Octupolar versus Néel Order in Cubic 5d2 Double Perovskites

Abstract

We report time-of-flight neutron spectroscopic and diffraction studies of the 5d 2 cubic double pervoskite magnets, Ba2M OsO6 (M = Zn, Mg, Ca). These cubic materials are all described by antiferromagnetically-coupled 5d 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 are argued to be consistent with low temperature octupolar order, and are discussed in the context of other 5d DP magnets, and theories for d 2 ions on a FCC lattice which predict exotic orders driven by multipolar interactions.

Multipolar magnetism in d -orbital systems: Crystal field levels, octupolar order, and orbital loop currents

“…This highlights a non-trivial effect of t 2g -e g mixing, showing that it can be important for nucleating multipolar order in 5d Mott insulators. However, this effect by itself may be too small to account for the spin gap observed in neutron scattering experiments 18,19 on Ba 2 ZnOsO 6 , Ba 2 CaOsO 6 , and Ba 2 MgOsO 6 . We next turn to an additional mechanism, which can cooperate to enhance this splitting, or perhaps even reverse the level ordering which we argue is important in certain other materials.…”

“…Motivated by experiments 18,23-26 on certain cubic double perovskite (DP) Mott insulators, Ba 2 ZnOsO 6 , Ba 2 CaOsO 6 , and Ba 2 MgOsO 6 , which host a 5d 2 configuration on Os, we have instead proposed 19 that their observed nontrivial phenomenology, such as entropy and a spin gap, could be captured by assuming that the five-fold J = 2 multiplet is weakly split, resulting in a ground state non-Kramers doublet carrying quadrupolar and octupolar moments. The lack of any observed crystal distortions in X-ray and neutron diffraction experiments appears to rule out quadrupolar order 18 . Uniform ferrooctupolar ordering in the low lying doublet manifold then provides the most viable route to further reconciling the cubic symmetry, the observation of time-reversal symmetry breaking seen via µSR oscillations 23 , and the apparent lack of any magnetic Bragg peaks in elastic neutron diffraction experiments 18 .…”

“…Optical second-harmonic generation experiments on Cd 2 Re 2 O 7 have found evidence for such an inversion broken quadrupolar ordered state below T c ∼ 200 K 16 . Other candidates for multipolar orders include proposed quadrupolar order in A 2 OsO 4 (with A = K,Rb,Cs) 17 .In recent work, we have studied d-orbital Mott insulators with large SOC and a d 2 configuration in a local octahedral environment, and proposed these systems as candidates for realizing ferro-octupolar order 18,19 . Previous studies of such d 2 quantum magnets 20-22 have argued that the combination of crystal field and interaction effects, leads to the stabilization of a state with total L = 1 and S = 1, which are locked by SOC into a J = 2 spin.…”

“…In recent work, we have studied d-orbital Mott insulators with large SOC and a d 2 configuration in a local octahedral environment, and proposed these systems as candidates for realizing ferro-octupolar order 18,19 . Previous studies of such d 2 quantum magnets [20][21][22] have argued that the combination of crystal field and interaction effects, leads to the stabilization of a state with total L = 1 and S = 1, which are locked by SOC into a J = 2 spin.…”

Diffraction by multipoles in a 5d2 rhenium double perovskite

“…The result is a cautionary note on a subtraction of Bragg diffraction patterns obtained for high and low temperature modifications of a sample to estimate the magnetic signal. Its use in earlier studies of 5d 2 double perovskites returned null results for magnetic Bragg spots [18][19][20][21]. The inferred magnetic crystal class for Ba 2 YReO 6 is mmm1 (no.…”