Role of ferromagnetic spin structure in magnetization reversal and exchange bias phenomena

Recently, the dependence of exchange bias (EB) on ferromagnetic layer thickness (t(FM)) and temperature has become a matter of controversy, triggering renewed research efforts to decipher the key aspects of these intriguing phenomena. To demonstrate these anomalous dependences linked to the ferromagnetic spin structures, a modified Monte Carlo method is used on models of NiFe/antiferromagnet and Fe/antiferromagnet bilayers where a twisted configuration is favored along the film depth in the ferromagnetic layer during magnetization reversals. It is found that the EB field deviates from the reciprocal relation with tFM towards a lower value as tFM increases, and the maximum value of the EB field with temperature appears at 97 K for NiFe and 123 K for Fe, not the lowest temperature. The decrease in the EB field with t(FM) may also depend on the thickness of the antiferromagnetic layer and the twisted angle. The twisted configuration formed in a thick ferromagnetic layer encourages the propagation of domain walls through the ferromagnetic film to decrease the external switching field, leading to a suppressed EB. On the other hand, this twisted configuration simultaneously widens the domain walls and removes the unstable pinning points at relatively high temperatures. As a result, EB is recovered.