Connection between magnetism and structure in Fe double chains on the Ir(100) surface

The magnetic ground state of nanosized systems such as Fe double chains, chains recently shown to form in the early stages of Fe deposition on Ir100, is generally nontrivial. Using ab initio density functional theory we find that the straight ferromagnetic FM state typical of bulk Fe as well as of isolated Fe chains and double chains is disfavored after deposition on Ir100 for all the experimentally relevant double chain structures considered. So long as spin-orbit coupling SOC is neglected, the double chain lowest energy state is generally antiferromagnetic AFM, a state which appears to prevail over the FM state due to Fe-Ir hybridization. Successive inclusion of SOC adds two further elements, namely, a magnetocrystalline anisotropy and a Dzyaloshinskii-Moriya DM spin-spin interaction; the former stabilizing the collinear AFM state and the latter favoring a long-period spin modulation. We find that anisotropy is most important when the double chain is adsorbed on the partially deconstructed Ir100—a state which we find to be substantially lower in energy than any reconstructed structure—so that in this case the Fe double chain should remain collinear AFM. Alterna- tively, when the same Fe double chain is adsorbed in a metastable state onto the 51 fully reconstructed Ir100 surface, the FM-AFM energy difference is very much reduced and the DM interaction is expected to prevail, probably yielding a helical spin structure.