High-Field Giant Magnetoresistance in Co-Cu Superlattices
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HIGH-FIELD GIANT MAGNETORESISTANCE IN Co-Cu SUPERLATIICES DARRYL BARLETT, FRANK TSUI, LINCOLN LAUHON, TUSHAR MANDREKAR, CTIRAD UHER AND ROY CLARKE University of Michigan, Harrison Randall Laboratory of Physics, Ann Arbor, MI 48109. ABSTRACT We present evidence for a new type of giant magnetoresistance in (111) cobaltcopper superlattices with atomically smooth interfaces. We propose that the lowered dimensionality of the structure leads to an enhancement of the scattering of conduction electrons from paramagnetic interfaces obeying a Langevin-like saturation at very high fields, well beyond the switching field of the Co layers. The findings help to explain similarities in magnetotransport behavior with recently reported granular systems as well as differences with antiferromagnetically coupled multilayers. INTRODUCTION Several groups1- 3 have reported magnetoresistance (MR) values in MBE-grown Co-Cu (111) superlattices approaching the 'giant' effects associated with antiferromagnetic (AFM) coupling in Fe-Cr samples 4 . Whether the MR behavior observed in Co-Cu (111) samples also originates from AFM coupling is somewhat unclear at this point: their magnetization seems to be predominantly ferromagnetic in character with only a small fraction of the sample showing indications of AFM coupling. 5 In another recent experiment 6, on Co-Cu (111) multilayers grown on Cu single crystal substrates, there was no consistent evidence of AFM coupling. Sample defects have been invoked as a possible explanation of why AFM may be masked in the Co-Cu (111) system. For example, it has been suggested that stacking 7 faults and pinholes 8 may lead to ferromagnetic bridging across neighboring layers. Wellcontrolled sample growth and detailed atomic-scale characterization are therefore crucial to understanding the magnetic behavior of these materials. Here we present MR and magnetization results on a series of (111) single crystal Co-Cu superlattices, prepared by molecular beam epitaxy techniques with atomically smooth interfaces. We observe the appearance of a new type of giant magnetoresistance, one which is not dependent on AFM coupling and is operative up to high magnetic fields. By careful control of the interfacial quality, and consequently the uniformity of the layering, we are able to probe in some detail the role of the interfaces. In the limit of atomically smooth interfaces, our results suggest that the lowered dimensionality of the interfaces dominates the behavior rather than sample defects. SAMPLE GROWTH AND CHARACTERIZATION The series of samples were grown by molecular beam epitaxy on Ge-buffered (110) GaAs substrates. Buffer layers of 15A (110) bcc Co, followed by 20A (111) Au, were deposited on the Ge to initiate layer-by-layer superlattice growth in the (111) orientation. The subsequent superlattice layers were of the form [Co(7.5ML)/Cu(xML)]n with x = 2 to 17 ML; n, the number of bilayers, was typically 30. The pressure during superlattice growth was
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