Structure and Magnetism in Mo/Co Multilayers
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C.L. FOILES, M.R. FRANKLIN AND R. LOLOEE Department of Physics and Center for Fundamental Materials Research, Michigan State University, East Lansing, MI 48824
ABSTRACT The magnetic and structural properties of Co in Mo/Co multilayers are an unresolved issue. Two independent studies report different structures for thicker Co layers in multilayers of comparable dimensions as well as significant differences in both the form and the layer thickness for a loss of ferromagnetism. Both studies base their structural claims on a limited number of Bragg lines. We have used x-ray diffraction, transmission electron diffraction, and EXAFS to probe the structure of Mo/Co multilayers having Co layer thicknesses from 4 to 63A. Our structural and magnetic results for these samples are compared to those of the above studies and are contrasted with results for Mo/Fe multilayers having comparable dimensions.
INTRODUCTION Non-equilibrium structures can be established in metallic multilayers and thereby alter macroscopic properties. The magnetic behavior of Co in Mo/Co is a possible but controversial example. A comparison of results in two published studies of sputtered Mo/Co multilayers produces limited points of agreement and significant differences. Reflection X-ray diffraction (XRD) in each study produces low angle peaks that confirm a layered structure. Both studies find highly disordered layers when the layer thickness is about 10A or less and a near loss of magnetism occurs in this same layer thickness range. Agreement ends with these features and the studies report structural and magnetization results with fundamental differences for thicker layers. Sato used magnetron sputtering and his higher angle XRD data give a single dominant peak whose location is dependent upon the relative portions of Mo and Co in the bilayer unit [1]. For multilayers having equal amounts of Mo and Co with bilayer thicknesses of less than 40k, this peak location is constant and by a thickness of 60A it has split into Bragg peaks characteristic of the Mo and Co forming the bilayer. Wang, et al. used focused ion sputtering and they find very different higher angle XRD results [2]. Their data yield isolated Mo and Co peaks for multilayers with thicknesses comparable to those of Sato. In addition, a line from s-Co 7 Mo 6 is observed for many samples and the peak for HCP Co is observed for thick Co layers. Transmission electron diffraction (TED) data reveal lines consistent with BCC Mo and 3 lines associated with s-Co 7 Mo 6 . The dependence upon layer thickness for the room temperature saturation magnetization's of the two studies is also different. Consider the results for samples with equal amounts of Co and Mo in the bilayer. The studies agree that IOk Co layers have a saturation magnetization less than 10% that of bulk Co but Sato reports a slow rise with increasing Co layer thickness, still less than 40% of bulk for 30A Co layers, while Wang, et al. report a rapid rise that reaches >80% for 20, Co layers and > 95% for 30, Co layers. The results for
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