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

The exchange bias effect of $\chem{Co/CoO}$ bilayers is found to significantly increase when an ultra-thin, non-magnetic insulating $\chem{AlO}$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 $H_{\ab{EB}}$ at a spacer thickness of 1.0$\un{nm}$. 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 $\chem{AlO}$x spacer layer.