Interdiffusion up to the eutectic composition and vitrification in Gd/Co multilayers

Abstract

We have performed composition depth profiles and other structural experiments on Gd/Co multilayers and bilayers. Analysis of Auger depth profiles indicates a strongly asymmetric diffusion, especially of Co into the Gd layers. In multilayers, it is found that Co diffuses throughout the Gd layers and forms a Gd1−xCox amorphous alloy (x=0.29–0.35) that is limited at nearly the eutectic composition (x=0.37). Therefore, if the Co layers are nominally not too thin (≳3 nm), the multilayer mainly comprises an amorphous layers of composition around Gd0.7Co0.3 and layers of pure Co. Thin Co layers (3–7 nm) present broad halos in x-ray diffraction scans at wave vectors characteristics of pure cobalt. Thick layers (≳10 nm) show crystalline peaks with a coherence size (ξ⊥) shorter than its own layer thickness, suggesting the existence of amorphous phases next to the interface. This can be explained by the growth of pure and amorphous Co up to a thickness of 2–4 nm on the amorphous Gd alloy seed, but further Co deposition consists of hcp crystalline grains.