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

TL;DR

This study uses density-functional theory to understand the electronic and magnetic properties of La-based double perovskites, revealing how electron interactions influence their magnetic states and explaining experimental observations.

Contribution

It provides a detailed theoretical analysis of the electronic structure and magnetic interactions in La$_2$MnNiO$_6$, La$_2$MnCoO$_6$, and La$_2$MnFeO$_6$, clarifying the origin of their magnetic behaviors.

Findings

Fe$^{3+}$ state stabilized by electron interactions promotes ferrimagnetism in La$_2$MnFeO$_6$

Hund's coupling and Jahn-Teller distortions significantly affect magnetic properties

Calculated transition temperatures align with experimental data

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.