Transport and Magnetic Studies on the Spin State Transition of Pr_<1-x>Ca_xCoO_3 up to High Pressure(Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

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Transport and magnetic measurements and structural and NMR studies have been carried out on (Pr_<1-y>R'_y)_<1-x>A_x-CoO_3 {R' = (rare earth elements and Y) ; A = (Ca, Ba and Sr)} at ambient pressure and high pressure. The system exhibits a phase transition from a nearly metallic to an insulating state with decreasing temperature T, where the low spin (LS) state of Co^<3+> is suddenly stabilized. For y = 0, we have constructed a T-x phase diagram at various values of the external pressure p. It shows that the (T, x) region of the low temperature phase, which is confined to a very narrow region around x = 0.5 at ambient pressure, expands as p increases. For the occurrence of the transition, both the Pr and Ca atoms seem to be necessary. The intimate relationship between the local structure around the Co ions and the electronic (or spin) state of Co^<3+> ions is discussed : For the smaller unit cell volume or the smaller volume of the CoO_6 octahedra and for the larger tilting angle of the octahedra, the temperature of the transition becomes higher. The role of the carriers introduced by the doping of the A atoms, is also discussed. By analyzing ^<59>Co-NMR spectra and magnetic susceptibility of Pr_<1-x>Ca_xCoO_3, the electronic-energy-separations among the different spin states of Co^<3+> and Co^<4+> are roughly estimated.
社団法人日本物理学会の論文
2004-07-15