Magnetism of Co Layers Grown on a W(001) Surface: a Density Functional Study

In this study, we investigated the electronic structure and the magnetism of a system of Co layers grown on a W(001) surface, which exhibited relatively larger exchange splitting compared to that in the bulk in a recent experiment. We used the full-potential linearized augmented plane wave (FLAPW) method based on density functional theory to examine the possibility of formation of a surface alloy. The general gradient approximation was employed to describe the exchange-correlation potential. In this study, we considered a Co ML on a W(001) surface [1Co/W(001)] and a checkboard type CoW surface alloy on a W(001) surface [1(CoW)/W(001)], and we relaxed the systems by calculating total energies, being guided by the calculated forces. We found from the calculated total energies of the clean W(001) surface, the 1Co/W(001), and the 1(CoW)/W(001) that the surface alloy was favored energetically and that the alloy formation energy was about 380 meV. The ferromagnetic (FM) state was more stable, based on an energy difference of about 28 meV per Co atom, compared to the antiferromagnetic (AFM) one for 1(CoW)/W(001) whereas the AFM state was more favorable, based on an energy difference of about 25 meV per Co atom pair, than the FM one in 1Co/W(001). We will discuss the roles of the band hybridization between Co and W atoms in determining the ground magnetic states from the calculated single-particle energy spectra.