Exotic magnetism on the frustrated FCC lattice of 4d and 5d double perovskites

In the search for new exotic quantum states, the impact of strong spin-orbit interaction has been recently underlined with the discovery of the J_eff = 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 [3]. Its non-distorted fcc lattice is decorated with magnetic Os⁵⁺ (5d³) ions which undergo a magnetic transition to a type I fcc long range ordered antiferromagnetic state below T_N = 70 K. Our inelastic data reveals a large spin gap to the spin-wave excitations ∆ = 18(2) meV, unexpected for an orbitally quenched, d³ electronic configuration. We will discuss our results in the context of other recently studied cubic double perovskites, where exotic magnetic and non-magnetic phases are expected.

[1] B. J. Kim et al., Phys. Rev. Lett. 101, 076402 (2008).

[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] E. Kermarrec et al., arXiv:1410.0725 (2014).

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