Magneto-optic spectroscopy perovskite-type La1-x(Ba,Sr) xMnO3

Epitaxial thin films of ferromagnetic perovskite-type La_1-x (Ba,Sr)_xMnO₃ have been studied at room temperature by means of polar Kerr rotation and ellipsometric measurements over a wide photon energy range. The Kerr-rotation spectra show two prominent features, one at 2 eV and a second, larger one around 3.2 eV but the spectra do not differ much with Sr or Ba substitution. This indicates that exclusively electronic transitions involving Mn-3d and O-2p electrons are magnetooptically active. The Kerr rotation reaches a maximum value of -0.23° in La_0.7Sr_0.3MnO₃ at a photon energy of 3.2 eV. The structure at 2 eV is tentatively assigned to a dipole-forbidden, spin-allowed intra-Mn transition between crystal-field split 3d levels. The feature at 3.2 eV arises from a charge-transfer transition from occupied O-2p into empty Mn-3d levels. At low Sr doping (x=0.1), a narrow peak appears in the Kerr spectra at 4.6 eV. The full width at half maximum is estimated to be 0.25 eV. This is a factor of three smaller than the width of the two structures at lower energies. From the narrow width it is concluded that well localized electronic states are involved. Yet, the origin of this structure remains unclear