Elucidation of the helical spin structure of FeAs

We present the results of resonant x-ray scattering measurements and electronic structure calculations on the monoarsenide FeAs. We elucidate details of the magnetic structure, showing the ratio of ellipticity of the spin helix is larger than previously thought, at 2.58(3), and reveal both a right-handed chirality and an out-of-plane component of the magnetic moments in the spin helix. We find that electronic structure calculations and analysis of the spin-orbit interaction are able to qualitatively account for this canting.