  The single crystal growth, structure, and basic magnetic properties of
ErMgGaO4 are reported. The structure consists of triangular layers of magnetic
ErO6 octahedra separated by a double layer of randomly occupied non-magnetic
(Ga,Mg)O5 bipyramids. The Er atoms are positionally disordered. Magnetic
measurements parallel and perpendicular to the c axis of a single crystal
reveal dominantly antiferromagnetic interactions, with a small degree of
magnetic anisotropy. A weighted average of the directional data suggests an
antiferromagnetic Curie Weiss temperature of approximately -30 K. Below 10 K
the temperature dependences of the inverse susceptibilities in the in-plane and
perpendicular-to plane directions are parallel, indicative of an isotropic
magnetic moment at low temperatures. No sign of magnetic ordering is observed
above 1.8 K, suggesting that ErMgGaO4 is a geometrically frustrated magnet.
