Magnetic, Electronic Transport, and Calorimetric Investigations of La _1-xCa_xCoO₃ in Comparison with La _1-xSr_xCoO₃

The magnetic and electronic transport properties and the specific heat of La_1-xCa_xCoO₃ (0.05 < x < 0.5) have been investigated. In the whole composition range, the Co atoms are in an intermediate-spin (IS) state, and a spin glass or a ferromagnetic ordered state appears at low temperatures. The conductivity increases with x, and shows a metallic temperature dependence for x > 0.4. The overall resemblance of the Ca substituted system with the Sr substituted one implies that the Ca substitution introduces holes into Co-O bonds as in the case of the Sr substitution. The present results reveal that the hole-doping, not the ionic size, is essential to stabilize the IS state of Co atoms in La _1-xM_xCoO₃ with M being a divalent ion. The quantitative difference, lower T_C and less conductivity for M = Ca than M = Sr with the same x, suggests less mobility and/or less number of holes in the system with M = Ca. The electronic specific heat coefficient, null for LaCoO₃, increases sharply around x = 0.2 for M = Sr and x = 0.25 for M = Ca, and saturates at higher concentrations. The observation indicates the presence of the Fermi surface in the deeply substituted specimens. The magneto-transport properties of the metallic specimens, especially a large amplitude of the side-jump scattering in the extraordinary Hall resistivity, are characteristic of itinerant ferromagnetic metals, which supports the double exchange mechanism in the present system.