Helimagnetism in CrBr_2 and CrIBr

In CrX_2 (X=Br, I), a Jahn-Teller effect distorts the octahedral configuration of the anions about Cr, resulting in a "ribbon chain" structure. We previously observed helimagnetism in CrI_2 propagating along the ribbon chains, with an 80-90^∘ rotation per Cr ion. Via neutron scattering, we report that CrBr_2 and the solid solution CrIBr are also helimagnetic, with Néel temperatures of 17 and 12 K, respectively, and signs of intermediate magnetic transitions in both compounds. The helical angle between spins on consecutive Cr ions along the ribbon chains increases with increasing Br substitution, from 89.7^∘ (CrI_2) to 116^∘ (CrIBr) to 147^∘ (CrBr_2), possibly as a result of decreasing longer-range intrachain spin interactions with the increased electron localization of the lighter anions.