Giant Magnetoresistance and Oscillation in Epitaxial Fe/Cr(111) Multilayers
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ABSTRACT We report on the first studies of the giant magnetoresistance and oscillatory coupling in epitaxial Fe/Cr(111) multilayers. A series of samples were grown on hydrogen terminated Si(111) substrates at room temperature by UHV electron beam evaporation; with the thickness of Fe layer fixed at 30 A, and the thickness of Cr layer varied from 10-47 A. Giant magnetoresistance (GMR) is observed at 4.2 K in these samples, with a maximum value of 13% for a Cr layer thickness of 13 A. The associated oscillatory coupling is comparable to that reported in other crystallographic orientations in terms of both its period and phase.
INTRODUCTION Sandwiches and superlattices of magnetic films separated by a thin layer of nonmagnetic material have recently attracted wide interest because of their unusual magnetic and transport properties. Baibich et al. first discovered that the Fe/Cr multilayers display unexpectedly high values of magnetoresistance (The resistance dropped by almost a factor of 2 in a magnetic field of 2 T for some samples) [1]. This so called giant magnetoresistance (GMR) has great potential for technical applications such as recording heads and magnetic sensors. The GMR has since then inspired a lot of experimental and theoretical investigations in many systems [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], with significant progress being made both in increasing the size of the GMR effect and in reducing the size of the field required to effect the change in R. The large reduction in sample resistance has its origins in the dependence of the resistance on the relative orientation of neighboring ferromagnetic (FM) layers within the sample (this being smallest for a parallel arrangement) [13]. The largest effects are seen in samples where neighboring FM layers are oriented anti-ferromagnetically (AFM; the configuration of maximum resistance) through indirect exchange coupling across the non-magnetic spacer layer. From this configuration the low resistivity parallel arrangement can be induced by an external field of sufficient strength to overcome the exchange coupling. A phenomenon of interest equal to the GMR effect itself is the observation that for many systems the coupling between FM layers alternates periodically between FM and AFM with changing spacer thickness. This phenomenon is very reminiscent of the RKKY coupling seen between isolated magnetic atoms in a non-magnetic host, but in many cases the observed value for the period is much longer than that expected from a simple free-electron model of the RKKY interaction (ir/kF) [14]. Moreover in some cases two different periods have been observed with one being comparable to r/kF and the other longer. Many variations on the RKKY theme have been put forward to explain the long periods with many authors pointing out that aliasing associated with the periodic sampling of the spacer thickness imposed by its crystalline structure affords an opportunity for long effective periods. Herman and Schrieffer[11] have pointed out that such a mechanism is very susceptible to
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