Masao Obata

An ab initio approach to free-energy reconstruction using logarithmic mean force dynamics

We present an ab initio approach for evaluating a free energy profile along a reaction coordinate by combining logarithmic mean force dynamics (LogMFD) and first-principles molecular dynamics. The mean force, which is the derivative of the free energy with respect to the reaction coordinate, is estimated using density functional theory (DFT) in the present approach, which is expected to provide an accurate free energy profile along the reaction coordinate. We apply this new method, first-principles LogMFD (FP-LogMFD), to a glycine dipeptide molecule and reconstruct one- and two-dimensional free energy profiles in the framework of DFT. The resultant free energy profile is compared with that obtained by the thermodynamic integration method and by the previous LogMFD calculation using an empirical force-field, showing that FP-LogMFD is a promising method to calculate free energy without empirical force-fields.

Possible origin of nonlinear magnetic anisotropy variation in electric field effect in a double interface system

We investigated the effect of an electric field on the interface magnetic anisotropy of a thin MgO/Fe/MgO layer using density functional theory. The perpendicular magnetic anisotropy energy (MAE) increases not only under electron depletion but also under some electron accumulation conditions, showing a strong correlation with the number of electrons on the interface Fe atom. The reverse variation in the MAE under the electric field is ascribed to novel features on the charged interface, such as electron leakage. We discuss the origin of the variation in terms of the electronic structures.

Improving the Description of Nonmagnetic and Magnetic Molecular Crystals via the van der Waals Density Functional

We have derived and implemented a stress tensor formulation for the van der Waals density functional (vdW-DF) with spin-polarization-dependent gradient correction (GC) recently proposed by the authors [J. Phys. Soc. Jpn. 82, 093701 (2013)] and applied it to nonmagnetic and magnetic molecular crystals under ambient condition. We found that the cell parameters of the molecular crystals obtained with vdW-DF show an overall improvement compared with those obtained using local density and generalized gradient approximations. In particular, the original vdW-DF with GC gives the equilibrium structural parameters of solid oxygen in the α-phase, which are in good agreement with the experiment.

Investigation of magnetic dipole-dipole interaction using magnetic density on solid oxygen based on first-principles approach

We have developed the computational method to estimate magnetic dipole-dipole interaction energy including magnetic spin density on three-dimensional materials, in the framework of density functional theory. We investigated the stable spin direction in oxygen molecule and α- and δ-phases of solid oxygen. Our method gave the result that the magnetic dipole-dipole interaction of realistic spin density distribution provides the stable magnetic moments perpendicular to the molecular axis, in contrary to the simple expectation from the isolated-spin moment model of the oxygen molecule. The reason has been explained by the spin density distribution of oxygen molecule and magnetic dipole field. Moreover, the result of magnetic anisotropy energy was found to be consistent with the experimental ones.We have developed the computational method to estimate magnetic dipole-dipole interaction energy including magnetic spin density on three-dimensional materials, in the framework of density functional theory. We investigated the stable spin direction in oxygen molecule and α- and δ-phases of solid oxygen. Our method gave the result that the magnetic dipole-dipole interaction of realistic spin density distribution provides the stable magnetic moments perpendicular to the molecular axis, in contrary to the simple expectation from the isolated-spin moment model of the oxygen molecule. The reason has been explained by the spin density distribution of oxygen molecule and magnetic dipole field. Moreover, the result of magnetic anisotropy energy was found to be consistent with the experimental ones.

Tunneling electroresistance of MgZnO-based tunnel junctions

We investigated the tunneling electroresistance (TER) in metal/wurtzite-MgZnO/metal junctions for applications in nonvolatile random-access memories. A resistive switching was detected utilizing an electric-field cooling at ±1 V and exhibited a TER ratio of 360%–490% at 2 K. The extracted change in the average barrier height between the two resistance states gave an estimation of the MgZnO electric polarization at 2.5 μC/cm2 for the low-temperature limit. In addition, the temperature-dependent TER ratio and the shift of the localized states energies at the barrier interface supported the ferroelectric behavior of the MgZnO tunnel-barrier. From the first-principles calculations, we found a similar effect of the barrier height change coming from the reversal of ZnO electric polarization. The possibility of using metal electrodes and lower growth temperatures, in addition to the ferroelectric property, make the ZnO-based memory devices suitable for CMOS integration.

Molecular Interactions for Modeling of Oxygen System Using van der Waals Density Functional Approach

We investigated oxygen molecular systems of T-type, X-type, and S-type using the method with van der Waals density functional and spin-polarization dependent gradient correction. In the S-type configuration, the magnetic interaction showed a characteristic behavior with respect to the structural parameter of the angle, θ, that is formed by the molecular axis and intermolecular axis. It was found that there was a range of θ in which the magnetic interaction vanished at short intermolecular distances.

First-Principles Study on Structural and Electronic Properties in Fe/MgO Double Interface

We carried out the first-principles density functional calculation of the slab systems, MgO(5ML)/Fe(5ML), Fe(5ML)/MgO(5ML)/Fe(5ML), and MgO(3ML)/Fe(3ML)/MgO(1 or 7ML). We investigated structural and electronic properties in the Fe/MgO interface. The slab of a tunnel junction Fe/MgO/Fe exceptionally has a short Fe-O distance and a large Mg displacement from the O layer. In the junction with a small in-plane lattice constant, the interface resonance state tends to shift to a low energy, resulting in a partially electronoccupied state.