Spin-driven ferroelectricity in ferroaxial crystals

Spin-driven ferroelectricity in most non-collinear magnets, such as TbMnO3, is induced by the so-called inverse Dzyalonshinskii-Moriya mechanism and requires a cycloidal magnetic structure, an ordered magnetic state that is not truly chiral (or lacks helicity). Conversely, in a truly chiral magnetic state (proper helix), the pseudo-scalar helicity can not couple directly to the electric polarization, and therefore can't induce ferroelectric order. However, in systems of specific crystal symmetry, named here ``ferroaxials,'' the presence of collective structural rotations mediates an indirect coupling between magnetic helicity and ferroelectricity. I will review our recent experimental results for new compounds of this class, obtained by magnetic X-ray and neutron diffraction techniques, including a clear demonstration that the magnetic helicity can be controlled by an electric field.