Monatomic Fe and Co wires at the Pt surface step edge: theory of the unconventional magnetic anisotropy

We report a first-principles investigation of the anomalous magnetic anisotropy of quasi-one-dimensional ferromagnetic Fe and Co chains decorating the Pt(1 1 1) step edge. Our calculations for both Fe and Co wires show that the easy magnetization is always confined in the plane perpendicular to the wire. The symmetry breaking at the step leads to an easy magnetization axis of the Co wire at an odd angle of ∼20∘ towards the Pt step, in agreement with the experiment. This canting becomes more pronounced for the Fe wire, for which we obtain an easy axis at 30∘ towards the Pt step. Also, the Fe and Co spin and orbital moments become noncollinear, even in the case of a collinear ferromagnetic spin arrangement.