19. 1. 2022 Farzin Abadizaman: Ellipsometry measurements on Lanthanum Cobaltites Abstract: We have performed spectroscopic ellipsometry measurements from room temperature down to 7 K on LaCoO3 and Sr-doped LaCoO3 thin films on LSAT to investigate the mechanism of ferromagnetism (FM). It has been known that while bulk LaCoO3 is non-magnetic, both doping and strain render the LaCoO3 ferromagnetic. Our ellipsometry data show that the magnetic phase transition in both strain-induced and doping-induced FM is accompanied by a transfer of the spectral weight from finite energies to the lower energies. In the case of doping-induced FM, the transfer of spectral weight is from wrong spin transition region toward the free electrons (Drude), which is the hallmark of double exchange interaction, whereas shift of the spectral weight in the strain-induced FM indicates the stabilization of the high-spin state, perhaps by saving the kinetic energy. Furthermore, we observe that increasing the doping concentration increases the magnetic moment per Co atom and decreases the energy of the wrong spin transition, a trend that defies the theoretical expectations.