Spin structure of exchange-biased NiFeFeMnNiFe trilayers

The magnetic hysteresis curves of NiFe (t) FeMn (15 nm) NiFe (5 nm) exchange-biased trilayers, with t ranging from 3.8 to 18 nm, show two clearly separated loops, which were interpreted as corresponding to the independent magnetic reversal of the two ferromagnetic (FM) NiFe layers. Polarized neutron reflectometry allowed the determination of the evolution of these two layer magnetizations at various fields along the hysteresis curve. For increasing t, the behavior of the magnetization evolved from one where both layer magnetizations were always collinear to the applied field to one where rotation of the magnetizations was observed at various fields. Measurements on the thickest field-annealed sample additionally showed that, during the reversal, the magnetizations of the two NiFe layers are close to being perpendicular to each other. This magnetic configuration is discussed in terms of the FM/antiferromagnetic (AFM) interactions and of a possible coupling between the two FM layers across the thick AFM FeMn spacer.