Theory of High T$_c$ Ferrimagnetism in a Multi-orbital Mott Insulator

TL;DR

This paper models the high-temperature ferrimagnetism in Sr$_2$CrOsO$_6$, revealing it as a Mott insulator with a novel transition criterion and explaining its magnetic properties through a frustrated Heisenberg model.

Contribution

It introduces a new Mott transition criterion for multi-orbital systems and explains the magnetic behavior of Sr$_2$CrOsO$_6$ using a combined electronic and spin model.

Findings

Sr$_2$CrOsO$_6$ is a Mott insulator with a record high T_c.

Derived a new criterion for the Mott transition involving Coulomb interactions and bandwidth.

Predicted magnetic structure factor peaks for neutron experiments.

Abstract

We propose a model for the multi-orbital material Sr$_2$CrOsO$_6$ (SCOO), an insulator with remarkable magnetic properties and the highest $T_c \simeq 725$ K among {\em all} perovskites with a net moment. We derive a new criterion for the Mott transition $(\widetilde{U}_{1} \widetilde{U}_{2})^{1/2}>2.5W$ using slave rotor mean field theory, where $W$ is the bandwidth and $\widetilde{U}_{1(2)}$ are the effective Coulomb interactions on Cr(Os) including Hund's coupling. We show that SCOO is a Mott insulator, where the large Cr $\widetilde{U}_{1}$ compensates for the small Os $\widetilde{U}_{2}$. The spin sector is described by a frustrated antiferromagnetic Heisenberg model that naturally explains the net moment arising from canting and also the observed non-monotonic magnetization $M(T)$. We predict characteristic magnetic structure factor peaks that can be probed by neutron experiments.