Nonlinear dielectric properties and polarization in ferroelectric P(VDF-TrFE) copolymer thin films

VDF-TrFE copolymer thin film of molar composition 70/30 and thickness below 70 nm has been prepared by spin coating on glass substrate covered with aluminum electrode. After polarization at room temperature by some hysteresis cycles, the linear and second order permittivities ε₁ and ε₂ have been studied over temperature in a heating and cooling cycle up to 120 °C i.e., above the Curie temperature. The temperature dependence of polarization is derived from ε₁ and ε₂. It is found that above Curie temperature and after cooling to room temperature a non-switchable polarization remains. This non-switchable polarization points from the bottom to the top electrode and is not influenced by the initial polarization direction.