Excitation Continuum in a Pyrochlore S=1 Heisenberg Magnet

The existence of a spin-liquid for Heisenberg spins on the pyrochlore lattice was first speculated by Jacques Villain nearly 40 years ago and it is now recognized that the ground state of this model is a Coulomb spin liquid. However, experimental realizations are scarce. In this talk, I will discuss a new pyrochlore lattice magnet, NaCaNi₂F₇, which realizes the isotropic Coulomb spin liquid with S=1 spins and the complication of random Na⁺ - Ca²⁺ charge disorder in the crystal structure. I will present neutron scattering and calorimetric measurements that were used to uncover the magnetic correlations in this material and determine the magnetic Hamiltonian. The ionic disorder creates a rugged energy landscape that acts to freeze a small fraction of the magnetic degrees of freedom. However, the energy scale set by this disorder is small, and the Heisenberg interactions prevail. Only a small fraction of the available moment is frozen, and the magnetism in NaCaNi₂F₇ is dominated by a persistently fluctuating component that appears as a broad continuum of magnetic signal in inelastic neutron scattering measurements. These measurements demonstrate a beautiful realization of the Coulomb spin liquid and provide new insight into the interplay between disorder and magnetic exchange interactions in highly frustrated magnets.