We report the observation of weak magnetism in superlattices of LaAlO3/SrTiO3 using beta-detected nuclear magnetic resonance. The spin lattice relaxation rate of Li-8 in superlattices with a spacer layers of 8 and 6 unit cells of LaAlO3 exhibits a strong peak near similar to 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 similar to 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 (similar to 0.002 mu(B)) in the two samples with a larger LaAlO3 spacer thickness. DOI: 10.1103/PhysRevLett.109.257207