Competing multipolar orders in a face-centered cubic lattice: Application to the osmium double perovskites

TL;DR

This paper investigates the complex multipolar magnetic orders in face-centered cubic lattice systems, specifically osmium double perovskites, revealing how intra- and interorbital exchanges influence the emergence of octupolar order.

Contribution

It introduces a comprehensive analysis of exchange interactions in fcc lattice systems, highlighting the role of interference effects and higher-order processes in stabilizing octupolar order.

Findings

Interference of intraorbital exchanges generates ferromagnetic octupolar interactions.

Osmium double perovskites are near the boundary between quadrupolar and octupolar orders.

Higher-order virtual processes may stabilize octupolar order.

Abstract

In 5$d^2$ Mott insulators with strong spin-orbit coupling, the lowest pseudospin states form a non-Kramers doublet, which carries quadrupolar and octupolar moments. A family of double perovskites where magnetic ions form a face-centered cubic (fcc) lattice was suggested to unveil an octupolar order offering a rare example in $d$-orbital systems. The proposed order requires a ferromagnetic (FM) octupolar interaction, since the antiferromagnetic (AFM) Ising model is highly frustrated on the fcc lattice. A microscopic model was recently derived for various lattices: for an edge-sharing octahedra geometry, AFM Ising octupolar and bond-dependent quadrupolar interactions were found when only dominant inter- and intraorbital hopping integrals were taken into account. Here we investigate all possible intra- and interorbital exchange processes and report that interference of two intraorbital exchanges generates a FM octupolar interaction. Applying the strong-coupling expansion results together with tight-binding parameters obtained by density functional theory, we estimate the exchange interactions for the osmium double perovskites, Ba$_2$BOsO$_6$ (B=Mg,Cd, and Ca). Using classical Monte Carlo simulations, we find that these systems are close to the phase boundary between AFM type-I quadrupole and FM octupole orders. We also find that exchange processes beyond second-order perturbation theory including virtual processes via pseudospin-triplet states may stabilize an octupolar order.