Origin of insulating weak-ferromagnetic phase in ultra-thin La0.67Sr0.33MnO3 films on SrTiO3 substrate

We investigate the origin of insulating weak-ferromagnetic phase in ultra-thin epitaxial La0.67Sr0.33MnO3 (LSMO) films on SrTiO3 substrate using density functional theory calculation together with X-ray linear dichroism (XLD). The calculations show that symmetry breaking of the crystal field at the LSMO surface largely lowers the energy level of Mn d3z2 orbital at the surface and leads to full occupancy of the d3z2 orbital in majority spin channel, and XLD spectra clearly show the preferential occupation of Mn d3z2 orbital at the surface. Such an orbital reconstruction and charge redistribution in the ultra-thin films largely suppresses double-exchange interaction and favors super-exchange interaction, resulting in G-type antiferromagnetic spin ordering and insulating state. The anisotropic exchange interaction due to spin-orbital interaction leads to spin canting, and thus the films show weak ferromagnetism.