Magnetic properties of the chromium dihalides CrBr₂ and CrI₂

CrI₂ and CrBr₂ are structurally versatile helimagnetic compounds, having been isolated as 3D bulk crystals, 2D monolayers, and 1D chains encapsulated within carbon nanotubes. Due to Jahn-Teller distortion, a "ribbon chain" structure forms, with the helical spin rotation propagating along the chain direction. The ribbon chains are arranged into layers, and a number of polytypes have been seen, including nearly-primitive and nearly-rhombohedral monoclinic stacking types for CrBr₂ and CrI₂, respectively, and an orthorhombic CrI₂ structure with alternating layer orientations that has been proposed to be ferroelectric. We will discuss a number of recent results, including the existence of a structural phase transition transforming both CrBr₂ and CrI₂ to the monoclinic CrI₂ structure at high temperature, the spin wave spectra of these compounds, and the magnetic susceptibility of CrI₂ encapsulated within carbon nanotubes.