Investigation of the oxidation state and orbital reconstruction at the interfaces La_0.67Sr_0.33MnO₃ ultrathin films

Full text: Complex oxides have received great attention in the current technological and scientific scenario for exhibiting promising multifunctional and emergent properties. These arise from the strong electron-electron correlation and the interplay between lattice, orbital, charge and spin ordering. Specifically, the La_0.67Sr_0.33MnO₃ (LSMO) is a ferromagnetic half metal, with perovskite structure, presenting almost 100% spin polarization at room temperature, what makes it very interesting for applications in spintronics and oxide-based devices. However, the formation of magnetic dead layers at thin films interfaces represents an obstacle for the development of new devices. Although previous studies have linked the formation of dead layer to strain, oxygen vacancies and cation non-stoichiometry, its microscopic origin is still not entirely clear [1,2,3]. In this work, we grow, by pulsed laser deposition, epitaxial ultrathin films of LSMO (thicknesses between 1.2 and 8.4 nm) on SrTiO₃ (001) single crystals in order to verify whether the oxidation state of manganese (Mn) and/or an orbital reconstruction at the LSMO interfaces could be related to the formation of the magnetic dead layer. The samples' electronic structure was investigated by Mn L2,3-edge and O K-edge X-ray absorption spectroscopy (XAS) and linear dichroism (XLD), and their magnetic properties by Mn L2,3-edge magnetic circular dichroism (XMCD). All measurements were acquired by total electron yield mode. Our results suggest a reduction in the Mn valence and a consequent decrease in the magnetic moment as the LSMO thickness decreases, i.e. at the interfaces. We also observe a breakdown in the eg orbitals degeneracy at the LSMO surface, leading to an orbital reconstruction which may be related to the loss of magnetism. (author)