Exotic phases induced by strong spin-orbit coupling in ordered double perovskites

TL;DR

This paper models the complex magnetic phases in double perovskites with strong spin-orbit coupling, revealing exotic magnetic and quadrupolar orders, and suggests potential quantum spin liquid states influenced by lattice distortions.

Contribution

It introduces a microscopic model capturing the interplay of spin-orbit coupling and orbital-dependent exchange, predicting novel magnetic and quadrupolar phases in double perovskites.

Findings

Identification of antiferromagnetic, ferromagnetic, and quadrupolar phases.

Prediction of quantum spin liquid states under certain conditions.

Analysis of the impact of lattice distortions on magnetic order.

Abstract

We construct and analyze a microscopic model for insulating rock salt ordered double perovskites, with the chemical formula A$_2$BB'O$_6$, where the B' atom has a 4d$^1$ or 5d$^1$ electronic configuration and forms a face centered cubic (fcc) lattice. The combination of the triply-degenerate $t_{2g}$ orbital and strong spin-orbit coupling forms local quadruplets with an effective spin moment $j=3/2$. Moreover, due to strongly orbital-dependent exchange, the effective spins have substantial biquadratic and bicubic interactions (fourth and sixth order in the spins, respectively). This leads, at the mean field level, to three main phases: an unusual antiferromagnet with dominant octupolar order, a ferromagnetic phase with magnetization along the $[110]$ direction, and a non-magnetic but quadrupolar ordered phase, which is stabilized by thermal fluctuations and intermediate temperatures. All these phases have a two sublattice structure described by the ordering wavevector ${\boldsymbol Q} =2\pi (001)$. We consider quantum fluctuations and argue that in the regime of dominant antiferromagnetic exchange, a non-magnetic valence bond solid or quantum spin liquid state may be favored instead. Candidate quantum spin liquid states and their basic properties are described. We also address the effect of single-site anisotropy driven by lattice distortions. Existing and possible future experiments are discussed in light of these results.