Low-moment magnetism in the double perovskites Ba2MOsO6 (M = Li, Na)

The magnetic ground states of the isostructural double perovskites Ba2NaOsO6 and Ba2LiOsO6 are investigated with muon-spin relaxation. In Ba2NaOsO6 long-range magnetic order is detected via the onset of a spontaneous muon-spin precession signal below T-c = 7.2 +/- 0.2 K, while in Ba2LiOsO6 a static but spatially disordered internal field is found below 8 K. A probabilistic argument is used to show from the observed precession frequencies that the magnetic ground state in Ba2NaOsO6 is most likely to be low-moment (approximate to 0.2 mu(B)) ferromagnetism and not canted antiferromagnetism. Ba2LiOsO6 is antiferromagnetic and we find a spin-flop transition at 5.5 T. A reduced osmium moment is common to both compounds, probably arising from a combination of spin-orbit coupling and frustration.