Pinned Co moments in a polycrystalline permalloy/CoO exchange-biased bilayer

We have measured element-specific magnetization depth profiles across the interface between a polycrystalline ferromagnet and an antiferromagnet in an exchange-biased bilayer of Py/CoO. Using soft x-ray resonant reflectivity we have identified a thin (0.5 nm) layer containing uncompensated Co magnetization at the interface with the Py. The majority of this magnetization follows the external field; however, ∼10% of the magnetization in this interfacial layer is pinned antiparallel to the cooling field used when biasing the sample, consistent with the negative exchange bias in this bilayer system, provided that the pinned Co spins are antiferromagnetically coupled to the ferromagnetic layer.