Results and Discussion

Figure 2a shows NiFe surface steps revealed in reflected light on the NiO/NiFe bilayer epitaxially deposited onto MgO (001). They are created by the intersection of screw dislocation slip planes with the top NiFe surface. Figure 2b shows a birefringence picture of the same area due to effective internal microstresses caused by edge dislocations aligned along (110) and (110) slip planes. Both screw and edge dislocations were introduced into the MgO substrate while cleaving the substrate prior to the bilayer deposition. The steps on the top NiFe surface indicate that these MgO dislocations propagated during the deposition through the NiO and NiFe layers and, hence, these screw dislocations also introduced steps into the interface of the bilayer.

The MOIF images shown in Fig. 3 illustrate the behavior of the domain structure in the biased MgO(001)/NiO/NiFe sample during reversal of its magnetization along the [110] unidirectional axis. The sample was first magnetized to saturation with the field µH = -30 mT aligned opposite the direction of the unidirectional anisotropy. The MOIF image of the saturated sample (not shown) was homogeneous and only stray magnetic fields at the sample edge were revealed (as a black band). As the applied field was decreased, incoherent rotation of the magnetization occurs (Fig. 3a), resulting in an inhomogeneous distribution of spins which is entirely associated with the dislocation structure of the sample (compare with Fig. 2). Upon increasing the applied field in the reverse sense, a large amount of domain nucleation occurs at the dislocation slip planes and at their intersections (Fig. 3b). These new domains grow slightly until domain wall annihilation occurs. At this point, the domains disappear, and an inhomogeneous distribution of magnetization similar to that shown in Fig. 3a appears.

For magnetization reversal in the opposite direction, the domain structure changes in a similar fashion to the above picture. However, nucleation of reversed-phase domains occurs at defferent local centers (Fig. 3c, d). In addition, nucleation starts at a larger absolute value of the reversed applied field because the applied field in this direction must overcome the unidirectional anisotropy. Consistent with this behavior, an asymmetric hysteresis loop is observed (Fig. 1a). This loop exhibits an exchange shift µH_E = 3.5 mT and coercive field µH_C = 4 mT.

In the NiO/NiFe bilayer having a [110] unidirectional anisotropy axis the most significant feature of the remagnetization process is the complicated domain structure resulting from the strong influence of edge and screw dislocations on both spin rotation and domain nucleation and growth. In fact, it is observed that the dislocations pin these domain walls so effectively that the domain sizes do not exceed the spacing between neighboring dislocation slip planes. It should be emphasized that while the screw dislocations introduce inhomogeneous internal stresses and additionally create steps at the NiO/NiFe interface (which can frustrate the interface NiO/NiFe magnetization vectors), the edge dislocations introduce only inhomogeneous internal stresses. This difference suggests that the induced inhomogeneous magnetic anisotropy due to magnetoelastic interactions plays an important role in the remagnetization of the bilayer. For the NiO/NiFe bilayer biased along the [110] direction, the edge dislocations also make complete saturation of the magnetization along the bias axis very difficult. Figure 4 shows for this case, that non-uniform magnetization always exists along the [110] slip planes. Remagnetization along the [110] axis is accompanied by the vertical motion of a vortex wall along the slip plane, resulting in a change in contrast on the slip plane, but still leaving behind light/dark contrast bands indicating changes in magnetization direction. This contrast banding remains up to higher fields. For remagnetization along a hard-axis no shift in the hysteresis loop was observed, and the coercivity was very small.

When an unidirectional anisotropy was induced along the [100] direction in the NiO/NiFe bilayer, the dislocations also stimulate domain nucleation and impede domain wall motion. However, in this case the domain walls can more easily overcome the dislocations barriers, and therefore, the domain sizes are larger (Fig. 5). Compare, for example, Fig. 2 and 5. It is obvious from Fig. 5 that the domains in the [100] biased bilayer can grow to sizes much larger than the dislocation slip planes spacing. The measured unidirectional exchange field (µH_E = 2 mT) and coercivity (µH_C = 2.6 mT) of this bilayer are smaller than that in the NiO/NiFe bilayer with a [110] unidirectional biased axis. This means that the reversal process depends on the relationship between the unidirectional and an induced anisotropies.

The MOIF images of the domain structure taken during the [110] easy-axis magnetization reversal of a free NiFe layer (grown on MgO (001) without a NiO buffer) in two opposite directions are shown in Fig. 6. The dislocation structure of this sample is also similar to the one shown in Fig. 2. As is obvious, there is no strong dislocation influence on the domain structure of the free NiFe layer. This is due to an almost zero magnetostriction in the NiFe. Domains nucleate and disappear, as a rule, at film edges for both field directions due to a minimization of stray fields at edges. Remagnetization proceeds by the growth of domains over the whole sample. Though these particular patterns were obtained for the 50 nm thickness NiFe, such a behavior is typical for free ferromagnetic films with different thicknesses.[10]

Comparison of the domain structure behavior in the free ferromagnetic film (Fig. 6) with that in the exchange-biased film (Fig. 3) led us to propose that dislocations in the NiO/NiFe bilayer influence primarily the spin configurations in the antiferromagnetic layer. This antiferromagnetic layer, however, affects the behavior of spins in the ferromagnetic layer. To confirm this proposition, we show in Fig. 7 the domain patterns taken during remagnetization of a polycrystalline NiO/NiFe bilayer (Fig. 7a) and a free NiFe layer (Fig. 7b) deposited on Si substrates. It is obvious that the domain structure in the polycrystalline NiO/NiFe bilayer is complicated and fine-scaled. On the contrary, in the polycrystalline NiFe layer, the remagnetization proceeds by the motion of almost rectilinear domain walls with cross ties over large distances. It does indicate that the polycrystalline structure in the NiO affects the magnetization reversal process in the bilayer.