Low-frequency dynamic hysteresis in exchange-coupled Ni₈₁Fe₁₉/Ir₂₂Mn₇₈ bilayers

The dynamics of hysteresis, including the training effect, the field sweep rate dependence, and the field strength dependence of the exchange bias and coercivity, is experimentally investigated in exchange-coupled Ni₈₁Fe₁₉/Ir₂₂Mn₇₈ bilayers in the low-frequency range. The dependence of the exchange field and the coercivity with the number of measurement cycles is well described by a power-law function in which the index varies with the Ir₂₂Mn₇₈ thickness. We have also found that the exchange bias depends upon the sweep rate according to a power law. This “training effect” and the dynamic response of the exchange biasing can be explained by a thermal fluctuation model in which the antiferromagnet is assumed to be composed of grains which undergo thermal fluctuations.