Training effect in ferro (F)/antiferromagnetic (AF) exchange coupled systems: Dependence on AF thickness

Systematic investigation on the training effect in polycrystalline ferro (F)/antiferromagnetic (AF) exchange coupled systems using sputtered NiFe/IrMn bilayers has revealed that the type of the training effect changes from one type (Type I) to another (Type II) with increasing the AF layer thickness. In Type I the hysteresis loop in the pinned direction shrinks from both sides with the cycle of the applied field, while in Type II it shifts toward the positive pinned direction accompanied by loop shrinkage. This phenomenon can be understood as the stabilization of both AF grains contributing to the enhancement of coercivity and to the exchange biasing with cycles of applied field. This conclusion is based on our recently proposed extended Fulcomer and Charap’s model in which direct positive and negative exchange coupling between the AF grains is incorporated.