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

Abstract

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 temperatures2-6 and electric-field-tuned metal-insulator and superconductor-insulator phase transitions3,6-8. Bulk magnetization and magnetoresistance measurements also indicate some form of magnetism depending on preparation conditions5,9-11 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 orbitals12,13. 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 measurements3,6,7. 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. © 2011 Macmillan Publishers Limited. All rights reserved.