Spin Hamiltonian, {Order Out of a Coulomb Phase} and Pseudo-Criticality in the Highly Frustrated Pyrochlore Heisenberg Antiferromagnet FeF$_{\bf 3}$}

Abstract: FeF$3$, with its half-filled Fe$^{3+}$ $3d$ orbital, hence zero orbital angular momentum and $S=5/2$, is often put forward as a prototypical highly-frustrated classical Heisenberg pyrochlore antiferromagnet. By employing {\it ab initio} density functional theory (DFT), we obtain an effective spin Hamiltonian for this material. This Hamiltonian contains nearest-neighbor antiferromagnetic Heisenberg, bi-quadratic and Dzyaloshinskii-Moriya interactions as dominant terms and we use Monte Carlo simulations to investigate the nonzero temperature properties of this minimal model. We find that upon decreasing temperature, the system passes through a Coulomb phase, composed of short-range correlated coplanar states, before transforming into an "all-in/all-out" (AIAO) state via a very weakly first order transition at a critical temperature $T_c\approx 22$ K, in good agreement with the experimental value for a reasonable set of Coulomb interaction $U$ and Hund's coupling $J{\rm H}$ describing the material. Despite the transition being first order, the AIAO order parameter evolves below $T_c$ with a power-law behavior characterized by a pseudo "critical exponent" $\beta \approx 0.18$ in accord with experiment. We comment on the origin of this unusual $\beta$ value.