  A detailed study of the evolution of the magnetoresistance was performed on
electrodeposited Co/Cu multilayers with Cu layer thicknesses ranging from 0.5
nm to 4.5 nm. For thin Cu layers (up to 1.5 nm), anisotropic magnetoresistance
(AMR) was observed whereas multilayers with thicker Cu layers exhibited clear
giant magnetoresistance (GMR) behaviour. The GMR magnitude increased up to
about 3.5 to 4 nm Cu layer thickness and slightly decreased afterwards.
According to magnetic measurements, all samples exhibited ferromagnetic (FM)
behaviour. The relative remanence turned out to be about 0.75 for both AMR and
GMR type multilayers. This clearly indicates the absence of an
antiferromagnetic (AF) coupling between adjacent magnetic layers for Cu layers
even above 1.5 nm where the GMR effect occurs. The AMR behaviour at low spacer
thicknesses indicates the presence of strong FM coupling (due to, e.g.,
pin-holes in the spacer and/or areas of the Cu layer where the layer thickness
is very small). With increasing spacer thickness, the pin-hole density reduces
and/or the layer thickness uniformity improves which both lead to a weakening
of the FM coupling. This improvement in multilayer structure quality results in
a better separation of magnetic layers and the weaker coupling (or complete
absence of interlayer coupling) enables a more random magnetization orientation
of adjacent layers, all this leading to an increase of the GMR. Coercive field
and zero-field resistivity measurements as well as the results of a structural
study reported earlier on the same multilayers provide independent evidence for
the microstructural features established here. The large GMR reported
previously on such Co/Cu multilayers at Cu layer thicknesses around 1 nm can be
attributed to the presence of a fairly large superparamagnetic (SPM) fraction
rather than being due to a strong AF coupling.
