Chromium trihalides were recognized as magnetic Van der Waals materials since 1920s. They were studied vigorously during 1950s to 70s when fundamental theories of magnetic ordering, exchange interactions, and magnetic anisotropy were developing. Recent advances in device fabrication and magneto-optical measurements lead to the discovery of 2D magnetic ordering in CrI3 monolayers with great potential for atomically thin devices. CrCl3, CrBr3, and CrI3 are stable in air and easy to exfoliate, therefore ideal for applications. Here, we present the chemical pathway to tune all physical properties of these materials by mixing different halide species. We use CrCl3-xBrx as a benchmark to show the design principles of mixed halides where isovalent substitution of Br for Cl occurs during a chemical vapor transport process. Remarkably, all physical properties including the van der Waals gap, the magnetic ordering temperature, the ferromagnetic exchange coupling, the optical gap, and the magnetic anisotropy are tuned linearly as a function of x. Our results demonstrate unprecedented control over the magneto-optical properties of CrCl3-xBrx required for a range of applications from spintronics to biomedical industries.
