Octupolar vs N\'{e}el Order in Cubic 5$d^2$ double perovskites

TL;DR

This study investigates the magnetic order in cubic 5d^2 double perovskites, revealing evidence for octupolar or quadrupolar order driven by multipolar interactions, despite the absence of magnetic Bragg peaks.

Contribution

It provides experimental evidence for non-dipolar magnetic order in 5d^2 double perovskites, highlighting the role of multipolar interactions in these materials.

Findings

Presence of thermodynamic anomalies at T*

Gapped magnetic excitation spectrum with type I AF order wavevectors

Muon spin resonance indicates time reversal symmetry breaking without magnetic Bragg peaks

Abstract

We report time-of-flight neutron spectroscopic and diffraction studies of the 5$d^2$ cubic double pervoskite magnets, Ba$_2$MOsO$_6$ ($M$ = Zn, Mg, Ca). These cubic materials are all described by antiferromagnetically-coupled 5$d^2$ Os$^{6+}$ ions decorating a face-centred cubic (FCC) lattice. They all exhibit thermodynamic anomalies consistent with phase transitions at a temperature $T^*$, and exhibit a gapped magnetic excitation spectrum with spectral weight concentrated at wavevectors typical of type I antiferromagnetic orders. While muon spin resonance experiments show clear evidence for time reversal symmetry breaking, no corresponding magnetic Bragg scattering is observed at low temperatures. These results, consistent with low temperature octupolar or quadrupolar order, are discussed in the context of other 5$d^2$ DP magnets, and theories for $d^2$ ions on a FCC lattice which predict exotic orders driven by multipolar interactions.