Competing magnetic interactions in perpendicular exchange-biased [Co/Pt]_y/FeMn multilayers

摘要

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 FMonmagnet 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.