Robust half-metallic antiferromagnets La$A$VOsO$_6$ and La$A$Mo$Y$O$_6$ ($A$ = Ca, Sr, Ba; $Y$ = Re, Tc) from first-principles calculations

TL;DR

This study uses first-principles calculations to design and analyze new half-metallic antiferromagnetic double perovskites, revealing their stability, electronic structures, and magnetic mechanisms, and identifying some as unconventional AFM insulators.

Contribution

The paper systematically predicts new half-metallic antiferromagnetic double perovskites using ab initio methods, including full structural optimizations and analysis of magnetic mechanisms, expanding the class of known AFM materials.

Findings

Identified La$A$VOsO$_6$, La$A$MoTcO$_6$, La$A$MoReO$_6$ as stable HM-AFM materials.

Discovered La$A$CrTcO$_6$ and La$A$CrReO$_6$ as unconventional AFM insulators.

Showed the importance of superexchange and double exchange mechanisms in magnetic properties.

Abstract

We have theoretically designed three families of the half-metallic (HM) antiferromagnets (AFM), namely, La$A$VOsO$_6$, La$A$MoTcO$_6$ and La$A$MoReO$_6$ ($A$ = Ca, Sr, Ba), based on a systematic {\it ab initio} study of the ordered double perovskites La$ABB'$O$_6$ with the possible $B$ and $B'$ pairs from all the 3$d$, 4$d$ and 5$d$ transtion metal elements being considered. Electronic structure calculations based on first-principles density-functional theory with generalized gradient approximation (GGA) for more than sixty double perovskites LaCa$BB'$O$_6$ have been performed using the all-electron full-potential linearized augmented-plane-wave method. The found HM-AFM state in these materials survives the full {\it ab initio} lattice constant and atomic position optimizations which were carried out using frozen-core full potential projector augmented wave method. It is found that the HM-AFM properties predicted previously in some of the double perovskites would disappear after the full structural optimizations. The AFM is attributed to both the superexchange mechanism and the generalized double exchange mechanism via the $B$ ($t_{2g}$) - O (2$p_{\pi}$) - $B'$ ($t_{2g}$) coupling and the latter is also believed to be the origin of the HM. Finally, in our search for the HM-AFMs, we find La$A$CrTcO$_6$ and La$A$CrReO$_6$ to be AFM insulators of an unconventional type in the sense that the two antiferromagnetic coupled ions consist of two different elements and that the two spin-resolved densities of states are no longer the same.