Relaxation-to-creep transition of domain-wall motion in a two-dimensional random-field Ising model with an ac driving field

Monte Carlo simulations of a two-dimensional, random-field Ising model with an ac driving field are used to study the relaxation-to-creep transition of domain-wall motion at low temperatures. The resultant complex susceptibility χ=χ′−iχ″ exhibits features in agreement with the experiments of ultrathin ferromagnetic and ferroelectric films: the semicircle and straight line in the χ′-χ″ plot are Cole-Cole signatures of relaxation and creep states, respectively. The exponent β describing the creep motion is measured, and an intermediate state between the relaxation and creep states is identified.