Onset of non-collinear magnetism in small Fe clusters

The onset of noncollinear magnetism in small Fe_N clusters is investigated by using a rotational invariant tight-binding Hamiltonian. The ground-state local magnetic moments, magnetic order and average magnetic moments are calculated as a function of the Coulomb exchange integral J. Representative results for Fe₃ and Fe₅ show that the noncollinear magnetic solutions are the most stable. A variety of qualitatively different self-consistent solutions is obtained as a function of J. This includes magnetic solutions with collinear and noncollinear spin arrangements. Our calculations are compared with previous density-functional results. Extensions and limitations of our work are also pointed out.