Influence of thermal vibrations on polarization switching in the model of local fields

Abstract

A microscopic model to simulate the polarization dynamics, the model of local fields, is improved by considering thermal vibrations. The model is based on a sequence of single dipole flips which are thermally activated. The time to flip a single dipole depends on its deterministic transition rate which depends on the local electric field and on a probabilistic factor. In each step, the dipole with the shortest flip time is switched. Thermal vibrations of the dipoles cause changes of the distances between the dipoles. The variation of distances effects variations of the local field at the dipoles. In the framework of the extended model, these variations are considered by multiplying the local fields in each step with a Gaussian distributed random number. The model is applied to simulate polarization switching and polarization hysteresis loops of two and of three dimensional systems based on the barium titanate structure. The simulations yield intrinsic dead layers close to the electrodes and around defects which cannot be switched even in very strong fields. These nonswitchable layers are nuclei for domains and thus nuclei for polarization switching. The switching time of the system vastly decreases with the amplitude of the thermal vibrations. Moreover, the thermal vibrations enable the polarization switching in low external fields and decrease the coercive fields.