Noriya Ichikawa

Anisotropic oxygen diffusion at low temperature in perovskite-structure iron oxides

Oxygen-ion conduction in transition-metal oxides is exploited in, for example, electrolytes in solid-oxide fuel cells and oxygen-separation membranes, which currently work at high temperatures. Conduction at low temperature is a key to developing further utilization, and an understanding of the structures that enable conduction is also important to gain insight into oxygen-diffusion pathways. Here we report the structural changes observed when single-crystalline, epitaxial CaFeO₂.₅ thin films were changed into CaFeO₂ by low-temperature reductions with CaH₂. During the reduction process from the brownmillerite CaFeO₂.₅ into the infinite-layer structure of CaFeO₂, some of the oxygen atoms are released from and others are rearranged within the perovskite-structure framework. We evaluated these changes and the reaction time they required, and found two oxygen diffusion pathways and the related kinetics at low temperature. The results demonstrate that oxygen diffusion in the brownmillerite is highly anisotropic, significantly higher along the lateral direction of the tetrahedral and octahedral layers.

Multiferroic Compounds with Double-Perovskite Structures

New multiferroic compounds with double-perovskite structures were synthesized. Bi2NiMnO6 was synthesized in bulk form by high-pressure synthesis and also in a thin-film form by epitaxial growth. The material showed both ferromagnetic and ferroelectric properties, i.e., the multiferroic property at low temperature. Bi2FeCrO6 was also fabricated in a (1 1 1) oriented BiFeO3/BiCrO3 artificial superlattice, with a 1/1 stacking period. The superlattice film showed ferromagnetic behavior and polarization switching at room temperature. In the compounds, Bi3+ ion, located at the A site in the perovskite structure, caused ferroelectric structural distortion, and the B-site ordering of the Ni2+ and Mn4+ ions (Fe3+ and Cr3+ ions) in a rock-salt configuration led to ferromagnetism according to the Kanamori-Goodenough rule.

Thermally formed conducting filaments in a single-crystalline NiO thin film

Effects of thermal annealing on resistance switching properties of a single-crystalline NiO thin film grown on a Pt0.8Ir0.2 bottom electrode were investigated. Annealing the NiO thin film above 430 °C produced conducting filaments in the single-crystalline NiO and the resulting low-resistance state changed to a high-resistance state at an electrical reset bias voltage. Rearrangements of crystalline defects such as Ni vacancies play a role in forming the conducting filaments. This “thermal forming” is essentially the same as “electrical forming” but does not need large bias voltages to make the initially ON state in unipolar switching devices.

Multiferroism at Room Temperature in BiFeO3/BiCrO3(111) Artificial Superlattices

We report the fabrication of BiFeO3/BiCrO3(111) artificial superlattices with 1/1 stacking in a layer-by-layer growth mode on atomically flat SrTiO3(111) surfaces. While BiFeO3 and BiCrO3 are antiferromagnets having Fe–O–Fe and Cr–O–Cr bonds, these superlattices contain Fe–O–Cr bonds, in which ferromagnetic interaction is expected. Magnetization measurements at 300 K revealed that the magnetic moment per transition metal ion was 1.7 µB, suggesting ferromagnetic spin order. Ferroelectric behavior at room temperature was confirmed by an analysis using a scanning non-linear dielectric microscope.

Reversible changes of epitaxial thin films from perovskite LaNiO3 to infinite-layer structure LaNiO2

An infinite-layer-structure epitaxial thin film of LaNiO2 was prepared by low-temperature reduction with CaH2 from a LaNiO3 epitaxial thin film grown on a SrTiO3(100) substrate. The oxygen content changed reversibly from the perovskite LaNiO3 to the infinite-layer LaNiO2 without losing the structural framework and the topotactic relationship. Consequently, the oxidation state of Ni ions in the film changed from trivalent to divalent to monovalent.

Comparison of stripe modulations in La1.875Ba0.125CuO4 and La1.48Nd0.4Sr0.12CuO4

We report combined soft and hard x-ray scattering studies of the electronic and lattice modulations associated with stripe order in La

Time-Resolved Luminescence of TiO2 Powders with Different Crystal Structures

_{1.875}

Two-Dimensional Charge Disproportionation of the Unusual High Valence State Fe4+ in a Layered Double Perovskite

Ba

Selective reduction of layers at low temperature in artificial superlattice thin films

_{0.125}

Strain-induced significant increase in metal-insulator transition temperature in oxygen-deficient Fe oxide epitaxial thin films.

CuO