Figure 1(a) shows magnetization curves for two Fe-Cu-Co sandwiches grown during the same run, but deposited at different substrate temperatures of 220 and 165 °C. As the figure illustrates, sandwiches grown at temperatures 220 °C and above reproducibly have square magnetization curves, indicating that the Fe and Co moments are reversing together. However, in the sandwiches deposited at 165 °C and below, the Co and Fe do indeed have different coercivities, as demonstrated by the presence of the shoulder in the magnetization curve at H = 0.1 kOe. For the increasing field part of the loop, the shoulder corresponds to the field region where the moment of the Fe film has already reversed, but the moment of the Co film is just beginning to rotate. The identification of the higher coercivity part of the loop with the Co film has been confirmed by a study on a series of samples with different Co thicknesses. Similar magnetization curves were reported in Ref. 5 for NiFe-Cu-Co superlattices. The disappearance of the shoulder in the magnetization curve between 165 and 220 °C is attributed to slight interdiffusion of the Cu and ferromagnetic layers. Recent Auger electron spectroscopy and thermal energy atom scattering studies of the Co/Cu system by de Miguel et al.[6] show that diffusion of Cu into Co begins at about 177 °C. No significant in-plane magnetic anisotropy is observed in the magnetization curves of these two samples despite the presence of bias fields up to 50 Oe during deposition.
Figure 1(b) shows magnetoresistance data for the same two sandwiches at room temperature. The magnetoresistance is here defined to be the ratio of the field-induced change in resistance to the zero-field resistance. The 220 °C sample has an MR curve that is typical for thin ferromagnetic films, with a peak of 0.5% near the coercive field of the magnetization curve. For the 165 °C sandwich the peaks in the magnetoresistance correspond well to the region of the magnetization curve where the Fe and Co moments are maximally antialigned, as expected. The magnitude of the transverse MR at magnetic saturation is 3.1% for this sandwich, comparable to the room-temperature MR previously found for uncoupled sandwiches.2,3 The anisotropic magnetoresistance (AMR) for the 165 °C sample was measured directly at 3 kOe to be 0.41%. The spin-valve component of the magnetoresistance is the transverse MR (applied field perpendicular to the current direction) minus half the AMR.[7] Therefore the component of the MR attributable to the spin-valve effect is 2.9%. A spin-valve magnetoresistance of over 3% at room temperature can be obtained even in simple Fe-Cu-Co sandwiches by optimizing the deposition temperature and the layer thicknesses.
Figure 2 shows magnetoresistance data for two Fe51Å/Cu55Å/Co48Å sandwiches grown simultaneously at 165 °C, one on glass and one on Si. The Fe films of both samples switch between 0 and about 0.05 kOe. However, the Co moment in the sample grown on glass reverses at a significantly higher field than the Co moment in the Si sample, as demonstrated by the corresponding M-H loops (not shown). This variability in the behavior of the Co layer is somewhat surprising given that it is the Fe layer which is deposited directly on the substrate. The deposition temperature variation in Figure 1 also principally affects the behavior of the Co layer. X-ray diffraction and cross-sectional transmission electron microscopy studies are currently underway to look for associated structural changes in the Co film or the interface quality.[8]