  We report torque magnetometry measurements of an oxide heterostructure
consisting of an amorphous Al$_2$O$_3$ thin film grown on a crystalline
SrTiO$_3$ substrate ($a$-AO/STO) by atomic layer deposition. We find a torque
response that resembles previous studies of crystalline LaAlO$_3$/SrTiO$_3$
(LAO/STO) heterointerfaces, consistent with strongly anisotropic magnetic
ordering in the plane of the interface. Unlike crystalline LAO, amorphous
Al$_2$O$_3$ is nonpolar, indicating that planar magnetism at an oxide interface
is possible without the strong internal electric fields generated within the
polarization catastrophe model. We discuss our results in the context of
current theoretical efforts to explain magnetism in crystalline LAO/STO.
