Yuji Umeda

Impact of local strain on Ti-L2,3 electron energy-loss near-edge structures of BaTiO3: a first-principles multiplet study

Identification of local strains is crucial because the local strains largely influence the ferroelectric property of BaTiO₃. The effects of local strains induced by external pressures on the Ti-L₂,₃ electron energy-loss near-edge structure (ELNES) of BaTiO₃ were theoretically investigated using first-principles multiplet calculations. We revealed that the effects appear in the position of the spectral threshold, namely the spectrum shifts to lower and higher energy sides by the tensile and compressive pressures, respectively. We concluded that conventional ELNES observations can identify only large strains induced by -10 GPa, and 0.1 eV energy resolution is required to identify ±2% of strains.

Ab-initio multiplet calculation of oxygen vacancy effect on Ti-L2,3 electron energy loss near edge structures of BaTiO3

The effect of oxygen vacancy on Ti-L2,3 electron energy-loss near-edge structures (ELNES) of BaTiO3 was theoretically investigated through ab initio multiplet calculation. The presence of an oxygen vacancy influences spectral features not only at the nearest neighbor Ti site but also at Ti sites further from the oxygen vacancy. The effects of different oxygen vacancy concentrations were also investigated. Based on this study, it was concluded that the detection limit for oxygen vacancy with Ti-L2,3 ELNES is approximately 1%.

Theoretical Analysis of Oxygen Vacancy Formation in Zr-Doped BaTiO3

One of the most serious problems for the development of multilayer ceramic capacitors (MLCCs) is that their electrical resistance decreases under long-term DC voltage. Oxygen vacancy migration in BaTiO3 is thought to be one cause of this deterioration. In this study, to understand this mechanism, quantitative analysis of the oxygen vacancy formation energy [Ef(VO)] in Zr-doped and undoped BaTiO3 was performed. The Ef(VO) of Zr-doped BaTiO3 was higher than that of undoped BaTiO3 because the valence of Ti in undoped BaTiO3 easily changed from +4 to +3 owing to oxygen vacancy formation, compared with that in Zr-doped BaTiO3. We also prepared undoped (BaTiO3) and Zr-doped (BaZr0.05Ti0.95O3) ceramic samples sintered under reducing atmosphere (T = 1573 K pO2 = 10-13 MPa). BaZr0.05Ti0.95O3 remained an insulator, but BaTiO3 showed semiconducting behavior. This experimental result corresponds well to theoretical results of first-principles calculations.

Effect of Strain on Magnetic Properties of Nd-Fe-B Thin Film Magnets on BaTiO 3

The influence of strain in the Nd-Fe-B was investigated by using the structural phase transition of BaTiO3 in films consisting of a Mo top layer (10 nm), Nd-Fe-B (30 nm), and a Mo bottom layer (20 nm) deposited onto BaTiO3 (001) substrates by sputtering. As a result, it was found that the magnetization along the easy axis jumps up in magnitude by 3% at around 280 K. This significant effect is certainly due to the lattice strain accompanying the structural phase transition of BaTiO3 from orthorhombic to tetragonal. First-principles calculations were applied to simulate the relationship between magnetization, magnetocrystalline anisotropy and the strain in the Nd-Fe-B main phase. It was also found that the magnetization along the easy axis increases as the lattice constant of the a-axis expands, which shows mostly good agreement with the experimental results under some assumptions.

Theoretical and experimental studies of formation and migration of oxygen vacancies in BaMxTi1−xO3(M = Zr, Ge)

The formation and migration energies of oxygen vacancies in pure BaTiO3, and BaM x Ti1− x O3 (M = Zr, Ge) are calculated by first-principles calculations to understand the effect of doping on the reliability of multilayer ceramic capacitors (MLCCs). The formation and migration energies of oxygen vacancies are found to be larger in BaZr x Ti1− x O3 than in BaTiO3. This finding could be one of the possible reasons behind the improved reliability of Zr-doped MLCCs materials. On the other hand, by substituting Ge, the migration energy of BaGe x Ti1− x O3 becomes larger than that of BaTiO3. This is despite the smaller oxygen vacancy formation energy in BaGe x Ti1− x O3 than in BaTiO3. Even though Zr and Ge are tetravalent in BaM x Ti1− x O3, their valence states are different after the formation of oxygen vacancies, providing an explanation for the differences in vacancy formation and migration energies between BaZr x Ti1− x O3 and BaGe x Ti1− x O3. Our theoretical results are further confirmed by experiments on these model systems.