Phonon-mediated spin-flipping mechanism in the spin ices Dy2Ti2 O7 and Ho2Ti2 O7

To understand emergent magnetic monopole dynamics in the spin ices Ho2Ti2O7 and Dy2Ti2O7, it is necessary to investigate the mechanisms by which spins flip in these materials. Presently there are thought to be two processes: quantum tunneling at low and intermediate temperatures and thermally activated at high temperatures. We identify possible couplings between crystal field and optical phonon excitations and construct a strictly constrained model of phonon-mediated spin flipping that quantitatively describes the high-temperature processes in both compounds, as measured by quasielastic neutron scattering. We support the model with direct experimental evidence of the coupling between crystal field states and optical phonons in Ho2Ti2O7.