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Perovskite-type manganites exhibit various interesting phenomena arising from complex interplay among spin, charge, orbital, and lattice degrees of freedom. One such example is the keen competition between phases with different spin/charge/orbital orders. Keen competition between antiferromagnetic metal and orbital-ordered insulator is found in the slightly electron-doped regime near Mn4+ state which is stabilized by the high oxygen-pressure condition. Another one is the emergence of ferroelectricity either induced by the magnetic ordering or independently of the magnetic ordering. As the respective examples, perovskite-type YMnO3 and Sr1−xBaxMnO3 are discussed. In the YMnO3, the ferroelectric lattice distortion associated with the E-type spin order is observed for the first time. Displacement-type ferroelectricity with off-center magnetic ions is discovered for Sr0.5Ba0.5MnO3, which shows both large polarization value and strong coupling between ferroelectricity and magnetism.

Single crystals of various transition metal oxides exhibiting fascinating physical properties were grown by flux method at high pressures of several GPa. The growth conditions were determined based on synchrotron powder X-ray diffraction studies at high-pressure and high-temperature conditions. Results on a S =1/2 one dimensional alternating exchange antiferromagnet (VO) 2 P 2 O 7 , perovskites PrNiO 3 , CaFeO 3 , BiMnO 3 and an oxychloride high- T C superconductor Ca 2− x Na x CuO 2 Cl 2 are reported.

The magnetic structure of a hexagonal layered cobalt oxide SrCo 6 O 11 , which exhibits a 1/3 magnetization plateau and a concomitant large negative megnetoresistance below 20 K, was studied by means of neutron diffraction measurements and Rietveld analysis. It has been found that only the trigonal bipyramidal site carries a localized moment, which corresponds to S =2, and that the moment has an Ising-like anisotropy along the c -axis. These moments are aligned ferromagnetically in the c -plane, while the interlayer coupling is field dependent. The 1/3 magnetization plateau reflects a field-induced ferrimagnetic alignment along the c -axis like “up–up–down–up–up–down”. We interpret this complex spin structure as resulting from competition of an antiferromagnetic superexchange interaction between the nearest ferromagnetic layers and carrier-mediated, ferromagnetic interactions.

We observed the pressure-induced metal-insulator (MI) transition in BiNiO3 and studied the structural features under high pressure by means of synchrotron X-ray diffraction (SXRD). The results show that the transition from the triclinic insulating phase to the higher symmetric metallic phase with a hysteresis occurs in the vicinity of 3 GPa, which is associated with a melting of the oxygen holes trapped and ordered in the Bi–O sublattice.

We report on low-temperature resistivity of the simplest misfit cobaltate [Sr 2 O 2− δ ] 0.53 CoO 2 under magnetic fields up to 5 T. Single crystalline samples were prepared under high pressure. The resistivity data show a kink around 10 K, below which part of Fermi surface is removed, and eventually obey a Fermi-liquid T 2 law. A heavy-mass Fermi-liquid state in [Sr 2 O 2− δ ] 0.53 CoO 2 signifies strong electron correlation near a Mott insulator.

The layered compound SrCo6O11 contains Ising-like spins showing unusual magnetic behavior such as 1/3 plateau in magnetization at low temperatures. In order to see how Ising-like spins couple to itinerant electrons, we investigate transport properties of SrCo6O11 using single crystalline samples. Resistivity measurements show metallic behavior and a relatively large anisotropy. Pertaining to magnetic ordering, both resistivity and thermopower show a sharp drop near 20 K. Although the magnetic layer and the conductive layer are separated in space, itinerant electrons interact strongly with localized Ising-like spins, which can be confirmed in Hall resistivity measurements.