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

Abstract

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 2-6 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 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. Coexistence of ferromagnetism and superconductivity in nature is rare 15-19. The LaAlO 3 /SrTiO 3 (LAO/STO) interface is a new system for studying this coexistence. LAO and STO are both perovskite band insulators with no magnetic order in their bulk form. For LAO grown on the TiO 2-terminated STO substrate, a high-mobility electron gas was observed at the interface 1. Electronic reconstruction, driven by the polar/nonpolar interface, is thought to move charge from the LAO layers across the interface into the STO, causing an effective electronic doping responsible for the observed conductivity 1. The interplay of this effect with oxygen vacancies and structural changes 20 , and the relative contribution of these three effects to the carrier concentration, remains a subject of debate. Significant variability in the physical properties in similar samples indicates that the ground state of this interface system is sensitive to small changes in growth conditions. Superconductivity 2-5 and features interpreted as interface magnetism 5,9,10 have been independently observed at the LAO/STO interface through transport and bulk magnetization measurements. One recent study inferred the existence of both LETTERS NATURE PHYSICS

Insulator-to-Metal Transition at Oxide Interfaces Induced by WO3 Overlayers

“…Gate tunable superconductivity [2,3], strong spin-orbit coupling [4,5] and magnetism [6,7] are some of the many phenomena observed. The origin of this 2DES is a long standing question in the solid state community and recent results indicate that a consistent picture should take into account both the built-in polar field and the presence of point defects [8][9][10].…”

Two-dimensional electron gas at the LaAlO3/SrTiO3 inteface with a potential barrier

“…[1][2][3] It has been shown, that even though the constituent oxides are ordinary band insulators with well-known electronic properties, 4 their interfaces can exhibit a variety of phenomena -from two-dimensional (2D) metallic conductivity, [5][6][7][8][9] and superconductivity, 10 to ferromagnetism 11,12 and coexistence of magnetic order and superconductivity. 1,13,14 Beginning with the paper by Ohtomo and Hwang, 5 the physical properties of the LaAlO 3 /SrTiO 3 interface have been intensively studied in recent years, both experimentally and theoretically. The main peculiarity of such an interface is its perfectly metallic conductivity, with a relatively high electronic mobility, that can be described by the model of 2D electron gas.…”

Theory of Multi-Orbital Superconductivity in SrTiO3/LaAlO3Interface

“…For instance, electron gases with a high carrier density on the order of 10 13 cm −2 have been realized in SrTiO 3 /LaAlO 3 (STO/LAO) interfaces [1], and SrTiO 3 (STO) surfaces [2]. The discovery of superconductivity [3], ferromagnetism [4][5][6], and their coexistence [5,6] shed light on innovating phenomena in these systems. These quantum condensed phases are controlled by a gate voltage through the change of carrier density [2,7,8].…”