Ab initio density functional calculations of ferromagnetism in low-dimensional nanostructures: From nanowires to nanorods

We present ab initio spin-density functional calculations of the electronic and magnetic properties of Fe and Ni nanostructures with a geometry varying between a straight linear wire and a three-dimensional nanorod. With decreasing tension along the axis of the nanostructure we find a series of transitions first from dimerized to periodic and zigzag wires, then to a planar triangular stripe, and further to a nanorod consisting of a periodic stacking of triangular antiprims. In all nanostructures atoms are in a high-moment state, with magnetic moments of about 3.1 mu(B) for Fe and about 1 mu(B) for Ni. A transition to a low-spin or nonmagnetic state is initiated at a fixed critical value of the interatomic distance, independent of dimension and coordination number. The analysis of the electronic structure shows that already for the one-dimensional nanostructures the ratio between exchange splitting and magnetic moment is close to the universal value I=Delta/M similar to 1 eV/mu(B) established for bulk itinerant magnets.