Experimental and theoretical evidence for a new universality class in FeF3: A 3D lattice with frustrated Heisenberg spins (abstract)

The pyrochlore form of FeF3 is a Heisenberg antiferromagnet [TN=15.2(1)K], in which the Fe3+ atoms form a high-symmetry lattice of corner-sharing tetrahedra. The low-temperature magnetic structure is noncollinear with four sublattices oriented 109° from each other. Neutron diffraction was used to determine the critical exponent, β=0.17(2), which does not correspond to any known universality class. Monte Carlo simulations with finite-size scaling analysis confirm the measured value of β and also provide the exponent ν=0.34(4). The existence of this new universality class supports Kawamura’s hypothesis that critical properties of a system depend on the symmetry properties (and not just the dimensionality) of the order parameter.