  Magneto optic measurements are a very powerful tool for investigating the
polarization of a conduction band as a function of temperature and are used
here to study the polarization of the mobile electrons in 50nm LSMO (x=0.3)
strained thin films grown epitaxially on single crystalline (001) LaAlO3 (LAO)
and (001) lattice matched substrate (LSAT). The magnetic circular dichroism
(MCD) has been investigated in magnetic fields up to 0.5 T and over a
temperature range (10 to 450 K). The MCD spectra of both the films show a peak
at the band gap at around 3 eV and the peak is found to be shifted towards
lower energy side with the increase of temperature. A separate polaron peak
(well known in insulating samples) appears at lower energy (about 1.8 eV) with
the increase of temperature in all these metallic films. The rapid decrease in
conduction band polarization in the film on LAO has strong implications for the
use of these manganites in room temperature spintronics.
