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 beta 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 beta and also determine that nu = 0.38(2), gamma = 1.1(1), and alpha = 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.