Anisotropic diffusion of3dmetals on W(110): Competition between crystalline structure and surface steps

Abstract

A dot of Fe, Co, Ni, or Cu was deposited on a clean W(110) surface by evaporation through a mask (\O{} 100 \ensuremath{\mu}m). Upon annealing at 720--1070 K we observed by scanning Auger microscopy that for Fe, Co, and Ni a 1-ML-thick film spreads across the surface, whereas for Cu the simultaneous spreading of 1 and 2 ML was found. For all materials the spreading is anisotropic, though significant differences between the elements were found. For Fe the fast spreading direction is along 〈110〉 and the anisotropy is caused by the crystallographic structure of the substrate. For Co and Ni the fast spreading direction is determined at low spreading temperatures $(<~820\mathrm{}\mathrm{K})$ by the steps on the substrate surface. For the higher spreading temperatures the influence of the steps becomes weaker and the crystallographic structure tends to determine the spreading anisotropy. For Cu, the fast spreading direction for the first ML is along the step direction whereas the crystallographic structure causes an anisotropic spreading of the second ML. The spreading behavior is discussed within the ``unrolling carpet'' model.