The loop-stage dependent exchange bias training effect in FeNi/FeMn bilayer

Recently, the exchange bias (EB) phenomenon in the ferromagnet/antiferromagnet (FM/AFM) bilayer and its inborn instability effect, i .e ., training effect [1], have attracted much attention for its great technological importance in spintronic devices . Usually, the study of training effect is performed by continuously recording the exchange field (H_E) and the coercivity (H_C) after measuring an entire magnetic hysteresis (M-H) loop . However, an M-H loop can be divided into four different magnetization stages, i .e . positive saturation stage, descending branch, negative saturation stage, ascending branch, as illustrated by `I´, `II´, `III´ and `IV´ in Fig . 1 . Different loop stages may have different contributions to the EB training effect, which has rarely been studied so far . Therefore, in this work, the loop-stage dependent EB training effect has been carefully investigated in the polycrystalline FeNi/FeMn bilayer.