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

Artificial multilayered films of niobium and gadolinium were prepared at different substrate temperatures T(S) on sapphire (1120BAR) 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-degrees-C to guarantee oriented growth and to prevent the chemical reaction of gadolinium with sapphire. Samples with a multilayer period LAMBDA = D(Nb) + D(Gd) = 50-300 angstrom 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 LAMBDA = 50 angstrom only for T(S) less-than-or-equal-to 100-degrees-C. The multilayers are structurally incoherent for all T(S) up to 700-degrees-C, no three-dimensional metallic superlattice is built up. ... mehrThe loss of long-range order in the growth direction is mainly due to interface roughness caused by the variations in growth behavior.