Anomalous Ferromagnetism of Monatomic Co Wire at the Pt(111) Surface Step Edge

A first-principles investigation of the anomalous ferromagnetism of a quasi-one-dimensional Co chain at the Pt(111) step edge is reported. Our calculations show that the symmetry breaking at the step leads to an easy magnetization axis at an odd angle of $\sim20^{\circ}$ {\em towards} the Pt step, in agreement with experiment [P. Gambardella {\em et al.}, {\em Nature} {\bf 416}, 301 (2002)]. Also, the Co spin and orbital moments become noncollinear, even in the case of a collinear ferromagnetic spin arrangement. A significant enhancement of the Co orbital magnetic moment is achieved when modest electron correlations are treated within LSDA+$U$ calculations.