Unconventional Phase Diagram of Two-Dimensional Electrons at the LaAlO3/SrTiO3 Interface

Controlling the coupling between itinerant electrons and localized spins can lead to exotic magnetic states. A novel model system is the interface between LaAlO3 and SrTiO3, where local magnetic moments are believed to coexist with extended two-dimensional electrons. When the density of the itinerant electrons is tuned above a universal critical density, nc, sub-bands with new orbital symmetries are populated, likely leading to a change in the coupling to the localized spins. In this talk I will present our measurements showing that strong, symmetry-dependent, coupling between itinerant electrons and localized moments leads to an unconventional phase diagram for the LaAlO3/SrTiO3 system. Using anisotropic magnetoresistance and anomalous Hall effect measurements, we identify two phases in the space of carrier density and in-plane magnetic field. At densities n>nc and high fields the system is strongly polarized and shows large crystalline anisotropy. Surprisingly, below a density-dependent critical field which is divergent at nc, the polarization and anisotropy vanish whereas the resistivity sharply rises. This behavior, unobserved in other coupled magnetic systems, could be understood within a model involving a coupling that depends on the symmetry of the itinerant electrons. The interplay between the two phases enables gate-tunable magnetism at the LaAlO3/SrTiO3 interface.