  The observation of ferroelectric, ferromagnetic and ferroelastic phases in
thin films of binary oxides attract the broad interest of scientists and
engineers. However, the theoretical consideration of observed behaviour
physical nature was performed mainly for HfO2 thin films from the first
principles, and in the framework of Landau-Ginzburg-Devonshire (LGD)
phenomenological approach with a special attention to the role of oxygen
vacancies in both cases. Allowing for generality of the LGD theory we applied
it to the group of binary oxides in this work. The calculations have been
performed in the assumption that oxygen vacancies, as elastic dipoles, can be
partially transformed into electric dipoles due to the defect site-induced
and/or surface-induced inversion symmetry breaking (via e.g. piezoelectric
effect), and can "migrate" entire the depth of an ultra-thin film. We
calculated the dependence of the film polarization on the applied voltage for
room temperature and different film thickness. Since the many films of binary
oxide are ferroelectric and ferromagnetic due to the same oxygen vacancies,
they can be multiferroics. Performed calculations have shown that thin films of
binary oxides can be considered as a new wide class of multiferroics with broad
spectra of physical properties useful for application in nanoelectronics and
nanotechnology. The properties can be controlled by the choice of oxygen
vacancies concentration, film thickness and special technological treatment,
such as annealing.
