Symmetry and asymmetry during magnetization reversal in exchange biased multilayers and bilayers

Abstract

We have studied the magnetization reversal process in continuous: ${[\mathrm{Co}∕\mathrm{Co}\mathrm{O}]}_{20}$ and separated: ${[\mathrm{Co}∕\mathrm{Co}\mathrm{O}∕\mathrm{Au}]}_{20}$ exchange-biased polycrystalline multilayers (MLs). For continuous ML, reversal proceeds sequentially starting with the bottom (top) Co layer for increasing (decreasing) field. Each Co layer remagnetizes symmetrically for both field branches in a nonuniform mode similarly as we have observed earlier for ${[\mathrm{Ir}\mathrm{Mn}∕\mathrm{Co}\mathrm{Fe}]}_{3∕10}$ MLs [Phys. Rev. B. 70, 224410 (2004)]. By polarized neutron reflectivity, we observe increasing exchange bias field strengths down the stack. However, usual asymmetric reversal is observed for the separated ML. We explain the different magnetization behavior within a simple and general model. The increased anisotropy energy for continuous ML is responsible for the nonuniform symmetric reversal as the angular dependencies for reversal are guided by the relative strengths of exchange, anisotropy, and Zeeman energies.