Fourfold oscillation in anisotropic magnetoresistance and planar Hall effect at the LaAlO_3/SrTiO_3 heterointerfaces: Effect of carrier confinement and electric field on magnetic interactions

摘要

The confinement of the two-dimensional electron gas (2DEG), preferential occupancy of the Ti 3d orbital, and strong spin-orbit coupling at the LaAlO_3/SrTiO_3 interface play a significant role in its emerging properties. Here we report a fourfold oscillation in the anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) at the LaAlO_3/SrTiO_3 heterointerface. We evaluate the carrier confinement effects on the AMR and find that the fourfold oscillation appears only for the case of 2DEG systems while it is twofold for the three-dimensional system. The AMR behavior is further found to be sensitive to applied electric field, emphasizing the significance of spin-orbit coupling at the interface. These confinement effects suggest that the magnetic interactions are predominant at the interface, and the gate electric field modulation of AMR shows the tunability of magnetic interactions. As the fourfold oscillation fits well to the phenomenological model for a cubic symmetry system, this suggests that the origin of the oscillation may be linked to the anisotropy in magnetic scattering arising from the interaction of electrons with the localized magnetic moments coupled to the crystal symmetry. The observed large PHE further indicates the in-plane nature of magnetic ordering that arises from the in-plane Ti 3d_(xy) orbitals.