Emergence of a ferromagnetic insulating state in LaMnO3/SrTiO3 heterostructures: Role of strong electronic correlations and strain

Inspired by the experimental findings of an exotic ferromagnetic insulating state in LaMnO3/SrTiO3 (LMO/STO) heterostructures, we calculate the electronic and magnetic state of LaMnO3/SrTiO3 superlattices with comparable thicknesses employing ab initio dynamical mean-field theory. Projecting on the low-energy subspace of Mn 3d and Ti 3d states, and solving a multi-impurity problem, our approach emphasizes local correlations at the Mn and Ti sites. We find that a ferromagnetic insulating state emerges due to the intrinsic effects of strong correlations in the system, in agreement with experimental studies. We also predict that, due to electronic correlations, the emerging two-dimensional electron gas is located at the LMO side of the interface. This is in contrast to density-functional theory results that locate the electron gas on the STO side. We estimate the transition temperature for the paramagnetic-to-ferromagnetic phase transition, which may be verified experimentally. Importantly, we also clarify that the epitaxial strain is a key ingredient for the emergence of the exotic ferromagnetic insulating state. This becomes clear from calculations on a strained LaMnO3 system, also showing ferromagnetism which is not seen in the unstrained bulk material.