Extreme sensitivity of the magnetic ground-state to halide composition in FeCl$_{3-x}$Br$_x$

Abstract

Mixed halide chemistry has recently been utilized to tune the intrinsic magnetic properties of transition-metal halides -one of the largest families of magnetic van der Waals materials. Prior studies have shown that the strength of exchange interactions, hence the critical temperature, can be tuned smoothly with halide composition for a given ground-state. Here we show that the groundstate itself can be altered by a small change of halide composition leading to a quantum phase transition in FeCl3−xBrx. Specifically, we find a three-fold jump in the Néel temperature and a sign change in the Weiss temperature at x = 0.08 corresponding to only 3% bromine doping. Using neutron scattering, we reveal a change of the ground-state from spiral order in FeCl3 to Atype antiferromagnetic order in FeBr3. Using first-principles calculations, we show that a delicate balance between nearest and next-nearest neighbor interactions is responsible for such a transition. These results support the proximity of FeCl3 to a spiral spin liquid state, in which competing interactions and nearly degenerate magnetic k-vectors may cause large changes in response to small perturbations.