Field dependent anisotropic magnetoresistance measurements on CoFe/IrMn bilayers

The exchange anisotropy field, along with the qualitative distribution of the local pinning field at the ferromagnetic/antiferromagnetic interface, has been studied in polycrystalline CoFe/IrMn bilayers. The exchange anisotropy was studied using both the anisotropic magnetoresistance (AMR) technique and hysteresis loop measurements. The AMR technique determines a larger value for the exchange anisotropy than the hysteresis loop technique. It is observed that H _ea extracted via the AMR technique for the majority of samples shows a quantifiable dependence on the applied measurement field, even at fields much less than the exchange bias measured by hysteresis loop. It is believed that a reversible measurement of the soft dispersive moments takes place at these low fields, and that the field dependence is indicative of a distribution of the local magnetization direction.