Résumé
Motivated by both fundamental questions and potential technological applications, an intense research field is nowadays concerned by the possibility of generating innovative materials with controlled, specifically tailored properties. Through the exaltation of surface or interface effects the reduction of dimensionality in nanostructured systems usually gives rise to original, unique properties that are often completely different from those observed in the bulk state. In particular, the epitaxial growth of nanoscale thin films on appropriate substrates can be used to stabilize unusual crystalline phases, allowing thereby the generation of artificial materials. It is the case of thin Mn films adsorbed on W surfaces for which it was recently demonstrated experimentally that they are magnetic and adopt at room temperature the δ bcc structure [1, 2], whereas this structure is stable in the bulk state only at temperatures much higher than any possible magnetic ordering temperature. In this talk we will show how an atomistic first-principles approach based on the density functional theory can fruitfully supplement the experiments in order to get a better insight in the intriguing properties of this system. In good agreement with experiments the pseudomorphic growth of up to a few Mn layers deposed on W surfaces is demonstrated. Thin films Mn/W(110) are found to be antiferromagnetic while our calculations for Mn/W(100) predict a remarkable transition from ferromagnetic to antiferromagnetic order between 2 and 3 Mn monolayers. Magnetism turns out to have a major contribution in stabilizing the pseudomorphic growth of the few first layers on W(110) for example. To get a more realistic approach of the experimental growth conditions we also performed an extensive study of the elementary dynamical processes occurring in the first stages of Mn growth. We show that the kinetics of Mn surface diffusion is governed by the hopping process while exchange with surface atoms is rather disfavored due to higher energy barriers. Magnetic interaction between adatoms and substrate leads to complex behaviors like for instance a remarkable decrease in the diffusion barriers or in some cases to interesting frustration effects. [1] M. Bode et al., Surf. Sci. 432, 8 (1999); Phys. Rev. B 66, 014425 (2002). [2] Y. Tian and F. Jona, J. Phys.: Condens. Matter 13, 1805 (2002).