Helimagnetism in the candidate ferroelectric CrI2

Although many other transition metal halides have been magnetically characterized, CrI${}_{2}$, a proposed ``sliding ferroelectric'', has been overlooked. Via neutron powder diffraction, the authors report here that a screw-like helimagnetic order develops below 17 K, corresponding to a ~90\textordmasculine{} spin rotation per Cr ion along the ``ribbon chains''. Calculations suggest that, as in the structurally and magnetically similar copper dihalides, the helimagnetism arises due to a sufficiently antiferromagnetic intrachain next-nearest-neighbor coupling.