Ga$^{3+}$-substitution effects in the weak ferromagnetic oxide LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$

TL;DR

This study investigates how Ga$^{3+}$ substitution affects the magnetic properties and spin states of LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$, revealing that Ga$^{3+}$ suppresses ferromagnetism and preferentially replaces high-spin Co$^{3+}$ ions.

Contribution

It provides new insights into the selective substitution effects of Ga$^{3+}$ on high-spin Co$^{3+}$, contrasting with Rh$^{3+}$ behavior, and elucidates the mixed spin state nature of LaCoO$_{3}$.

Findings

Ga$^{3+}$ substitution suppresses ferromagnetic order.

One Ga$^{3+}$ reduces 4.6 μ_B per formula unit.

Ga$^{3+}$ preferentially replaces high-spin Co$^{3+}$.

Abstract

Magnetization and x-ray diffraction have been measured on polycrystalline samples of LaCo$_{0.8-y}$Rh$_{0.2}$Ga$_{y}$O$_{3}$ for $0 \leq y \leq 0.15$ in order to understand the spin state of Co$^{3+}$ through the Ga$^{3+}$ substitution effect. The ferromagnetic order in LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$ below 15 K is dramatically suppressed by the Ga$^{3+}$ substitution, where the ferromagnetic volume fraction is linearly decreased. The normal state susceptibility also systematically decreases with the Ga content, from which we find that one Ga$^{3+}$ ion reduces 4.6 $\mu_{\rm B}$ per formula unit. We have evaluated how the concentration of the high-spin state Co$^{3+}$ changes with temperature by using an extended Curie-Weiss law, and have found that the substituted Rh$^{3+}$ ion stabilizes the high-spin state Co$^{3+}$ ion down to low temperatures. We find that Ga$^{3+}$ preferentially replaces the high-spin state Co$^{3+}$, which makes a remarkable contrast to our previous finding that Rh$^{3+}$ preferentially replaces the low-spin state Co$^{3+}$. These results strongly suggest that the magnetically excited state of LaCoO$_{3}$ at room temperature is a mixed state of high-spin state Co$^{3+}$ and low-spin state Co$^{3+}$.