  Magnetotransport and superconducting properties are investigated for
uniformly La-doped SrTiO3 films and GdTiO3/SrTiO3 heterostructures,
respectively. GdTiO3/SrTiO3 interfaces exhibit a high-density two-dimensional
electron gas on the SrTiO3-side of the interface, while for the SrTiO3 films
carriers are provided by the dopant atoms. Both types of samples exhibit
ferromagnetism at low temperatures, as evidenced by a hysteresis in the
magnetoresistance. For the uniformly doped SrTiO3 films, the Curie temperature
is found to increase with doping and to coexist with superconductivity for
carrier concentrations on the high-density side of the superconducting dome.
The Curie temperature of the GdTiO3/SrTiO3 heterostructures scales with the
thickness of the SrTiO3 quantum well. The results are used to construct a
stability diagram for the ferromagnetic and superconducting phases of SrTiO3.
