Origin of spontaneous magnetization reversal in exchange biased heterostructures

The dependence of thermally induced spontaneous magnetization reversal on time-dependent cooling protocols implies that this reversal is energetically favored, contrary to the prediction of existing positive exchange bias models. Slow cooling through and long waiting near the N\'eel temperature of the antiferromagnet enhances the magnitude of the magnetization reversal. Further, cycling the temperature around the N\'eel temperature leads to a thermal training effect that causes the reversal magnitude to incrementally increase to a maximum value. An extended model of exchange bias that explicitly considers domain walls forming parallel to the interfaces in either the ferromagnet or antiferromagnet is shown to predict spontaneous reversal, the sign of exchange bias, and to allow a qualitative description of the associated dynamics.