Direct imaging of the coexistence of ferromagnetism and superconductivity at the LaAlO₃/SrTiO₃ interface

LaAlO₃ and SrTiO₃ are insulating, non-magnetic oxides, yet the interface between them exhibits a two-dimensional electron system with high electron mobility¹, superconductivity at low temperatures^2-6 and electric-field-tuned metal-insulator and superconductor-insulator phase transitions^3,6-8. Bulk magnetization and magnetoresistance measurements also indicate some form of magnetism depending on preparation conditions^5,9-11 and a tendency towards nanoscale electronic phase separation¹⁰. Here we use local imaging of the magnetization and magnetic susceptibility to directly observe a landscape of ferromagnetism, paramagnetism and superconductivity. We find submicrometre patches of ferromagnetism in a uniform background of paramagnetism, with a non-uniform, weak diamagnetic superconducting susceptibility at low temperature. These results demonstrate the existence of nanoscale phase separation as indicated by theoretical predictions based on nearly degenerate interface sub-bands associated with the Ti orbitals^12,13. The magnitude and temperature dependence of the paramagnetic response indicate that the vast majority of the electrons at the interface are localized¹⁴, and do not contribute to transport measurements^3,6,7. In addition to the implications for magnetism, the existence of a two-dimensional superconductor at an interface with highly broken inversion symmetry and a ferromagnetic landscape in the background indicates the potential for exotic superconducting phenomena.