Magnetic tunability of Fe-based van der Waals materials via halide composition

Transition-metal halides are van der Waals materials that can be tuned via mixed halide chemistry. Despite exciting applications in ultrathin magnetic logic devices, little is known regarding the tunability of their magnetic properties. In this talk, I report the sensitivity of the magnetic ground state in the series of compounds FeCl_3-xBr_x to minute changes in non-magnetic ligand size. Fe³⁺ halides in the high-spin, half-filled configuration are expected to host unknown competition between ferromagnetic (FM) and antiferromagnetic (AFM) interactions according to the Goodenough-Kanamori rules for super-exchange interactions. For very small amounts of Br doping, drastic changes in critical temperature as well as critical field are observed, both of which evolve continuously with increasing Br content. This novel tunability of critical field has not been discussed in previous mixed halide materials. Neutron diffraction is used to solve the zero-field magnetic structure of FeBr₃ and previous neutron results for FeCl₃ allow us to assign the appropriate magnetic ordering for intermediate Br content compounds. First principle calculations are performed to highlight the effect of altered lattice parameters on exchange coupling.