Curie-Weiss-type law for the strain and stress effects on the dielectric response of ferroelectric thin films

Variations of the dielectric properties of ferroelectric thin films with the misfit strain S-m in the film/substrate system and the associated biaxial stress sigma inside the film are analyzed theoretically. Calculations are performed in a mean-field approximation for the dielectric response displayed in a plate-capacitor setup by single crystalline films epitaxially grown on tensile substrates (S-m> 0). It is shown that, in the absence of misfit-strain-induced phase transitions, the film dielectric susceptibility eta(33) obeys the Curie-Weiss-type law eta(33)(S-m) = epsilon(0)K(S)/(S-m-S-m*), eta(33)(sigma) = epsilon(0)K(sigma)/(sigma-sigma*). Theoretical predictions are compared with the measured stress dependence of the dielectric response of polycrystalline Ba0.7Sr0.3TiO3 films grown on Pt/SiO2/Si. The theory explains the observed dielectric behavior and predicts the existence of in-plane polarization state in this film/substrate system. (C) 2000 American Institute of Physics. [S0003-6951(00)03542-7].