The switching time and the coercive field in the polarization reversal of ferroelectric (FE) films have been investigated theoretically using the Landau Devonshire free energy expression and the Landau Khalatnikov dynamic equation in this work. Our numerical data show that the switching time is an exponential function of the applied field, and the function implies that there is a definite coercive field in the switching of a FE film. The effects of the thickness and the surface parameter δ on the switching time and the coercive field have been studied, and we found that the coercive field and the switching time could either decrease or increase with decreasing film thickness in film with surface conditions of positive δ or negative δ, respectively. These results are consistent with the switching phenomena reported in experiments regarding the film thickness dependence of the coercive field and the switching time.
The Landau-Khalatnikov theory is used to study the switching phenomenon of weak magnetoelectric (ME) coupling in proper multiferroics. We investigate two different types of ME multiferroics: one with a bare magnetic transition temperature below the Curie temperature of ferroelectricity, and the other type is when the Curie temperature of ferroelectricity is below the bare magnetic transition temperature. We adopt a fourth-order potential for ferromagnetics and sixth-order potential of ferroelectrics in the Landau free energy based on experimental reports. We found that the electric field alone cannot switch the magnetization and that the magnetic field alone cannot switch the polarization in weak ME multiferroics. Some interesting results were found: at a normalized transition temperature, the magnetization drops abruptly. Two switching current peaks appear when magnetic and electric fields are simultaneously applied to the system.
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