Daiki Yoshikawa

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.

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.

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.