Abrupt changes and hysteretic behavior of90°domains in epitaxial ferroelectric thin films with misfit dislocations

Abstract

The influence of misfit dislocations on the statics and dynamics of 90° domains (twins) in epitaxial thin films is studied theoretically. The misfit dislocations create additional potential barriers, which hinder the movements of domain walls along the film/substrate interface. For ferroelastic 90° walls, these barriers are considerable because an elastic interaction exists between 90° domains and misfit dislocations owing to the coupling of the dislocation stresses to the spontaneous strains generated at the proper or improper ferroelastic phase transition. The energy of elastic interaction is evaluated for a single 90° domain embedded into a tetragonal film containing a periodic array of edge misfit dislocations. On this basis. the equilibrium width of a single embedded 90° domain is calculated as a function of the misfit strain in the film/substrate systems. It is found that, at some threshold values of the misfit strain, the gradual variation of the domain width is interrupted by steplike changes. The evolution of the 90° domain pattern during the polarization switching in ferroelectric films with misfit dislocations is analyzed in detail. The calculations show that the electric-field-induced growth of an embedded 90° domain involves three different stages. Remarkably, at some critical value of the applied electric field, one of the 90° domain walls jumps over the neighboring misfit dislocation, which is accompanied by abrupt changes of the domain width and position in the film. The steplike movements of 90° domain walls over the misfit dislocations are proposed as a possible microscopic mechanism of the Barkhausen effect occurring in ferroelectric thin films during the polarization reversal. The hysteretic behavior of 90° domains at the cyclic variation of an applied electric field is also demonstrated.