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

Abstract

We report measurements and analysis of magnetization, resistivity, and thermopower of polycrystalline samples of the perovskite-type Co/Rh oxide La${}_{0.8}$Sr${}_{0.2}$Co${}_{1\ensuremath{-}x}$Rh${}_{x}$O${}_{3\ensuremath{-}\ensuremath{\delta}}$. 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 ${\ensuremath{\mu}}_{B}$, which implies that one Rh${}^{3+}$ ion makes a few magnetic Co${}^{3+}$ ions nonmagnetic (the low-spin state) and causes disorder in the spin state and the highest occupied orbital. In this disordered composition ($0.05\ensuremath{\leqslant}x\ensuremath{\leqslant}0.75$), 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 $\ensuremath{\mu}$V/K${}^{2}$, which is as large as that of heavy-fermion materials such as CeRu${}_{2}$Si${}_{2}$.