Direct observation of induced ferromagnetism in SrTiO$_{\mathrm{3}}$ quantum wells confined in GdTiO$_{\mathrm{3}}$ heterostructures

Complex oxide thin film heterostructures with charge transfer at the interface provide a uniquely tunable environment in which to study the physics of highly-correlated and quantum-confined electron systems. Previous studies using magnetoresistance measurements had suggested that thin SrTiO$_{\mathrm{3}}$ layers become ferromagnetic when grown between thicker GdTiO$_{\mathrm{3}}$ layers. Here we report the direct observation of induced ferromagnetism in SrTiO$_{\mathrm{3}}$ quantum wells using polarized neutron reflectometry (PNR). Four GdTiO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ superlattice structures with varying SrTiO$_{\mathrm{3}}$ layer thickness were grown epitaxially on LSAT (001) substrates by hybrid molecular beam epitaxy. Chemical and magnetic depth-profiles were refined using a combination of x-ray and polarized neutron reflectivity measurements taken at temperatures ranging from 4-300K. We observed a critical thickness below which the SrTiO$_{\mathrm{3}}$ layer have non-zero magnetism in the center of the well. These results are in excellent agreement with the previous magnetoresistance measurements and provide the first direct observation of induced-magnetism in this system.