Electronic and Magnetic Properties of double-perovskites La$_2$MnRuO$_6$ and Hole-Doped La$_2$MnFeO$_6$ and their Potential for Magnetic Refrigeration

TL;DR

This study uses density functional theory to explore double-perovskites for magnetic refrigeration, finding that certain doped LaMnFeO6 compounds could operate effectively at room temperature.

Contribution

It demonstrates that doping LaMnFeO6 with Ba, Ca, or Sr can induce ferromagnetism suitable for magnetic refrigeration, expanding the range of potential magnetocaloric materials.

Findings

La2MnRuO6 is ferrimagnetic with higher magnetic moment than La2MnFeO6.

Doubly-ordered LaBaMnFeO6 and LaSrMnFeO6 are predicted to be ferromagnetic at room temperature.

Monte Carlo simulations suggest these compounds are promising for magnetic refrigeration.

Abstract

Magnetic refrigeration at room-temperature is a technology that could potentially be more environmentally-friendly, efficient and affordable than traditional refrigeration. The search for suitable materials for magnetocaloric refrigeration led to the study of double-perovskites La$_2$MnNiO$_6$, La$_2$MnCoO$_6$ and La$_2$MnFeO$_6$. While La$_2$MnNiO$_6$ and La$_2$MnCoO$_6$ are ferromagnets with near room-temperature $T_C$s, previous theoretical study of double-perovskite La$_2$MnFeO$_6$ revealed that this material is a ferrimagnet due to strong electronic interactions in Fe-$d$ orbitals. Here, we investigate the double-perovskites La$_2$MnRuO$_6$ and LaA''MnFeO$_6$ (A'' = Ba, Ca and Sr) with density functional theory (DFT) as materials that can counteract the effects the strong repulsion present in the in Fe-$d$ shells of La$_2$MnFeO$_6$ and lead to a ferromagnetic state. Our study reaveals that while La$_2$MnRuO$_6$ is also a ferrimagnet, but with a higher net magnetic moment per formula than La$_2$MnFeO$_6$, doubly-ordered LaA''MnFeO$_6$ are ferromagnets. By mapping the total energy of the LaA''MnFeO$_6$ compounds obtained from DFT calculations to the Ising model, we also calculate their magnetic exchange couplings. This allows us to estimate the trend in $T_C$ of the three doped La$_2$MnFeO$_6$ materials with classical Monte-Carlo calculations and predict that doubly-ordered LaBaMnFeO$_6$ and LaSrMnFeO$_6$ could be suitable materials for room-temperature magnetic refrigeration.