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.

8 pages, 8 figures, submitted to Phys. Rev. B

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