Enhanced exchange bias due to an ultra-thin, non-magnetic insulator spacer layer

The exchange bias effect of Co/CoO bilayers is found to significantly increase when an ultra-thin, non-magnetic insulating AlO_x spacer is added at the ferromagnet/antiferromagnet interface. The dependence on the spacer thickness is non-monotonic with a maximum two-fold enhancement of the exchange bias field H_EB at a spacer thickness of 1.0 nm. These observations are explained in terms of a discontinuous layer structure of the spacer and its thickness evolution. There is no indication of a magnetic interaction across the AlO_x spacer layer.