Structural, magnetic, and chemical properties of thin Fe films grown on Rh(100) surfaces investigated with density functional theory

Abstract

The structural, magnetic, and chemical properties of ultrathin Fe films grown on Rh(100) films have been investigated using ab initio density-functional calculations. The particular interest in this system stems from the fact that in view of the size mismatch between face-centered cubic (fcc) Rh and body-centered cubic (bcc) Fe, it is expected (and actually confirmed by the calculations) that the Fe films will form a tetragonal structure intermediate between bcc $\ensuremath{\alpha}$- and fcc $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Fe}$, leading to a conflict between the ferromagnetic character of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}$ and the antiferromagnetism of $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Fe}$. Indeed the calculations find a single monolayer of Fe on Rh(100) (which had been characterized on the basis of XMCD experiments as ``magnetically dead'') to order antiferomagnetically, while thicker Fe films have a ferromagnetic ground state. Independent of the film thickness we find that the magnetic energy differences between the magnetic ground state and possible excited magnetic states are always much smaller than the energy difference between the ground state and the nonmagnetic state. This suggests a lower magnetic ordering temperature than in bulk bcc Fe (in agreement with experiment) and the existence of a disordered local-moment state at higher temperatures. The chemical properties of the films have been tested by the adsorption of CO molecules. We find that the adsorption properties are strongly dependent on the magnetic state of the substrate---the breakdown of magnetic order always leads to a significant enhancement of the adsorption energy. At low coverages we predict the existence of a tilted adsorption geometry similar to that reported for the (100) surface of $\ensuremath{\alpha}$ Fe.