Enhancement of exchange bias field in top-pinned FeMn/Py bilayers with Ta/Cu hybrid underlayers

We compared the exchange bias field H_ex of bottom-pinned Py/FeMn and top-pinned FeMn/Py bilayers with Ta/Cu hybrid underlayers for both as-deposited and annealed cases. Top-pinned bilayers show higher H_ex than bottom-pinned bilayers. It is more than two times higher than that of bottom-pinned bilayers and even more than one order of magnitude higher than that of top-pinned bilayer with Ta single underlayers. Furthermore, top-pinned bilayers exhibit enhanced H_ex after field cooling, while bottom-pinned bilayers remain almost unchanged. Dramatic increase in intensity of FeMn peak due to hybrid underlayer and FeMn/Py interface roughness dominantly result in the enhanced exchange bias field in the as-deposited case. On the other hand, the microstructural change like interface morphology and FeMn chemical composition at FeMn/Py local interface rather than crystallographic texture of FeMn layer play a key role in enhancing the exchange bias field in annealed case. Our results cannot be explained by the results from the previous studies on IrMn/CoFe bilayers. We suggest that the stoichiometric Fe ₅₀Mn₅₀ ratio around FeMn/Py interface as well as FeMn(111) texture should be responsible for high H_ex of top-pinned bilayers with Ta/Cu hybrid underlayers.