Magnetic phase transitions in $Ln_{1-x}$Sr$_{x}$Co$_{0.5}$Mn$_{0.5}$O$_{3}$ ($Ln$ = La, Pr, Nd, and Eu)

The system of La$_{1-x}$Sr$_{x}$Co$_{0.5}$Mn$_{0.5}$O$_{3}$ solid solutions have been studied by neutron diffraction and the measurement of the magnetization. The composition with $x$ = 0 has the monoclinic structure (space group $P$2$_{1/n}$); the compositions with 0.15 $\le x\le$ 0.6 are rhombohedral $(R\bar3 c)$, and the composition with $x$ = 0.75 is cubic $(Pm\bar3 n)$. All the compositions with $x\le$ 0.75 are shown to contain a ferromagnetic component. The stoichiometric composition with $x$ = 0.5 undergoes a sharp ferrromagnet–paramagnet transition at 240 K; however, no coherent magnetic contribution to the neutron diffraction has been detected. The magnetic data are interpreted, assuming that the ferromagnetism at $x\le$ 0.15 is due to positive superexchange Co$^{2+}$–O–Mn$^{4+}$ interaction, while, at $x\ge$ 0.25, the superexchange interaction between ions Co$^{3+}$ in a high-spin state and ions Mn$^{4+}$ is responsible for the ferromagnetic component.