Figure 3 is a more informative way of presenting the data of Figure 2. Here both the spin-valve Dr, the numerator of the spin-valve MR, and r(H=0), its denominator, are displayed separately. The resistivity is higher for the lowest Fe film thickness (tFe = 16Å), but is essentially constant for thicker Fe films within the precision of the resistivity measurement and the reproducibility of the total Cu thickness (97 ± 8 Å). The numerator, Dr, clearly is responsible for most of the variation of the MR with Fe thickness. Separate comparisons of the thickness dependence of Dr and r with the semiclassical theory[3] should prove more helpful than fits to the ratio Dr/r in quantitatively understanding the origin of the spin-valve effect.
In all the existing theories of the spinÐvalve effect, the magnitude of the magnetoresistance is determined by the relative carrier lifetimes for spin-dependent and spin- independent scattering processes. For example, it has been proposed that the origin of the spin-dependent scattering is interfacial roughness,[2] but clearly interfacial disorder will also contribute to spin-independent scattering. One might therefore expect that both Dr and r will increase with increasing interfacial disorder, and there are x-ray diffraction results that support this conjecture.[10] As a preliminary result, the data of Figure 3 (and a similar plot for samples with different Co thicknesses[8]) do not show a correlation between Dr and r for the Fe-Cu-Co sandwiches. This lack of correlation may be due to the dominance of bulk scattering over interfacial scattering, as suggested by the peak in the MR versus Fe thickness, or it may have a deeper origin. A detailed structural study of the Fe-Cu-Co sandwiches should help determine whether the important scattering events are due to interdiffusion and roughness at the interfaces, or are influenced more by finite grain size and impurities within the ferromagnetic layers.
We thank D. King for the layer thickness determinations. This research was supported by DARPA and ONR.