Loop bifurcation and magnetization rotation in exchange-biased Ni∕FeF₂

Exchange-biased Ni∕FeF₂ films have been investigated using vector coil vibrating-sample magnetometry as a function of the cooling field strength H_FC. In films with epitaxial FeF₂, a loop bifurcation develops with increasing H_FC as it divides into two sub-loops shifted oppositely from zero field by the same amount. The positively biased sub-loop grows in size with H_FC until only a single positively shifted loop is found. Throughout this process, the negative and positive (sub)loop shifts maintain the same discrete value. This is in sharp contrast to films with twinned FeF₂ where the exchange field gradually changes with increasing H_FC. The transverse magnetization shows clear correlations with the longitudinal subloops. Interestingly, over 85% of the Ni reverses its magnetization by rotation, either in one step or through two successive rotations. These results are due to the single-crystal nature of the antiferromagnetic FeF₂, which breaks down into two opposite regions of large domains.