Correlation of magnetotransport and structure in sputtered Co/Cu multilayers

Abstract

Magnetic multilayer structures of Co/Cu prepared by dc magnetronsputtering are studied with respect to changing number of bilayers(N)for different thicknesses of the Cu spacer layer corresponding to different couplingconditions according to the oscillatory interlayer exchange coupling. X-ray reflectivityand diffuse scattering show that the multilayers become smoother with increasingN. Thegrowth exponent of the roughness is found to be lower for a multilayerthan for a single-layer film of similar thickness. The roughness ofsubsequent interfaces along the stack is conformal, and the lateralcorrelation does not change with the period number, but depends on thethickness of the spacer layers. The improved layer structure for largerN increasesthe antiferromagnetic coupling fraction as inferred from magneto-optic Kerr effectmeasurements and thereby increases the giant magnetoresistance (GMR) ratio up to 35%for N = 10.Thus, the first few bilayers do not contribute to the GMR but act as a buffer toimprove the growth conditions for the following bilayers. The first about fivebilayers can be replaced by a bottom Co layer of equivalent thickness which alsoimproves the layer structure for a subsequently deposited lower number of bilayerswithout much loss in the GMR ratio. This smoothening effect due to theincreasing of the thickness of the bottom-most layer is related to thesimultaneously decreasing grain size.