  Geometric magnetic frustration (GMF) has attracted substantial interest due
to the exotic physics and rich phase diagrams revealed by the cancellation of
normally-dominant magnetic interactions, giving impetus for the search for
novel frustrated systems, most often based on antiferromagnetic correlations
between magnetic ions decorating triangular or tetrahedral lattices. We report
here low-temperature magnetic susceptibility and muon spin relaxation results
on Li4MgOsO6 and Li3Mg2OsO6, members of the A5BO6 "rock salt ordered" family of
frustrated materials. In Li3Mg2OsO6 we find spin freezing below 12K. In
Li4MgOsO6, which can crystallize into either orthorhombic Fddd or monoclinic
C2/m crystal symmetries depending on synthesis conditions, we find magnetism
consistent with glassy-like behavior dominating below 2K, with partial ordering
and evidence for dynamics at somewhat higher temperatures.
