Geometrical origin of ferrimagnetism and superparamagnetism in Fe-based double perovskite multiferroics

Abstract: We show that a superstructure of antiferromagnetically interacting Fe$^{3+}$ ($S=5/2$) ions in double perovskites AFe${1/2}$M${1/2}$O${3}$ exhibits a ferrimagnetic ordering below $T{fe} \approx 5.6J_1$ ($J_1/k_B \sim 50$~K), which is close to room temperature. Small clusters of the same structure exhibit a superparamagnetic behavior at $T \lesssim T_{fe}$. The possibility of formation of such clusters explains the room-temperature (superpara)magnetism in 3$d$-metal based oxides.