The Case for Octupolar Order in Cubic 5d2 Double Perovskites

We have carried out neutron spectroscopic and diffraction studies on powder samples of the cubic 5d² family of double perovskites, Ba₂ZnOsO₆, Ba₂MgOsO₆, and Ba₂CaOsO₆. These materials all display thermodynamic phase transitions at T*=30 K, 49 K, and 50 K, respectively, where ~ R ln(2) in entropy is released on passing through T*. All three display Curie-Weiss susceptibilities with antiferromagnetic Curie-Weiss constants of ~ -150 K, and show oscillations in low temperature zero longitudinal μSR, indicating time reversal symmetry breaking. Our inelastic neutron scattering shows the development of a gapped spin excitation spectrum, where a gap of ~ 15 meV develops below T*. This spectral weight is centred at wavevectors typical of type I AF order, however no magnetic Bragg diffraction is observed at low temperatures. Our neutron powder diffraction measurements place an upper limit on any possible ordered dipole moment of ~ 0.1 μ_B. To understand these results, we introduce a model for orbital repulsion between the two 5d electrons in Os⁶⁺ in these materials. This results in a non-Kramer's doublet and excited triplet of states for the ion, and ferro-octupolar order below T*, consistent with the full set of experimental observations. We believe this likely constitutes the first compelling case for octupolar order in a d-orbital Mott insulators.