Monte Carlo simulations of the electrocaloric effect in relaxor ferroelectrics

By combination of the canonical and microcanonical Monte Carlo simulations a direct evaluation of the electrocaloric (ECE) effect is carried out in relaxor ferroelectrics. A Hamiltonian is introduced, which includes a thermal energy, a Ginzburg-Landau static ground state term, a dipole-dipole interaction energy, a domain wall energy that arising from the short-range interaction and an electrostatic energy contribution describing the coupling to external and random fields. By incorporating the frozen random dipoles random fields are induced to reproduce the relaxor behavior. The influence of the density of the frozen dipoles on the hysteresis and ECE is investigated. The hysteresis begins to resemble a relaxor-type with 20% density of the frozen dipoles. Upon increasing the density of the frozen random dipoles, the ECE peak position shifts to a lower temperature but the temperature variation is reduced. In relaxor ferroelectrics the ECE is maximum at the freezing temperature where the nonergodic-to-ergodic transition takes place. Our results, especially the evolving domain structures, imply that the entropy variation in an ECE cycle is reduced since the random frozen dipoles destabilize the polarization of their neighbor sites.