Magnetic and transport properties of the spin-state disordered oxide La0.8Sr0.2Co_{1-x}Rh_xO_{3-\delta}

TL;DR

This study investigates how substituting Rh for Co in La0.8Sr0.2CoO3 affects its magnetic, structural, and thermoelectric properties, revealing a transition from ferromagnetic to paramagnetic states and an anomalous thermopower enhancement.

Contribution

It provides new insights into the spin-state disorder and transport phenomena in La0.8Sr0.2Co_{1-x}Rh_xO_{3- ext{delta}} across the full substitution range.

Findings

Magnetic ground state shifts from ferromagnetic to paramagnetic near x=0.25.

Rh substitution reduces saturation magnetization significantly.

Thermopower is anomalously enhanced below 50 K, especially at x=0.5.

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$_{1-x}$Rh$_x$O$_{3-\delta}$. 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 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 ($0.05\le x \le 0.75$), 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 CeRu$_2$Si$_2$.