of a coherent tunneling process in the MTJs. First principles calculations of band dispersion relations and tunneling transmittance in a Mn3Ga/MgO/Mn3Ga structure were also performed. The results revealed the existence of Δ1-bands in Mn3Ga and demonstrated the possibility of a coherent tunneling process existing in the MTJ.