  Independently exchange biased subsystems can coexist in FeF2/Ni bilayers
after various field-cooling protocols. We find double hysteresis loops for
intermediate cooling fields, while for small or large cooling fields a
negatively or positively shifted single loop, respectively, are encountered.
Both the subloops and the single loops have the same absolute value of the
exchange bias field, mu_0 H_E = 0.09 T. This suggests that the antiferromagnet
breaks into two magnetic subsystems with opposite signs but equal magnitude of
bias acting on the ferromagnet. In this case the ferromagnet does not
experience an average bias from the antiferromagnet but rather two independent
subsystems ('bi-domain' state). This idea is confirmed by micromagnetic
simulations including the effect of the antiferromagnet. We also present
experiments, where thermally activated motion of these antiferromagnetic
'domain' boundaries can be achieved.
