Spin-polarized two-dimensional electron gas at the EuO/SrTiO$_3$ interface*

The development of novel nano-oxide spintronic devices could benefit greatly from interfacing with emergent phenomena at oxide interfaces. However, due to complicating factors such as thermodynamics and band alignment, the formation of such an interface is nontrivial. In this paper, we integrate highly spin-split semiconductor EuO on SrTiO$_3$ (001) by molecular beam epitaxy. Using density functional theory, we predict a spin-polarized two dimensional electron gas at the EuO/SrTiO$_3$ interface. Even at room temperature, out-diffusion of oxygen from SrTiO$_3$ during EuO epitaxy creates a highly conductive layer of oxygen vacancies on the SrTiO$_3$ side of the interface. The films are ferromagnetic with a Curie temperature of 70 K and display giant magnetoresistance below the transition temperature. Leveraging this approach to offers an as-yet unexplored route to seamlessly integrate ferromagnetism and the oxide two-dimensional electron gas.