Transport and magnetic properties of R1-xAxCoO3. (R = La, Pr and Nd; A = Ba, Sr and Ca)

Transport and magnetic measurements have been carried out on perovskite Co-oxides R1-xAxCoO3 (R = La, Pr, and Nd; A = Ba, Sr and Ca; 0 ≤ x ≤ 0.5: All sets of the R and A species except Nd1-xBaxCoO3 have been studied.). With increasing the Sr- or Ba-concentration x, the system becomes metallic ferromagnet with rather large magnetic moments. For R = Pr and Nd and A = Ca, the system approaches the metal-insulator phase boundary but does not become metallic. The magnetic moments of the Ca-doped systems measured with the magnetic field H = 0.1 T are much smaller than those of the Ba- and Sr-doped systems. The thermoelectric powers of the Ba- and Sr-doped systems decrease from large positive values of lightly doped samples to negative ones with increasing doping level, while those of Ca-doped systems remain positive. These results can be understood by considering the relationship between the average ionic radius of R1-xAx and the energy difference between the low spin and intermediate spin states. We have found the resistivity-anomaly in the measurements of Pr1-xCaxCoO3 under pressure in the wide region of x, which indicates the existence of a phase transition different from the one reported in the very restricted region of x∼0.5 at ambient pressure [Tsubouchi et al. Phys. Rev. B 66 (2002) 052418.]. No indication of this kind of transition has been observed in other species of R. (author)