Octupolar versus Néel Order in Cubic 5d^{2} Double Perovskites

• Published in: Physical review letters Vol. 124; no. 8; p. 087206
• Main Authors: Maharaj, D D, Sala, G, Stone, M B, Kermarrec, E, Ritter, C, Fauth, F, Marjerrison, C A, Greedan, J E, Paramekanti, A, Gaulin, B D
• Format: Journal Article
• Language: English
• Published: United States 28-02-2020
• ISSN: 1079-7114
• DOI: 10.1103/PhysRevLett.124.087206

We report time-of-flight neutron spectroscopy and neutron and x-ray diffraction studies of the 5d^{2} double perovskite magnets, Ba_{2}MOsO_{6} (M=Zn,Mg,Ca). These materials host antiferromagnetically coupled 5d^{2} Os^{6+} 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 ferro-octupolar 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.