From the bulk to monatomic wires: An ab-initio study of magnetism in Co systems with various dimensionality

A systematic ab-initio study within the framework of the local-spin-density approximation including spin-orbit coupling and an orbital-polarization term is performed for the spin and orbital moments and for the X-ray magnetic circular dichroism (XMCD) spectra in hcp Co, in a Pt supported and a free standing Co monolayer, and in a Pt supported and a free standing monatomic Co wire. When including the orbital-polarization term, the orbital moments increase drastically when going to lower dimensionality, and there is an increasing asymmetry between the L_2 and L_3 XMCD signal. It is shown that spin and orbital moments can be obtained with good accuracy from the XMCD spectra via the sum rules. The <T_z> term of the spin sum rule is surprisingly small for the wires, and the reason for this is discussed.