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 spinorbit 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 L2 and L3 XMCD signals. It is shown that spin and orbital moments can be obtained with good accuracy from the XMCD spectra via the sum rules. The ^Tz& term of the spin sum rule is surprisingly small for the wires, and the reason for this is discussed.