In this paper, we study the thermally activated reversal of IrMn/CoFe exchange-coupled structures using Lorentz microscopy and magnetometry. An asymmetry and a training effect were found on the hysteresis loops both with and without holding the film at negative saturation of the ferromagnetic layer. Holding the film at negative saturation results in the hysteresis loop shifting toward zero field. We believe that, in this system, two energy barrier distributions with different time constants coexist. The large-time-constant thermally activated reversal of the antiferromagnetic layer contributes to a increasing shift of the entire hysteresis loop toward zero field with increased period of time spent at negative saturation of the ferromagnetic layer. The small-time-constant thermal activation contributes to asymmetry in the magnetization reversal and training effects
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