Exotic Magnetism on the FCC Lattice of 5dn Double Perovskites

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In the search for new exotic quantum states, the impact of strong spin-orbit interaction has been recently underlined with the discovery of the Jeff = ½ spin orbital Mott state in the 5d5 layered perovskites iridates [1]. The double perovskite structure, where the magnetic ions form a face-centered-cubic (fcc) sublattice, can accommodate a large variety of 5d transition metal elements, and therefore offers an ideal playground for systematic studies of the exotic magnetic and non-magnetic ground states stabilized by strong spin-orbit coupling [2]. Here, we report time-of-flight neutron scattering measurements on the antiferromagnetic, frustrated, cubic double perovskite system Ba2YOsO6. Its non-distorted fcc lattice is decorated with magnetic Os5+ (5d3) ions which undergo a magnetic transition to a long range ordered antiferromagnetic state below TN = 70 K, as revealed by magnetic Bragg peaks occuring at the [100] and [110] positions. Our inelastic data reveals a large spin gap to the spin-wave excitations Δ = 19(2) meV, unexpected for an orbitally quenched, d3 electronic configuration. We compare this result to the recent observation of a Δ=5 meV spin gap in the related cubic double perovskite Ba2YRuO6 (Ru5+, 4d3) [3], and conclude to a stronger spin-orbit coupling present in the heavier, 5d, osmate system.

[1] B. J. Kim et al., Science 323, 1329 (2009).
[2] G. Chen, R. Pereira and L. Balents, Phys. Rev. B 82, 174440 (2010);
G. Chen and L. Balents, Phys. Rev. B 84, 094420 (2011).
[3] J. P. Carlo et al., Phys. Rev. B 88, 024418 (2013).