Electronic and Magnetic Properties of the Candidate Magnetocaloric-Material Double Perovskites La${}_2$MnCoO${}_6$, La${}_2$MnNiO${}_6$ and La${}_2$MnFeO${}_6$

Abstract

The search for room-temperature magnetocaloric materials for refrigeration has led to investigations of double perovskites. In particular, a puzzle has appeared in the La${}_2$MnNiO${}_6$, La${}_2$MnCoO${}_6$ and La${}_2$MnFeO${}_6$ family of compounds. They share the same crystal structure, but while La${}_2$MnNiO${}_6$ and La${}_2$MnCoO${}_6$ are ferromagnets below room temperature, La${}_2$MnFeO${}_6$, contrary to simple expectations, is a ferrimagnet. To solve this puzzle, we use density-functional theory calculations to investigate the electronic structure and magnetic exchange interactions of the ordered double perovskites. Our study reveals the critical role played by local electron-electron interaction in the Fe-$d$ orbital to promote the Fe${}^{3+}$ valence state with half-filled $d$-shell over Fe${}^{2+}$ and to establish a ferrimagnetic ground state for La${}_2$MnFeO${}_6$. The importance of Hund's coupling and Jahn-Teller distortion on the Mn${}^{4+}$ ion is also pointed out. Exchange constants are extracted by comparing different magnetically ordered states. Mean-field and classical Monte-Carlo calculations on the resulting model give trends in $T_C$ that are in agreement with experiments on this family of materials.

accepted in Physics. Rev. B