Biocompatible Ferromagnetic Cr-Trihalide Monolayers

Cr with an electronic configuration of 3d$^{\mathrm{5}}$4s$^{\mathrm{1}}$ possesses the largest atomic magnetic moment (6\textmu $_{\mathrm{B}})_{\mathrm{\thinspace }}$of$_{\mathrm{\thinspace }}$all elements in the 3d transition metal series. Furthermore, the trivalent chromium (Cr$^{\mathrm{3+}})$ is biocompatible and is widely found in food and supplements. Here using first principles calculations combined with Monte Carlo simulations based on Ising model, we systematically study a class of 2D ferromagnetic monolayers CrX$_{\mathrm{3}}$ (X$=$ Cl, Br, I). The feasibility of exfoliation from their layered bulk phase is confirmed by the small cleavage energy and high in-plane stiffness. Spin-polarized calculations, combined with self consistently determined Hubbard U that accounts for strong correlation energy, demonstrate that CrX$_{\mathrm{3}}$ (X$=$Cl, Br, I) monolayers are ferromagnetic and Cr is trivalent and carries a magnetic moment of 3\textmu $_{\mathrm{B}}$, the resulting Cr$^{\mathrm{3+}}$ ions are biocompatible. The corresponding Curie temperatures for CrCl$_{\mathrm{3}}$ CrBr$_{\mathrm{3}}$ CrI$_{\mathrm{3\thinspace }}$are are found to 66, 86, and 107 K, respectively, which can be increased to 323, 314, 293 K by hole doping. The biocompatibility and ferromagnetism render these Cr-containing trichalcogenide monolayers unique for applications.