Ferromagnetism and unconventional impurity effects in Rh- and Ga- doped LaCoO$_{3}$

The perovskite oxide LaCoO$_{3}$ has been long investigated because of a dramatic change of its spin state for Co$^{3+}$ ions with temperature variation. The Co$^{3+}$ ions (3$d^{6}$) in LaCoO$_{3}$ takes the non-magnetic low-spin state of \textit{t}$_{2g}^{6}$ at low temperature. The spin state of the Co$^{3+}$ ions in this system is sensitive to the chemical substitutions; we have found a weak ferromagnetism in a solid solution of LaCoO$_{3}$ and LaRhO$_{3}$ [S. Asai \textit{et al}., JPSJ 80, 104705 (2011).]. Since the two oxides are non-magnetic at low temperature, our finding is an example of ``order by disorder,'' where a non-magnetic impurity makes a non-magnetic oxide ferromagnetic. We have further investigated the magnetization and x-ray diffraction of LaCo$_{0.8-y}$Rh$_{0.2}$M$_{y}$O$_{3}$ (M = Rh, Ga) [S. Asai \textit{et al}., PRB 86, 014421 (2012).]. The magnetization decreases by the Ga$^{3+}$ substitution much more drastically than by the Rh$^{3+}$ substitution. It indicates that at least two kinds of Co$^{3+}$ ions exist in LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$; one is nonmagnetic, and the other is magnetic. In this talk, we will also discuss the change of the lattice volume with the Rh$^{3+}$ and Ga$^{3+}$ substitution [S. Asai \textit{et al}., JPSJ 82, 114606 (2013).].