Hiroyuki Kimura

A significant negative thermal expansion (NTE) in an ilmenite-type MgVO₃ ceramic is reported in this paper. MgVO₃ undergoes V–V dimerization at 500–550 K accompanied by a structural phase transition from the triclinic (low-temperature) to rhombohedral (high-temperature) phase. This transition induces a large anisotropic crystal structure deformation and results in a large NTE in the ceramic sample, which is caused by the microstructural effect. The NTE was observed over a temperature range of 450–630 K, and the total volume shrinkage reached approximately −0.7%. This study demonstrates that controlling the chemical bonding is a promising method for the creation of NTE materials.

Reversible, thermally controlled, spontaneous magnetization switching (reversible TCSMS) is demonstrated in the absence and presence of an external magnetic field in polycrystalline perovskite-type CaMn_0.95Sb_0.05O₃. The spontaneous magnetization value is retained throughout a cyclic process even in bias magnetic fields. Neutron diffraction and theoretical studies indicate that two weak ferromagnetic phases, namely, spin-canted G- and A-type antiferromagnetic phases, coexist and that their ratio changes during magnetization switching. This spin structure transition is attributable to growth of e_g orbital ordering clusters. The coexistence of two weak ferromagnetic phases plays a key role in exhibiting reversible TCSMS.

Solid solutions of (1- x )BiFeO 3 – x BaTiO 3 ( x = 0.0–1.0) were studied by the combination of X-ray and neutron powder diffraction methods in order to investigate structural and magnetic properties over a wide temperature region. The structure analyses revealed that there exists a hexagonal phase over the wide range of x , and a coexistence of a cubic or a tetragonal phase was revealed. The compositions of the two coexisting phases differ from each other, and Ba and Ti were found to favor the cubic or tetragonal phase. As x increases, the relaxation of the rhombohedral distortion in the hexagonal phase was observed. The decreases of the ferroelectric and the antiferromagnetic phase transition temperatures were also observed leading the suppression of the polarization and the magnetic moment of Fe 3+ ion at room temperature. As a whole, the ferroelectricity and ferromagnetism in this material were considered to arise from the remaining hexagonal phase.