Bi-domain state in the exchange bias system FeF2/Ni

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.