Experimental Results

Figure 1a shows the easy and hard axis magnetization of a NiO(500Å)/NiFe(100Å) deposited on Si with an amorphous Al₂O₃ film. The NiO film is untextured with a grain size in the growth direction, derived from the FWHM of the NiO(200) Bragg peak, of 80Å. The easy axis loop is offset by H_E = 66 Oe which corresponds to an interface exchange energy of J = 0.046 erg/cm². The largest value we observed was J = 0.065 erg/cm². The energies are in agreement with those observed by [1] . NiCoO/NiFe, and NiO/Co bilayers with promising exchange fields have also been fabricated using this new IBS sputtering technique Figure 1b shows the magnetization of a bilayer deposited simultaneously with the films shown in Figure 1a, but on a polished single crystal (001) oriented MgO substrate. XRD shows both the NiO and the NiFe are highly (001) textured. The magnetization of the NiFe shows evidence of both a induced uniaxial anisotropy along the H_b axis, and unidirectional anisotropy defined by the direction of H_b. Figure 1b shows M(H) in the in-plane (100) directions, one parallel to H_b and one perpendicular to H_b. The exchange field H_E is 20 Oe (J = 0.015 erg/cm²).

The dependence of H_E on the thickness of the IBS grown NiO and NiFe layers was measured. Consistent with other studies [7] , the exchange field agrees with t_NiFe^-1 behavior for 300Å > t_NiFe > 50Å and t_NiO = 500Å, as expected from the interfacial origin of the interaction. The H_E is approximately constant for t_NiO > 400Å, and decreases to zero at a critical thickness of about 175Å. The easy axis coercivity peaks near the critical thickness. The optimum NiO thickness where the difference between H_E and H_c is maximum is between 400Å and 500Å. Finally we find the training effect, which is the reduction of H_E after multiple field cycles, is largest for NiO thicknesses near the critical thickness. No training effect was observed for NiO films greater than 400Å.

The morphology of the NiO deposited on an amorphous buffer layer is sensitive to the detailed deposition conditions. We have studied the effect of substrate deposition temperature, beam voltage, deposition rate and substrate voltage bias on the texture of 500Å thick NiO layers. We observe NiO(111), (200) and (220) Bragg reflections with various intensities (the positions indicate the NiO lattice is expanded relative to bulk by 1%). We find, however, that H_E is not sensitive to the changes in the bulk NiO morphology detected at this level. Though we find variations in the strength of H_E they are not correlated to variation on the NiO texture. This conclusion is illustrated in Figure 2 where the interface exchange energy, J = H_E M_s t_NiFe, for a wide range of NiO(500Å)/NiFe(tNiFe) films is plotted as a function of the ratio of the NiO(111) to the NiO(200) Bragg peak intensities. The data are space-filling indicating that the bulk texture of the NiO layer is not a good predictor of the strength of the interface coupling.

The Neel transition temperature, T_N, for bulk NiO is 250°C. The H_E of NiO/NiFe bilayers typically drops to zero at a lower blocking temperature, Tb, which also varies with the interface and bulk morphological properties of the bilayer couple [7] . The temperature dependence of H_E and H_c of an IBS grown NiO(500Å)/NiFe(50Å) coupled film is shown in Figure 3. H_E drops approximately linearly with increasing temperature reaching a blocking temperature of T_b = 200°C in this film. The slope of the decrease in H_E with temperature is approximately the same for all of the films measured, indicating that the room temperature exchange field is a good predictor of T_b. H_c also decreases with increasing temperature but at a slower rate reaching the room temperature value of a free NiFe layer (1-2 Oe) at 230°C. Temperature cycles above TN with subsequent cooling in modest fields reduced H_c of the NiFe from 84 Oe to 60 Oe, but did not change H_E significantly.

In some bilayer films no offset was produced in the NiFe magnetization loop by the NiO buffer, but a large room temperature coercivity as well as a clear uniaxial anisotropy was observed, (H_c = 35 Oe and H_s = 80 Oe as compared to H_c = 1-2 Oe and H_s = 5 Oe in a free NiFe layer). This enhanced H_c indicates that interface exchange coupling is present, but that it averages to zero [8],[9] . The temperature dependence of H_ce in such a film composed of NiO(355Å)/NiFe(100Å) is also shown in Figure 3. The coercivity drops with increasing temperature similar to the behavior seen in films with H_E >0, reaching 1-2 Oe at 130°C. The hard axis saturation field has similar temperature dependence. The reduced temperature where H_ce goes to zero may be a result of a reduced T_N for the thinner NiO layer, however it seems more likely that the reduction is linked in the same way as the lack of H_E to the interfacial properties.