Study of equilibrium carrier transfer in LaAlO₃/SrTiO₃ from an epitaxial La_1−xSr_xMnO₃ ferromagnetic layer

Using x-ray magnetic circular dichroism and ab-initio calculations, we explore the La_1−xSr_xMnO₃/LaAlO₃/SrTiO₃ (001) heterostructure as a mean to induce transfer of spin polarized carriers from ferromagnetic La_1−xSr_xMnO₃ layer into the 2DEG (two-dimensional electron gas) at the LaAlO₃/SrTiO₃ interface. By out-of-plane transport measurements, the tunneling across the LaAlO₃ barrier is also analyzed. Our results suggest small or vanishing spin-polarization for the 2DEG: magnetic dichroism does not reveal a neat signal on Ti atoms, while calculations predict, for the pristine stoichiometric interface, a small spin-resolved mobile charge of 2.5 × 10¹³ cm⁻² corresponding to a magnetic moment of 0.038 μ _B per Ti atom, tightly confined within the single SrTiO₃ layer adjacent to LaAlO₃. Such a small magnetization is hard to be detected experimentally and perhaps not robust enough to survive to structural disorder, native doping, or La_1−xSr _xMnO₃ dead-layer effects. Our analysis suggests that, while some spin-diffusion cannot be completely ruled out, the use of ferromagnetic La_1−xSr_xMnO₃ epilayers grown on-top of LaAlO₃/SrTiO₃ is not effective enough to induce robust spin-transport properties in the 2DEG. The examined heterostructure is nevertheless an excellent test-case to understand some fundamental aspects of the spin-polarized charge transfer in 2D wells. (paper)