Microscopic model, spin wave theory and competing orders in the double perovskites

TL;DR

This paper develops a microscopic model for carrier-induced ferrimagnetism in metallic double perovskites, revealing phase transitions and quantum effects near critical points relevant for magnetotransport applications.

Contribution

It introduces a Kondo-like Hamiltonian approach to describe magnetic phases and quantum effects in double perovskites, highlighting phase separation and spin wave behavior.

Findings

Ground state varies with carrier density, being ferrimagnetic or antiferromagnetic.

Ferrimagnetic state exhibits half-metallicity.

Quantum effects are enhanced near the phase transition.

Abstract

We present a microscopic theory of carrier-induced ferrimagnetism in metallic double perovskite compounds such as ${\rm Sr}_{2}{\rm FeMoO}_{6}$ and ${\rm Sr}_{2}{\rm FeReO}_{6}$ which have recently attracted intense interest for their possible applications to magnetotransport devices. The theory is based on an effective "Kondo-like" Hamiltonian treated here within the large-$S$ expansion. We find that depending on the value of the carrier density the ground state is either a ferrimagnet or a layered antiferromagnet. The ferrimagnetic state has a robust half-metallic electronic structure. The transition to antiferromagnetic phase is first order accompanied with the regime of phase separation. We study spin wave spectrum including quantum corrections and find strongly enhanced quantum effects in the vicinity of zero-temperature phase transition.