The hysteresis loop and MOIF images of domain structure observed during the magnetization reversal of the NiO/NiFe bilayer grown on the (001) MgO substrate are shown in Fig. 1. The magnetic field was applied along the [110] direction or along reversed one. The [110] direction coincides with the direction of a magnetic field applied during the bilayer growth. Letters on the hysteresis loop refer to the conditions of the corresponding MOIF patterns. The hysteresis loop exhibits an exchange shift (HE = 35 Oe) and substantial coercivity (HC = 40 Oe). The MOIF patterns display that the magnetization reversal of the bilayer proceeds by nucleation and subsequent growth of domains with new magnetization orientations. Moreover, an asymmetry is revealed in the activity of the domain nucleation centers. When the magnetic field is aligned along the unidirectional anisotropy axis, the nucleation of domains occurs in central regions of the field of view at some defects (Fig. 1b). However, when the field is aligned in the opposite direction, the domain nucleation starts at other centers (Fig. 1f). Moreover, the reversal is finished at the former nucleation centers (Fig. 1h).
In contrast to the above picture, no asymmetry in the reversal of NiFe grown on MgO without NiO buffer is observed. Domains nucleate and disappear at film edges for both field directions parallel to the easy axis. The magnetization reversal proceeds by means of motion of almost rectilinear domain walls with cross ties over the whole sample (Fig. 2a, b). There is also no shift in the hysteresis loop of this free NiFe layer, and its coercive force is very small (approximately 2 Oe). For comparison, examples of domain structures created during magnetization reversal of the deformed section of the same NiO/NiFe bilayer as described in Fig. 1 are shown in Figs. 2c and 2d. Dislocation slip planes are shown in Figs. 2e and 2f. Fig. 2e shows NiFe surface steps revealed in reflected light on the NiFe/NiO bilayer. Fig. 2f shows microstresses from edge dislocations as revealed by a photoelasticity method.[9] Both the edge and screw dislocations were introduced into the MgO substrate during cleaving before the bilayer deposition. The same dislocation structure was found in the unbiased NiFe film described above. Note the weak influence of dislocations on the reversal of the unbiased NiFe film in Figs. 2a and 2b. This is due to the small permalloy magnetostriction.
In contrast, the magnetization reversal in the deformed NiO/NiFe bilayer depends essentially on the dislocation structure. The dislocations not only impede domain wall motion, but also play the role of domain nucleation centers. As a result, the specific head-to-head domain walls are disposed, as a rule, along the dislocation slip planes (Fig. 2c). Moreover, anisotropy inhomogeneities caused by dislocation stresses affect strongly the magnetization rotation processes at the saturation stage of the reversal (Fig. 2d).
A very important conclusion can be made from the analysis of the above data. Dislocations in the NiO/NiFe bilayer influence primarily the spin configuration of the AF layer. The spin behavior in the FM layer is affected only through its interaction with the AF layer.
Domain structures observed during the [1-10] hard-axis magnetization reversal of the plastically deformed NiO/NiFe bilayer are shown in Fig. 3. The hysteresis loop (Fig. 3a) has a symmetric shape and demonstrates a very small coercivity. In this case the magnetization reversal proceeds primarily by the incoherent rotation of magnetization vectors. However, unexpected nucleation and growth of domains has also been revealed. Both of these processes proceed simultaneously and are strongly governed by the dislocation structure. Elongated domains arise along screw dislocation slip planes, the most remarkable inhomogeneities of the spin rotation are observed along the slip planes of edge dislocations.
For comparison, Fig. 4 shows typical domain structures for a polycrystalline NiO/NiFe bilayer (a) and for a polycrystalline free NiFe film (b) grown on Si. The magnetic field was applied along the [110] unidirectional and along the [110] easy axis, respectively. Similar to the above results, imperfections in the NiO crucially affect the domain structure of the bilayer. As a result, the bilayer displays a complicated fine-scaled domain structure (Fig. 4a). In the polycrystalline NiFe film grown on Si without NiO buffer, the domain structure is simpler and the remagnetization proceeds by the motion of almost rectilinear domain walls with cross ties over large distances (Fig. 4b).