We report the observation of weak magnetism in superlattices of ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ using $\beta$-detected nuclear magnetic resonance. The spin lattice relaxation rate of ${}^8\mathrm{Li}$ in superlattices with a spacer layers of 8 and 6 unit cells of ${\mathrm{LaAlO}}_3$ exhibits a strong peak near $\sim 35\mathrm{K}$, 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 ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ 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 $\sim 4–6$ 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 ($\sim 0.002{\mu }_B$) in the two samples with a larger ${\mathrm{LaAlO}}_3$ spacer thickness.

• Received 9 January 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.257207