Broken vector spin chirality in biatomic Fe chains on Ir(001)*

We investigate from {\em ab initio} the magnetism of biatomic Fe chains, which form due to self-organization on the (5$\times$1)-reconstructed Ir(001) surface [1,2]. Using the {\tt FLEUR} code [3], we calculate the magnetic properties and exchange interactions in this system, finding a very small Heisenberg exchange along the chain of the order of 10 meV/Fe-atom. Upon including spin-orbit coupling we obtain the contribution from the Dzyaloshinskii-Moriya interaction and find that it leads to a 120$^{\circ}$ spin-spiral ground state of the Fe chains with a unique rotational sense. The results of the Monte-Carlo simulations based on the parameters from {\it ab initio} are in a very good agreement to STM experiments on the system. Moreover, simulations indicate a robustness of the spin chiral order parameter, which decays with temperature much slower than the scalar spin correlation, in analogy to a vector spin chiral liquid state. We discuss possible applications of the magnetism in these chains with respect to the transfer of information on the nanoscale.\newline [1] L. Hammer {\it et al.}, Phys. Rev. B {\bf 67}, 125422 (2003). [2] Y. Mokrousov \textit{et al.}, Phys. Rev. B \textbf{80}, 195420 (2009). [3] www.flapw.de