Growth Morphology of Thin Fe Films on Al(100)

Abstract

The mechanism of growth and the atomic structure of ultrathin Fe films, vapor deposited in ultra--high vacuum onto Al(100) kept at room temperature, have been investigated with different surface--sensitive techniques. Initially, Al(100) shows short--range (SRO) as well as long--range order (LRO), determined using secondary--electron imaging [1] and low--energy electron diffraction, respectively. LRO on the surface is destroyed after deposition of minute amounts of Fe, that is less than 1 atomic layer (AL), while no SRO is detected in the growing Fe film. The chemical composition at the surface is determined by the combination of Auger electron and ion--scattering spectroscopy. The results show that during the initial stages of growth Fe diffuses into the substrate and forms a surface alloy with Al. After an Fe deposition that amounts to an equivalent of 4\,AL, SRO in the atomic structure of the surface alloy is detected in form of bcc clusters. According to the bulk phase diagram of the Al--Fe system [2], only pure Fe and/or Al dissolved in Fe have the bcc structure. Therefore, we conclude that a dealloying is responsible for the formation of the bcc clusters. Atomically, they are rotated by 45$^\circ$ with respect to the fcc substrate, as expected by considering the lattice constants of pure Al and Fe metals. A continuous Fe film is formed at an equivalent Fe deposition of 6\,AL which shows LRO. The transition to an ordered surface film after the apparently disordered structure of the surface species is mediated by different Al--Fe surface alloys that act as a seed for the development of structural LRO in the Fe film. Ferromagnetic ordering at the surface could not be detected, observing magneto--optical Kerr rotation during Fe deposition, before structural LRO develops in the Fe film.