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

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 ground state itself can be altered by a small change of halide composition in ${\mathrm{FeCl}}_{3-x}{\mathrm{Br}}_x$. Specifically, we find a threefold 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 ${\mathrm{FeCl}}_3$ to $A$-type antiferromagnetic order in ${\mathrm{FeBr}}_3$. From first-principles calculations, we show that a delicate balance between nearest and next-nearest neighbor interactions is responsible for such a transition. These results demonstrate how varying the halide composition can tune the competing interactions and change the ground state of a spiral spin liquid system.

Figure 1

(a) Schematic illustration of the honeycomb $ab$ planes in the mixed halide system ${\mathrm{FeCl}}_{1.5}{\mathrm{Br}}_{1.5}$. The $J_1$ and $J_2$ exchange paths are highlighted in orange and blue colors, respectively. (b) Layered (VdW) structure of ${\mathrm{FeCl}}_{1.5}{\mathrm{Br}}_{1.5}$ viewed from the [210] direction. (c) SEM image and EDX color maps confirming the homogeneous distribution of halides in an ${\mathrm{FeCl}}_{1.33}{\mathrm{Br}}_{1.67}$ crystal. (d) Phase diagram of the spiral and FM states with ${\mathrm{FeCl}}_3$ and ${\mathrm{FeBr}}_3$ across the phase boundary. Here, the $(q,q,0)y$ label is the degenerate magnetic wave vector for the SSL state, not the final spiral ground state [11]. ${\mathrm{FeBr}}_3$ is not in the SSL phase and its order is FM in 2D.

Figure 2

(a) Magnetic susceptibility as a function of temperature measured under ZFC (open circles) and FC (full circles) conditions with $H\parallel c$. (b) Same as in (a) but with $H\perp c$. (c) Magnetization curves with $H\parallel c$ showing metamagnetic (MM) transitions in ${\mathrm{FeCl}}_{3-x}{\mathrm{Br}}_x$ samples with $x\ge 0.08$. (d) The MM transitions are absent when $H\perp c$. (e) $T_{\text{N}}$ as a function of Br content ($x$) showing an initial jump followed by a linear decrease. (f) ${\mathrm{\Theta }}_{\text{W}}$ as a function of $x$ showing an initial sign change followed by a linear decrease. (g) ${\mu }_{\text{eff}}$ estimated from the Curie-Weiss analysis (Fig. S1). Error bars in panels (e), (f), and (g) are mainly due to the uncertainty in evaluating the mass of thin VdW crystals. (h) The critical field of MM transitions as a function of $x$ showing an initial jump followed by a smooth increase. Error bars reflect the width of the transition (inset).

Figure 3

(a) 2D neutron diffraction scan at 4.8 K and zero field in the $H-L$ plane at $K=\overline{1}$, showing strong nuclear reflections at integer $L$ and weak magnetic reflections at half-integer $L$. (b) A 1D cut through the data in panel (a) showing nuclear (strong) and magnetic (weak) Bragg peaks that identify the magnetic propagation vector $\mathbf{k}=(0,0,1.5)$. (c) 2D scan in the $\left(H\overline{1}L\right)$ plane showing another set of nuclear and magnetic reflections. (d) 1D cut through the data in panel (c) showing Bragg peaks along $\left(1\overline{1}L\right)$ direction. (e) 2D scan in the $\left(H0L\right)$ plane showing the absence of magnetic reflections along the $\left(00L\right)$ direction. (f) Temperature dependence of the intensity of the magnetic reflection $\mathbf{Q}=(2,\overline{1},4.5)$ with a power-law fit to extract $T_{\text{N}}$ and the critical exponent $\beta$. (g),(h),(i) Candidate magnetic ground states of ${\mathrm{FeBr}}_3$ with $R_l$ symmetry. (j),(k),(l) Candidate ground states with $P_s$ symmetry. The ground state is determined as $A$-type AFM shown in panel (g).

Figure 4

(a) NN ($J_1$) and NNN ($J_2$) coupling constants computed from first principles and plotted as a function of in-plane lattice parameter $a$ in stoichiometric ${\mathrm{FeCl}}_3$. The left and right boundaries of the $x$ axes in panels (a)–(d) correspond to the $a$ axis of ${\mathrm{FeCl}}_3$ and ${\mathrm{FeBr}}_3$, respectively. (b) The ratio $|J_1/J_2|$ plotted as a function of $a$. (c),(d) Same as in (a),(b) but for the stoichimetric ${\mathrm{FeBr}}_3$. (e) $J_1$ and $J_2$ traced as a function of bromine content $x$ in ${\mathrm{FeCl}}_{3-x}{\mathrm{Br}}_x$ using virtual lattice approximation. (f) $|J_1/J_2|$ as a function of $x$.