Nonlinear Dielectric Properties and Polarization in Thin Ferroelectric P(VDF-TrFE) Copolymer Films

Thin VDF-TrFE copolymer films of molar composition 70/30 and thickness from 25 nm to 175 nm have been prepared by spin coating on a glass substrate covered with an aluminum electrode. The linear, second and third order permittivities epsilon(1), epsilon(2) and epsilon(3) of poled and unpoled samples have been studied versus temperature in heating and cooling cycles up to 120 degrees C, i.e., above the Curie temperature. The polarization and its temperature dependence are derived from epsilon(1) and epsilon(2). This allows a non-destructive readout of the polarization state. It is found that above the 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.