Magnetic Study of Metal-Complex-Based Molecular Conductors and Magnets under Pressure

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The main purpose of this research project is to develop techniques for measurements of static magnetization under pressure, as being applicable to study of electronic states of molecular conductors and magnets based on metal-complexes. In spite of various remarkable pressure-induced phenomena in molecular materials, magnetic study under pressure has not been appreciated so far, since such compounds exhibit small susceptibility of about 10^<-4> to 10^<-3> emu/mol. It has been demonstrated in this study that careful choice of materials used for the clamp-type slender pressure cell to minimize undesirable effect of the background contributions, can appreciably improves the precision of the magnetization measured under pressure. This enables us to measure the temperature dependence of paramagnetic susceptibility of about 10^<-4> emu/mol, which is satisfactory for the detection of magnetic phase transitions of a typical molecular conductor. This technical development has been applied to the magnetic study of a series of [Pd(dmit)_2] salts under pressure up to about 0.5 GPa. Magnetic studies of the [Pd(dmit)_2] salts, including those under ambient pressure, has revealed variety of novel phase transitions associated with the frustrated behavior of quantum spins on the triangular lattice, such as the valence instability due to orbital-dimer distortion-charge transfer interplay, the antiferromagnetic transitions induced by spatial anisotropy, and the valence bond ordering. Therefore, application of pressure is expected to give more improved understanding of the physical origin of such behavior. Pressure-induced bulk superconductivity has been found in the EtMe_3P salt (P2_1/m phase) : T_c = 5 K at 0.2 GPa. Through the insulator-to-metal Mott transition, no remarkable change in susceptibility has been found, which indicates that the electrons are strongly correlating even in the metallic state.