Theoretical Investigation of the Magnetic Order in FeAs

The magnetic structure of the iron monoarsenide FeAs is studied using first-principles calculations. We consider the collinear and non-collinear (spin-spiral wave) magnetic ordering and magnetic anisotropy. It is analitically shown that a magnetic triaxial anisotropy results in a sum of two spin-spiral waves with opposite directions of wave vectors and different spin amplitudes, so that the magnetic moments in two perpendicular directions do not equal each other.