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.
Categories: cond-mat.str-el cond-mat.mtrl-sci