Hysteresis loop shape of field-driven antiferromagnetic-ferromagnetic bilayers in experimental conditions

We simulate the field-driven magnetization reversal in the exchange-coupled antiferromagnetic-ferromagnetic bilayers, adapting the weakly disordered random-field Ising model to mimic experimental conditions with finite temperatures and driving field rates. Our results reveal how the theoretical ideal hysteresis loop shape with a central loop and side branches with multiple small steps corresponding to a given architecture at low temperatures and slow driving becomes systematically deformed under the combined impact of the elevated temperatures and driving speed. Notably, the loop becomes smoother with the number of steps diminishing as temperature and driving rate increase. The coercive field demonstrates a systematic increase with the driving rate, characterized by a power law with a universal exponent of 1/2, known in random ferromagnets; meanwhile, the nonuniversal factor depends on temperature and the antiferromagnetic layer thickness. For moderate driving rates, the coercivity decays exponentially with the increasing temperature, and the central loop eventually collapses above the ferromagnetic transition temperature, with two side branches dominating the loop shape. However, for substantial driving rates, the incomplete spin relaxation before the new field is applied prevents such thermal effects, and the loop remains almost rectangular, with a step on each branch. Predicting a range of parameters where the desired hysteresis shape is stable, these findings can stimulate further experimental research and applications of the field-driven bilayers.