2D auxetic material with intrinsic ferromagnetism: a copper halide (CuCl₂) monolayer

The discovery of ferromagnetism in monolayer transition metal halides exemplified by CrI₃ has opened a new avenue in the field of two-dimensional (2D) magnetic materials, and more such 2D materials are waiting to be explored. Herein, using an unbiased structure search combined with first-principles calculations, we have identified a novel CuCl₂ monolayer, which exhibits not only intrinsic ferromagnetism but also auxetic mechanical properties originating from the interplay of lattice and Cu-Cl tetrahedron symmetries. The predicted Curie temperature of CuCl₂ reaches ∼47 K, and its ferromagnetism is associated with the strong hybridization between the Cu 3d and Cl 3p states in the configuration. Moreover, upon biaxial tensile strain or carrier doping, the CuCl₂ monolayer can be converted from ferromagnetic to non-magnetic and from half-metal to metal. These properties endow this CuCl₂ monolayer with great potential for applications in auxetic/spintronic nanodevices.