Resistivity, dc magnetization, ac susceptibility, and heat-capacity measurements were carried out for polycrystalline ${\mathrm{Pr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{CoO}}_3$ $(0\mathrm{}<~x<~0.55).$ Insulating, metallic, and metal-insulator transition (MIT) regions appear successively with increasing x. The boundary compositions of neighboring regions are between $x=0.1\mathrm{}$ and 0.2 and between $x=0.45\mathrm{}$ and 0.5. In the insulating region, the sample is an insulator and shows no phase transition in the range of 2–300 K. In the metallic region, the sample is a metal and shows a ferromagnetic ordering below about 80 K. In the MIT region, the sample shows a metal-insulator transition above about 80 K and a ferromagnetic ordering at around 5 K. The electronic and spin states of each phase are discussed based on the experimental results.

The possibility of the new class ferroelectric materials of wurtzite structure simple chalcogenide was discussed using modern first-principles calculation technique. Ferroelectricity in the wurtzite structure (P6₃mc) can be understood by structure distortion from centrosymmetric P6₃/mmc by relative displacement of cation against anion along c-axis. Calculated potential surface of these compounds shows typical double well between two polar variants. The potential barriers for the ferroelectric polarization switching were estimated to be 0.25 eV/f.u. for ZnO. It is slightly higher energy to the common perovskite ferroelectric compound PbTiO₃. Epitaxial tensile strain on the ab-plane (0001) is effective to lower the potential barrier. The potential barrier decreased from 0.25 to 0.15 eV/f.u. by 5% ab-plane expansion in wurtzite structure ZnO. Epitaxial ZnO thin film with donor type defect reduction should be a possible candidate to confirm this ferroelectricity in wurtzite structure simple chalcogenide.

Perpendicularly magnetized layers are used widely for high-density information storage in magnetic hard disk drives and nonvolatile magnetic random access memories. Writing and erasing of information in these devices is implemented by magnetization switching in local magnetic fields or via intense pulses of electric current. Improvements in energy efficiency could be obtained when the reorientation of perpendicular magnetization is controlled by an electric field. Here, we report on reversible electric-field-driven out-of-plane to in-plane magnetization switching in Cu/Ni multilayers on ferroelectric BaTiO₃ at room temperature. Fully deterministic magnetic switching in this hybrid material system is based on efficient strain transfer from ferroelastic domains in BaTiO₃ and the high sensitivity of perpendicular magnetic anisotropy in Cu/Ni to electric-field-induced strain modulations. We also demonstrate that the magnetoelectric coupling effect can be used to realize 180° magnetization reversal if the out-of-plane symmetry of magnetic anisotropy is temporarily broken by a small magnetic field.

Magnetic susceptibility and heat capacity due to the spin-state transition in $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ were calculated by a molecular-field model in which the energy-level diagram of high-spin state reported by Ropka and Radwanski [Phys. Rev. B 67, 172401 (2003)] is assumed for the excited state, and the energy and entropy of mixing of high-spin Co ions and low-spin Co ions are introduced phenomenologically. The experimental data below $300\mathrm{K}$ were well reproduced by this model, which proposes that the high-spin excited state can be populated even if the energy of high-spin state is much larger than that of low-spin state, because the negatively large energy of mixing reduces the net excitation energy. The stability of each spin state including the intermediate-spin state is discussed based on the present results and other reports.