Stationary antiferromagnetic domains during magnetization reversal in an exchange-biased FeMn/Fe76Mn6C18 bilayer

Abstract

Domain processes were observed at 300 K using the magneto-optic indicator film technique (MOIF) in an exchange-coupled ferromagnet (FM)/antiferromagnet (AF) bilayer Fe76Mn6C18 (150 Å)/FeMn (100 Å) deposited under the presence of a 0.4 mT magnetic field (H) applied in the plane of the sample. The hysteresis loop for this sample was comprised of two half-loops symmetrically shifted in opposite directions from the origin. At H=0, MOIF observations showed the presence of domains in the FM with magnetization (M) vectors along the axis of the preparation field separated by 180° walls. Upon field application along that axis, saturation of the FM was achieved by the nucleation and growth of domains. In this state, at not very high fields, it was possible to observe an unusual MOIF contrast at the location of the original FM domain walls in the as-prepared ground state, associated with the intersection of domain walls in the AF with the FM. Upon field reduction M reversed only in regions which had reversed during the prior field application, so that at H=0 the domain structure was the same as that in the original ground state. If H was off-axis, during the original field application domain growth occurred in that same direction; upon field reduction, the preferred domain growth direction was at an angle with reversed sign. These results prove the AF domain walls do not move during the motion of FM domain walls, and that an exchange spring is created parallel to the AF/FM interface as the ferromagnetic layer reverses.