The magnetic tri-halide compounds continue to attract great interests has hosts of an array of fascinating quantum phenomena. In this talk, I will focus on three canonical members of this class of materials: FeCl₃, YbCl₃ and CrCl₃. Although all comprise a honeycomb lattice of magnetic ions, details of the exchange coupling and magnetic moment yield striking differences in physical behavior. In the absence of an applied magnetic field YbCl₃ has a long range ordered antiferromagnetic phase with a broad continuum of excitations. Inelastic neutron scattering measurements along with a spin wave theory accounting for both longitudinal transverse fluctuations reveal that YbCl₃ is perhaps the best realization to date of a nearest neighbor Heisenberg model on the honeycomb lattice and further reveal features in the continua which have long been expected in quantum magnets, but which have previously eluded observation. On the other hand, while CrCl₃ is also an example of a 2D honeycomb lattice, each honeycomb lattice plane contains ferromagnetically aligned spins and exhibits an excitation spectrum with well-defined Dirac magnons. This material serves as a test case for determining topological magnons and provides as clear demonstration of how to accurately determine a topological magnetic gap using inelastic neutron scattering. Details of experimental resolution functions must be incorporated in the analysis to accurately understand the energy scale of anisotropy terms in the Hamiltonian. While FeCl₃ also has a honeycomb structure, remarkably strong further neighbor interactions drive completely different behavior from the previous examples. We find evidence for the existence of a spiral spin-liquid state in this compound above the ordering temperature.

[A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory.]