Origin of Interface Magnetism in BiMnO3/SrTiO3 and LaAlO3/SrTiO3 Heterostructures

Possible ferromagnetism induced in otherwise nonmagnetic materials has been motivating intense research in complex oxide heterostructures. Here we show that a confined magnetism is realized at the interface between SrTiO₃ and two insulating polar oxides, BiMnO₃ and LaAlO₃. By using polarization dependent x-ray absorption spectroscopy, we find that in both cases the magnetism can be stabilized by a negative exchange interaction between the electrons transferred to the interface and local magnetic moments. These local magnetic moments are associated with magnetic Ti³⁺ ions at the interface itself for LaAlO₃/SrTiO₃ and to Mn³⁺ ions in the overlayer for BiMnO₃/SrTiO₃. In LaAlO₃/SrTiO₃ the induced magnetism is quenched by annealing in oxygen, suggesting a decisive role of oxygen vacancies in this phenomenon.