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

The magnetic ground states of the isostructural double perovskites Ba${}_2$NaOsO${}_6$ and Ba${}_2$LiOsO${}_6$ are investigated with muon-spin rotation. In Ba${}_2$NaOsO${}_6$ long-range magnetic order is detected via the onset of a spontaneous muon-spin precession signal below $T_{$\mathrm{c}$}=7.2\pm 0.2;$\mathrm{K}$$,whileinBa$_2$LiOsO$_6$ a static but spatially-disordered internal field is found below 8 K. A novel probabilistic argument is used to show from the observed precession frequencies that the magnetic ground state in Ba${}_2$NaOsO${}_6$ is most likely to be low-moment ($\approx 0.2$,${\mu }_{\mathrm{B}}$) ferromagnetism and not canted antiferromagnetism. Ba${}_2$LiOsO${}_6$ 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.