Observation of helimagnetism in the candidate ferroelectric CrI${}_2$

CrI${}_2$ is a quasi-one dimensional (1D) van der Waals (vdW) system that exhibits helimagnetism that propagates along the ribbons. This was determined from neutron time-of-flight diffraction measurements. Below $T_N=17$ K, a screw-like helimagnetic order develops with an incommensurate wavevector of $\mathbf{q}\approx (0.2492,0,0)$ at 8 K. Using density functional theory (DFT)$+U$ calculations, the $J_1$-$J_2$ model was leveraged to describe the helimagnetism, where $J_1(>0)$ and $J_2(<0)$ correspond, respectively, to a ferromagnetic nearest neighbor (NN) and antiferromagnetic next-nearest neighbor (NNN) intrachain interaction. The DFT$+U$ calculations predict that bulk CrI${}_2$ in the orthorhombic $Cmc{2}_1$ crystal structure satisfies the $\mathrm{\mid }{J}_2\mathrm{\mid }>\mathrm{\mid }{J}_1\mathrm{\mid }\mathrm{/}4$ condition, which favors formation of helimagnetic order.