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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 HE = 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 Hb axis, and unidirectional anisotropy defined by the direction of Hb. Figure 1b shows M(H) in the in-plane (100) directions, one parallel to Hb and one perpendicular to Hb. The exchange field HE is 20 Oe (J = 0.015 erg/cm2).

The dependence of HE on the thickness of the IBS grown NiO and NiFe layers was measured. Consistent with other studies [7] , the exchange field agrees with tNiFe-1 behavior for 300Å > tNiFe > 50Å and tNiO = 500Å, as expected from the interfacial origin of the interaction. The HE is approximately constant for tNiO > 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 HE and Hc is maximum is between 400Å and 500Å. Finally we find the training effect, which is the reduction of HE 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 HE is not sensitive to the changes in the bulk NiO morphology detected at this level. Though we find variations in the strength of HE they are not correlated to variation on the NiO texture. This conclusion is illustrated in Figure 2 where the interface exchange energy, J = HE Ms tNiFe, 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, TN, for bulk NiO is 250°C. The HE 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 HE and Hc of an IBS grown NiO(500Å)/NiFe(50Å) coupled film is shown in Figure 3. HE drops approximately linearly with increasing temperature reaching a blocking temperature of Tb = 200°C in this film. The slope of the decrease in HE with temperature is approximately the same for all of the films measured, indicating that the room temperature exchange field is a good predictor of Tb. Hc 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 Hc of the NiFe from 84 Oe to 60 Oe, but did not change HE 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, (Hc = 35 Oe and Hs = 80 Oe as compared to Hc = 1-2 Oe and Hs = 5 Oe in a free NiFe layer). This enhanced Hc indicates that interface exchange coupling is present, but that it averages to zero [8],[9] . The temperature dependence of Hce 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 HE >0, reaching 1-2 Oe at 130°C. The hard axis saturation field has similar temperature dependence. The reduced temperature where Hce goes to zero may be a result of a reduced TN for the thinner NiO layer, however it seems more likely that the reduction is linked in the same way as the lack of HE to the interfacial properties.



next up previous
Next: Discussion Up: Title page Previous: Title page Figures References

alchaiken@gmail.com (Alison Chaiken)
Sat Oct 14 13:45:11 PDT 1995