Electronic and magnetic properties of the candidate magnetocaloric-material double perovskites

The search for room-temperature magnetocaloric materials for refrigeration has led to investigations of double perovskites. In particular, a puzzle has appeared in the La2MnNiO6, La2MnCoO6, and La2MnFeO6 family of compounds. They share the same crystal structure, but while La2MnNiO6 and La2MnCoO6 are ferromagnets below room temperature, La2MnFeO6, 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 Fe3+ valence state with half-filled d shell over Fe2+ and to establish a ferrimagnetic ground state for La2MnFeO6. The importance of Hund’s coupling and Jahn-Teller distortion on the Mn4+ 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 TC that are in agreement with experiments on this family of materials.