Competing Magnetic Interactions in Perpendicular Exchange-Biased [Co/Pt]y/FeMn Multilayers

Perpendicular exchange bias in multilayers arises from a complex interplay between unidirectional anisotropy at the terminating ferromagnet (FM)/antiferromagnet (AFM) interface, the perpendicular anisotropy of the FM/nonmagnet multilayer stack, and the overall magnetostatic energy of the structure. Exchange bias field ( H eb ) and coercivity ( H c ) of [Co/Pt] y /FeMn with perpendicular anisotropy have been investigated by varying the thickness of a top Co layer in direct contact with the FeMn or number of Co/ Pt bilayers. An unusual dependence of H eb and H c on these parameters has been observed. As the top Co layer thickness of [Co/Pt] y /FeMn multilayer varies, both H eb and coercivity H c show a peak in values and decrease when the top Co is too thin or too thick. H eb of [Co/Pt] y /FeMn is inversely proportional to the number of Co/Pt bilayers, y for 2 ≤ y ≤ 5, while H c increases. For y > 5, H eb increases and H c decreases with y until both of them reach constant values. These observations have been attributed to the role of the effective perpendicular anisotropy of the FM multilayer, especially the FM layer adjacent to the AFM layer, in maintaining the perpendicular exchange bias.