Critical properties of highly frustrated pyrochlore antiferromagnets

The pyrochlore form of FeF3 (pyr-FeF3) exhibits an unusual noncoplanar form of long-range magnetic order below 16 K. This is a result of the topological frustration inherent in the lattice of corner-sharing tetrahedra formed by the iron atoms. Neutron-diffraction experiments fix the critical exponent β at 0.18(2), which does not correspond to any known universality class. Monte Carlo simulations on the same lattice with Heisenberg spins confirm this value of β and also determine that ν=0.38(2), γ=1.1(1), and α=0.6(1). The power of Ferrenberg and Swendsen’s histogram method of Monte Carlo data analysis is discussed, as well as careful checks for weak first-order behavior in the transition. The recently discovered universality classes in triangular-lattice antiferromagnets are compared to the situation in pyr-FeF3.