Different types of spin structures relevant for type-II multiferroics. (a) Sinusoidal spin density wave, in which spins point along one direction but vary in magnitude. This structure is centrosymmetic and consequently not ferroelectric. (b) The cycloidal spiral with the wave vector and spins rotating in the (x,z)-plane. It is in this case where one finds nonzero polarization, . (c) In a so-called “proper screw” the spins rotate in a plane perpendicular to . Here the inversion symmetry is broken, but most often it does not produce polarization, although in certain cases it might .