Pinned magnetic moments in exchange bias: Role of the antiferromagnetic bulk spin structure

Magneto-optical Kerr effect measurements of epitaxial AFM/FM bilayers and FM/AFM/FM trilayers on Cu₃Au(001), where ``AFM'' stands for a Ni₂₅Mn₇₅ antiferromagnetic layer, and ``FM'' for ferromagnetic layers that are either Ni or Ni/Co with out-of-plane or in-plane easy axis of magnetization, show that trilayers with collinear magnetization directions of both FM layers exhibit always a much lower exchange bias field Heb at a fixed temperature compared to bilayers of the same Ni₂₅Mn₇₅ thickness. At the same time, the blocking temperature for exchange bias Tb is strongly reduced. In trilayers with orthogonal easy axes of the two FM layers (in-plane and out-of-plane), in contrast, both Heb and Tb are nearly identical to that of the corresponding bilayers. Such a behavior can be explained by pinned magnetic moments inside the bulk of the AFM layer that coexist independently for orthogonal spin directions, but have to be equally shared between both interfaces in the case of collinear spin directions. This result thus also confirms a 3Q-like noncollinear spin structure of Ni₂₅Mn₇₅.