The study of magnetic properties of ultrathin random films is important for spintronics applications as well as from the fundamental point of view. The parameter-free determination of the Curie temperature of low-dimensional systems is much more demanding as compared to bulk systems, among other things surface exchange interactions decay with the distance more slowly. As a model study, we investigated electronic structure and thermodynamical properties of Fe and Co overlayers on the fcc-Cu(001) substrate.
The effective two-dimensional Heisenberg Hamiltonian was constructed from first principles and properties of exchange interactions were investigated. The Curie temperatures were estimated using the Monte-Carlo simulations, and compared with a simplified approach based on the RPA method combined with the virtual crystal approximation. Our calculations indicate a weak maximum of the Curie temperature as a function of composition of the iron-cobalt overlayer. In the case of incomplete coverage we obtain percolation effect. We also present the study of the effect of alloy disorder on the shape of magnon spectra of random overlayers.