Perpendicular Resistance of Co/Cu Multilayers Prepared by Molecular Beam Epitaxy
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N.J. LIST, W.P. PRATT JR.*, M.A. HOWSON, J. XU, M.J. WALKER, B.J. HICKEY and D. GREIG Department of Physics, University of Leeds, Leeds LS2 9JT, UK, *Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA ABSTRACT Results are presented of the magnetoresistance of MBE-grown (111) Co/Cu multilayers measured with the current perpendicular to the plane of the layers (CPP). Although for measurements made with the more common geometry of current in the plane of the layers (CIP) there are large differences between the results on samples made by sputtering and those prepared by MBE, for these new CPP data the results on samples made by the two techniques are very much alike. For copper layers with thicknesses between 0.9nm to 6nm the magnetoresistance shows oscillations with copper thickness that were almost non-existent in the earlier CIP data. At the second peak the magnetoresistance in the CPP geometry is an order of magnitude greater than that in the CIP configuration. Although the interfaces in these samples have been shown to be very sharp, they appear to form a mosaic structure with the antiferromagnetic regions embedded in a ferromagnetic structure. It is argued that for CIP measurements the GMR is greatly reduced by these ferromagnetic correlations over length scales long compared to the electron mean free path. For CPP measurements, on the other hand, it is the spin diffusion length that is the determining factor with the mean free path no longer a key parameter and with values of the GMR virtually independent of the growth process. INTRODUCTION In nearly all studies of the so-called 'giant' magnetoresistance (GMR) observed in magnetic multilayers the measurements have been made with the current directed along the layers in the so-called Current-In-Plane (CIP) mode. Experimentally the resistance is of the order of ohms and easily measured, but on the other hand the detailed analysis of the magnetoresistance in the CIP configuration is complicated by the non-uniform distribution of current between the layers. A GMR is observed only when the mean free path of the electrons, X, is considerably greater than LM, the length scale of the magnetic inhomogeneity of the specimen. The minimum value of LM is clearly the bilayer spacing. This condition is essential to ensure that over the length of a mean free path conduction electrons of a given spin can sample regions of opposing magnetisation that can be changed on application of an external field. This is the criterion for the existence of the GMR. On the other hand when the mean free path is very short (X.< LM) the above condition is no longer true and the magnetoresistance from this source is effectively zero. More recently an alternative method of studying the resistance of the multilayers by measuring the resistance through the layers in the Current-Perpendicular-to-Plane (CPP) geometry has gained in prominence. With the CPP arrangement current density is uniform 329
Mat. Res. Soc. Symp. Proc. Vol. 384 ©1995Materials Research Soci
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