Presence and absence of antiferromagnetic coupling and giant magnetoresistance in sputtered and evaporated permalloy/copper multilayers

In multilayered thin films consisting of permalloy (Ni80Fe20) and copper, a strong antiferromagnetic coupling and a related giant magnetoresistance can be found which oscillates with the thickness of the copper spacer layers. This, however, is usually detected only in sputtered films, whereas evaporated multilayers often show no or only a very weak coupling and giant magnetoresistance. Here, we report on a comparative study of sputtered and evaporated multilayers. In the first maximum of the antiferromagnetic coupling, i.e. at a copper thickness of 0.9 nm, our sputtered films exhibit a saturation field of up to 1000 Oe and values of the giant magnetoresistance of up to 22% at room temperature at a permalloy thickness of about 2 nm. In contrast to this, the corresponding evaporated multilayers do not show any sign of antiferromagnetic coupling. A subsequent analysis of the films by cross-sectional transmission electron microscopy imaging indicates, in small regions, a good and a poor multilayered structure for sputtered and evaporated films, respectively. X-ray reflectivity measurements and atomic force imaging of the surfaces pointed to an increased interface roughness in the evaporated films. Thus we conclude that the absence of the antiferromagnetic coupling in the evaporated films should be related to an increased roughness of the layer interface and not to interdiffusion. (C) 1998 Elsevier Science B.V. All rights reserved.