  The crystal growth, structure, and basic magnetic properties of TmMgGaO4 are
reported. The Tm ions are located in a planar triangular lattice consisting of
distorted TmO6 octahedra, while the Mg and Ga atoms randomly occupy
intermediary bilayers of M-O triangular bipyramids. The Tm ions are
positionally disordered. The material displays an antiferromagnetic Curie Weiss
theta of ~ -20 -25 K, with no clear ordering visible in the magnetic
susceptibility down to 1.8 K; the structure and magnetic properties suggest
that ordering of the magnetic moments is frustrated by both structural disorder
and the triangular magnetic motif. Single crystal magnetization measurements
indicate that the magnetic properties are highly anisotropic, with large
moments measured perpendicular to the triangular planes. At 2 K, a broad
step-like feature is seen in the field-dependent magnetization perpendicular to
the plane on applied field near 2 Tesla.
