  Metal-insulator transition is observed in the La0.8Sr0.2MnO3 thin films with
thickness larger than 5 unit cells. Insulating phase at lower temperature
appeared in the ultrathin films with thickness ranging from 6 unit cells to 10
unit cells and it is found that the Mott variable range hopping conduction
dominates in this insulating phase at low temperature with a decrease of
localization length in thinner films. A deficiency of oxygen content and a
resulted decrease of the Mn valence have been observed in the ultrathin films
with thickness smaller than or equal to 10 unit cells by studying the
aberration-corrected scanning transmission electron microscopy and electron
energy loss spectroscopy of the films. These results suggest that the existence
of the oxygen vacancies in thinner films suppresses the double-exchange
mechanism and contributes to the enhancement of disorder, leading to a decrease
of the Curie temperature and the low temperature insulating phase in the
ultrathin films. In addition, the suppression of the magnetic properties in
thinner films indicates stronger disorder of magnetic moments, which is
considered to be the reason for this decrease of the localization length.
