Density-functional study of the CO adsorption on the ferromagnetic fcc Co(0Â 0Â 1) film surface

Experimental results on CO adsorption at the thin fcc Co(0 0 1) films have been obtained recently [L. Tskipuri, R.A. Bartynski, Surf. Sci. 603 (2009) 802] whereas theoretical analysis of this system is not available. In the present paper, we consider regular p(2×2) and c(2×2) CO overlayers adsorbed in the atop, bridge or hollow sites above the five-layer ferromagnetic fcc Co(0 0 1) film deposited at the Cu(0 0 1) surface. By using first-principles density-functional calculations with the correlation-exchange functional in the PBE form, we calculate the CO relaxed geometries, adsorption energies, stretching CO vibrational frequencies, work function and local magnetic moments on and near CO. We consider also an empirical adsorption-energy correction that might reduce the well-known flaw of many density-functional forms when applied to CO. We show also that in this correction, instead of the previously used stretching vibration frequency also the C-O distance can be used. The calculations suggest that the stable chemisorption site for the CO coverage θ=0.25 or 0.50 is the atop position. Together with the calculated CO stretching vibrational frequencies this conclusion compares well with the experimental findings. The CO molecule acquires very small magnetic moment that couples antiferromagnetically to moments of cobalt atoms. The magnetization at Co atoms close to CO is slightly lowered, the effect being strongest for the atop adsorption. The overall picture is similar to that for the Co(0 0 0 1) and Co(1 1 1) surfaces.