Measurement of internal stresses via the polarization in epitaxial ferroelectric films

The relations between electrical and mechanical properties of constrained f erroelectric films are analyzed. It is shown that the internal stresses and the elastic constants can be determined through the measurement of the ele ctrical response. The change in the polarization is proportional to interna l stresses due to film-substrate misfit, whereas the linear electrical and electromechanical responses to external field do not depend on the misfit a nd are determined by the film constraint. The theoretical results are succe ssfully applied to PbZr0.2Ti0.8O3 firms on (001) LaAlO3 substrate which exh ibit a considerable increase in the saturation polarization due to epitaxia l stresses. Significant recovery in the piezoelectric constant and suscepti bility is theoretically predicted and experimentally verified for specific film configurations which reduce the degree of constraint. The concept pres ented in this Letter can be expanded to constrained ferromagnetic and super conductor films.