The magnetic and electronic transport properties and the specific heat of La 1Àx Ca x CoO 3 (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 1Àx M x CoO 3 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 LaCoO 3 , increases sharply around x ¼ 0:2 for M = Sr and x ¼ 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.

Simultaneous measurements of the magnetoresistance, magnetization, Hall effect, thermoelectric power and specific heat of a modified FeRh compound where 3.5 at.% Ni is doped at the Fe sites are investigated in order to elucidate the origin of the giant-magnetoresistance phenomena through the metamagnetic transition from the antiferromagnetic (AF) to the ferromagnetic (F) state. The Hall coefficient changes its sign through the metamagnetic transitions. The sign of the thermoelectric power also changes from positive to negative through the transition from the AF to the F state. The electronic specific heat coefficient increases remarkably through the metamagnetic transition from the AF to the F state. These results imply that the giant-magnetoresistance effect in FeRh compounds results from the reconstruction of the Fermi surface across the metamagnetic transition. At low temperatures, two-step metamagnetic transitions were found, suggesting an uncompensated AF or ferrimagnetic state in the intermediate-field region around 2-3 T, only in the process of increasing the field.