Polarized neutron reflectivity characterization of weakly coupled Co/Cu multilayers

Room temperature resistivity studies on (1 1 1) Co(6 nm)/Cu(6 nm) multilayers with weak interlayer coupling yield a giant magnetoresistance (GMR) of several percent for the as-prepared state relative to the saturated (i.e., large magnetic field) state. After application of a magnetic field, the magnetoresistance for the coercive state is only half to two-thirds as large. Using specular and off-specular polarized neutron reflectivity, we have determined the magnetic structure of these multilayers in the as-prepared and coercive states. Measurements of as-prepared samples show a strong antiparallel correlation of in-plane ferromagnetic Co domains across the Cu interlayers. At the coercive field, the Co domains are uncorrelated along the growth direction. Thus, the larger GMR for the as-prepared state arises from long-range antiparallel magnetic order along the growth-axis direction that is destroyed upon application of a magnetic field. For both the as-prepared and coercive states, the size of the in-plane ferromagnetic domains is approximately 0.5-1.5 μm. These domains give rise to pronounced diffuse scattering in rocking curves through the antiferromagnetic peak position.