Growth and characterization of Nb/Gd multilayers for different substrate temperatures

Abstract

Artificial multilayered films of niobium and gadolinium were prepared at different substrate temperatures T S on sapphire (11 2 0) by electron-beam evaporation in ultrahigh vacuum. Prior to the sequential deposition of gadolinium and niobium films, a single-crystal niobium buffer layer was deposited at T S = 750 ° C to guarantee oriented growth and to prevent the chemical reaction of gadolinium with sapphire. Samples with a multilayer period Λ = D Nb + D Gd = 50−300 A ̊ were prepared. The growth morphology, chemical composition, and structure of the samples were analyzed by reflection high energy electron diffraction. Auger depth-profiling, Rutherford backscattering spectrometry, and X-ray scattering. Smooth growth of gadolinium on niobium is observed in contrast to an island growth of niobium on gadolinium at all temperatures T S. It is found that the resulting film morphology strongly depends on T S and film composition, yielding samples with well separated individual layers down to Λ = 50 A ̊ only for T S ⩽ 100 °C. The multilayers are structurally incoherent for all T S up to 700 °C, no three-dimensional metallic superlattice is built up. The loss of long-range order in the growth direction is mainly due to interface roughness caused by the variations in growth behavior.