  We report the observation of weak magnetism in superlattices of LaAlO3/SrTiO3
using beta-detected nuclear magnetic resonance. The spin lattice relaxation
rate of 8 Li in superlattices with a spacer layers of 8 and 6 unit cells of
LaAlO3 exhibits a strong peak near ~35 K, whereas no such peak is observed in a
superlattice with spacer layer thickness of 3 unit cells. We attribute the
observed temperature dependence to slowing down of weakly coupled electronic
moments at the LaAlO3/SrTiO3 interface. These results show that the magnetism
at the interface depends strongly on the thickness of the spacer layer, and
that a minimal thickness of ~4-6 unit cells is required for the appearance of
magnetism. A simple model is used to determine that the observed relaxation is
due to small fluctuating moments (~0.002 muB) in the two samples with a larger
LaAlO3 spacer thickness.
