Asymmetric recovery effect of exchange bias in polycrystalline NiFe/FeMn bilayers

Abstract

For exchange bias in polycrystalline NiFe/FeMn bilayers, the hysteretic behavior of the angular dependence and the recovery effect has been studied. In particular, the pinning direction (PD) at the ending remanent state of each hysteresis loop is identified. In the hysteretic behavior, in addition to the coercivity, the PD also demonstrates different angular dependence between clockwise and counterclockwise rotations of the external magnetic field. Measurements of the recovery effect consist of two major steps. In the first step, the PD is deviated from the initial one by using its hysteretic effect and training effect. For polycrystalline NiFe/FeMn bilayers, the rotated PD is located at the maximal angle θPD0 of ±22° with respect to the initial ones. As for the second step, an external magnetic field is applied at a specific orientation θH−RE and then switched off at the same orientation. For the negative θPD0, the recovery effect only occurs for 0<θH−RE<180° with the maximal effect at θH−RE=90° and vanishes for 180°<θH−RE<360°, and vice versa for the positive θPD0. Therefore, the recovery effect shows an asymmetric angular dependence on θH−RE. The recovery effect of the PD also depends on the magnitude and the application time of the recovery magnetic field. For the exchange field and the coercivity, similar recovery behaviors are observed and attributed to the recovery effect of the PD. These phenomena clearly show that the motion of antiferromagnet spins not only obeys the thermally activated transition but also strongly depends on the magnetization reversal mechanism of the ferromagnet layer.