Growth Temperature Dependence of Magnetoresistance in Co/Cu(111) Wedged Superlattices
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Growth Temperature Dependence of Magnetoresistance in Co/Cu(111) Wedged Superlattices G. R. Harp, S. S. P. Parkin, R. F. C. Farrow, R. F. Marks, M. F. Toney, Q. H. Lam, T. A. Rabedeau, A. Cebollada, and R. J. Savoy IBM Almaden Research Center, 650 Harry Rd., San Jose, CA 95120-6099
Abstract The magnetoresistance of MBE deposited Co/Cu(111) superlattices as a function of Cu layer thickness is studied at various growth temperatures. Wedged superlattices are fabricated with uniform Co layers but with Cu layers whose thickness varies with position along the length of the substrate. Cu thickness is varied over the range 7-75A. Only one maximum in magnetoresistance (MR) is observed for a Cu layer thickness 10A for all growth temperhA atures studied (00, 1500, 200°C). The optimal magnetoresistance values are achieved with 150°C growth temperature.
1
Introduction
The Co/Cu multilayer system has received much attention because record values of magnetoresistance (65% at room temperature) are observed in polycrystalline-sputtered Co/Cu multilayers[1]. This giant magnetoresistance (GMR) is associated with antiferromagnetic exchange coupling between alternating Co layers, and the exchange coupling is itself oscillatory with Cu layer thickness, oscillating between antiferromagnetic (AFM) and ferromagnetic (FM) coupling with increasing Cu layer thickness[2, 3]. Such sputtered multilayers are weakly textured in the (111) orientation. However, GMR and oscillatory exchange coupling have proven elusive in single-orientation (111) multilayers of Co/Cu grown by MBE. Prior to 1992, a number of groups published results showing neither GMR nor oscillatory exchange coupling in (111) Co/Cu multilayers[4, 5, 6]. Then in 1992, GMR was observed by several groups[7, 8, 9, 10, 11], but only at one Cu layer thickness (!- 10A), so there was no evidence for oscillatory exchange coupling. It was found that subtle defects appear to quench the AFM coupling (and hence GMR) so that even samples deposited under nominally identical conditions on similar substrates could show large variations in GMR[11]. The search for oscillatory exchange coupling in Co/Cu(111) continues in 1993 with several groups having made more in-depth studies of the magnetic and transport properties of such multilayers[12, 13, 14, 15, 16, 17]. The results from these studies are not at all in agreement, and naturally highlight different growth and/or measurement techniques. Our particular approach focuses on improving reproducibility in sample preparation, and on structural characterization of our samples to try to identify defects which might inhibit AFM coupling.
2
Experiment
The substrates for film growth are basal-plane sapphire strips (Insaco Inc.). Single samples were deposited on 25mm diameter substrates, and wedged samples onto rectangular substrates about 50mm long by l1mm wide. The back sides of the wedge substrates are scribed to a depth of 0.25mm (one half their thickness) at 2mm intervals across the along the length
Mat. Res. Soc. Symp. Proc. Vol. 313. ©1993 Mate
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