Octupolar versus Neel Order in Cubic 5d2 Double Perovskites

We report time-of-flight neutron spectroscopy and neutron and x-ray diffraction studies of the 5d(2) double perovskite magnets, Ba2MOsO6 (M = Zn, Mg, Ca). These materials host antiferromagnetically coupled 5d(2) Os6+ ions decorating a face-centered cubic (fcc) lattice and are found to remain cubic down to the lowest temperatures. They all exhibit thermodynamic anomalies consistent with a single phase transition at a temperature T*, and a gapped magnetic excitation spectrum with spectral weight concentrated at wave vectors typical of type-I antiferromagnetic orders. However, while muon spin resonance experiments show clear evidence for time-reversal symmetry breaking below T*, we observe no corresponding magnetic Bragg scattering signal. These results are shown to be consistent with ferrooctupolar symmetry breaking below T*, and are discussed in the context of other 5d double perovskite magnets and theories of exotic orders driven by multipolar interactions.