Nitrogen-induced reconstructions on the Cr(001) surface

We use spin-polarized density functional theory to describe the surface reconstructions formed when the Cr(001) surface is exposed to nitrogen. Our primary goal is to clarify the atomic structure of the N induced c(2x2) Cr (001) reconstruction and to study its properties. After analyzing the relative stability of several models with a 1/2 monolayer of N atoms on the surface, it was found that only two models were stable for different growth conditions. The first stable model shows two N atoms (per cell) adsorbed on hollow sites following a diagonal pattern. In the second, N atoms replace Cr atoms of the first monolayer, and after optimization, they end up in almost second layer positions. To compare with scanning tunneling microscopy experiments, simulated STM images, using the Tersoff-Hamann approximation, were obtained. Our two models can explain the two different c(2x2) Cr(001) reconstructions observed experimentally. The magnetic properties, such as magnetic moments and alignments, are discussed compared with experimental values.