Hysteresis and the dynamic phase transition in thin ferromagnetic films

Hysteresis and the nonequilibrium dynamic phase transition in thin magnetic films subject to an oscillatory external field have been studied by Monte Carlo simulation. The model under investigation is a classical Heisenberg spin system with a bilinear exchange anisotropy Λ in a planar thin film geometry with competing surface fields. The film exhibits a nonequilibrium phase transition between dynamically ordered and dynamically disordered phases characterized by a critical temperature T_cd, whose location is determined by the amplitude H₀ and frequency ω of the applied oscillatory field. In the presence of competing surface fields the critical temperature of the ferromagnetic-paramagnetic transition for the film is suppressed from the bulk system value T_c to the interface localization-delocalization temperature T_ci. The simulations show that in general T_cd<T_ci for the model film. The profile of the time-dependent layer magnetization across the film shows that the dynamically ordered and dynamically disordered phases coexist within the film for T<T_cd. In the presence of competing surface fields, the dynamically ordered phase is localized at one surface of the film.