Magnetic and transport properties of the spin-state disordered oxide La0.8Sr0.2Co1-xRhxO3-δ

We report measurements and analysis of magnetization, resistivity, and thermopower of polycrystalline samples of the perovskite-type Co/Rh oxide La0.8Sr0.2Co1-xRhxO3-delta. This system constitutes a solid solution for a full range of x, in which the crystal structure changes from rhombohedral to orthorhombic symmetry with increasing Rh content x. The magnetization data reveal that the magnetic ground state immediately changes upon Rh substitution from ferromagnetic to paramagnetic with increasing x near 0.25, which is close to the structural phase boundary. We find that one substituted Rh ion diminishes the saturation moment by 9 mu(B), which implies that one Rh3+ ion makes a few magnetic Co3+ ions nonmagnetic (the low-spin state) and causes disorder in the spin state and the highest occupied orbital. In this disordered composition (0.05 <= x <= 0.75), we find that the thermopower is anomalously enhanced below 50 K. In particular, the thermopower of x = 0.5 is larger by a factor of 10 than those of x = 0 and 1, and the temperature coefficient reaches 4 mu V/K-2, which is as large as that of heavy-fermion materials such as CeRu2Si2.