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

When the insulators lanthanum aluminate and strontium titanate are brought together, the interface between them forms a two-dimensional superconductor. Moreover, magnetic imaging of this interface shows that superconductivity and ferromagnetism coexist in separated nanoscale domains. LaAlO 3 and SrTiO 3 are insulating, non-magnetic oxides, yet the interface between them exhibits a two-dimensional electron system with high electron mobility 1 , superconductivity at low temperatures 2 , 3 , 4 , 5 , 6 and electric-field-tuned metal–insulator and superconductor–insulator phase transitions 3 , 6 , 7 , 8 . Bulk magnetization and magnetoresistance measurements also indicate some form of magnetism depending on preparation conditions 5 , 9 , 10 , 11 and a tendency towards nanoscale electronic phase separation 10 . 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 localized 14 , 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.