Electronic and magnetic properties of La_0.67Sr_0.33MnO₃ ultrathin films

Full text: One of the most studied ferromagnetic oxides is the La_0.67Sr_0.33MnO₃ (LSMO). It has a perovskite structure, high electric conductivity and is a half metal with almost 100 % spin polarization at room temperature. These characteristics make LSMO thin films very interesting for applications in spintronics and oxide-based devices. However, the formation of layers that exhibit no magnetism at the interfaces of LSMO epitaxial films, called magnetic dead layers (MDL), is an obstacle for the development of new devices. Although previous studies have linked the formation of MDL to strain, oxygen vacancies and cation non-stoichiometry, its microscopic origin is still not entirely clear. In this work, we grew, 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 MDL. The samples' electronic structure was investigated by Mn L2,3- edge X-ray absorption spectroscopy (XAS) and X-ray linear dichroism (XLD). The magnetic properties were investigated by Mn L2,3-edge X-ray magnetic circular dichroism (XMCD) and vibrating sampling magnetometer. Our results suggest a reduction in the Mn valence and a consequent decrease in the magnetic moment as the LSMO thickness decreases. We also observe a breakdown in the eg orbitals degeneracy at the LSMO surface, leading to an orbital reconstruction. Our results corroborate the formation of MDL at the surfaces of the ultrathin films of LSMO. (author)