The polarization properties in ferroelectric nanofilms investigated by molecular dynamics simulation

The polarization switching and strain effects on the ferroelectric characters of BaTiO₃ films are studied using molecular dynamics method based on a shell model. We observed the strip domains and the polarization switching in nanofilms, and found that the surface and inner polarization switch under the same electric field, although the surface polarization is weaker than the inner value. The strain effect is also studied from microscopic view. It is found that crystal cell distortions due to the in-plane strain will aggravate or relieve the separation of negative and positive charge centers. As a result, the in-plane strains can significantly influence the spontaneous polarization and even critical thickness of ferroelectric films. A logarithm relation between the in-plane strains and spontaneous polarizations is obtained via our simulations. A quantitative relation between the in-plane strains and critical thicknesses is also given out and then compared with Landau-Ginzburg theory, which displays a good agreement. This provides a possible interpretation on the discrepancies among the experimental measurements of the critical thicknesses.