Figure 2 shows that Hs for the tCr =16Å MBE-grown sandwich increases by 0.48 kOe between 300 K and 77 K. A previous FMR study of single Fe films grown on GaAs found that K1/Ms increases by about 0.06 kOe when T decreases from 300 to 77 K.[7] Assuming that K1(T) is similar for the Fe-Cr sandwiches grown on ZnSe epilayers, the change of K1/Ms with temperature can account for only a 0.12 kOe increase in Hs from 300 K to 77 K. Therefore the increase of Hs(T) in the MBE-grown samples, while coming partly from the temperature dependence of K1/Ms, is dominated by A12(T), just as in the polycrystalline and superlattice samples. Currently, efforts are underway to determine A12(T) and K1(T) independently by fitting the HJ(T) and Hs(T) data of Figure 2.
The temperature dependence of the magnetoresistance in the Fe-Cr sandwiches is more complex. Like Hs, Dr/r has two terms, one arising from the intrinsic anisotropy of the Fe layers, and another arising from effects related to the interlayer electron transport.[8] The Fe-layer contribution, which is known as the anisotropic magnetoresistance (AMR), is about the same size as in bulk Fe.[8] The AMR is a sinusoidal function of the angle between H and I. It is the second term, however, which is responsible for the anomalously large magnetoresistance reported in Ref. 2. This anomalous contribution, which has been called the spin-valve magnetoresistance, depends only on the relative orientation of the Fe moments, and not on their absolute orientation.[9] Therefore, the spin-valve magnetoresistance should be isotropic with respect to the applied field orientation. This fact allows one to separate the AMR and spin-valve contributions to the H||I and H perpendicular to I data of Figure 3(b): the AMR is the difference between the H||I and H perpendicular to I traces, while the spin-valve component is half the sum of the two data sets. The temperature dependences of the separate AMR and spin-valve terms are shown in Figure 4. The AMR is nearly constant with temperature, but the spin-valve part of the magnetoresistance, while constant below about 70 K, has a linear decrease at higher temperatures.
Barthelemy et al. have suggested that the decrease of the magnetoresistance with increasing temperature is due in part to spin-wave scattering.[2] Interlayer spin fluctuations could also be responsible for the linear decrease of A12 with T. In this regard it is relevant to point out that Dr, the field-induced change in resistivity, is itself a function of temperature. The temperature dependence of the spin-valve effect is therefore not just a trivial consequence of the temperature variation of the resistivity. The data shown in Figure 4 thus contain real information about the scattering processes responsible for the spin-valve magnetoresistance. At the present time, however, the true nature of these scattering processes remains unknown.
There are still many unanswered questions about the Fe-Cr system. For example, Table I shows that the magnitude of the magnetoresistance in the Fe-Cr sandwiches does not appear to be correlated with either the residual resistivity ratio or A12. Nonetheless, significant progress has been made towards understanding the antiferromagnetic exchange and giant magnetoresistance of the Fe-Cr superlattices and sandwiches.[3,4,5] The linearity of the temperature dependence of the antiferromagnetic exchange and spin-valve magnetoresistance should prove helpful in understanding the role of spin waves and spin-flip scattering in the Fe-Cr system.
The authors would like to thank J.R. Cullen, K.B. Hathaway, and A.C. Ehrlich for useful discussions, W.S. Rupprecht for help with data analysis, and D.R. King and F. Kovanic for technical assistance.