Ultrasonic investigations of the spin ices Dy2Ti2O7 and Ho2Ti2O7 in and out of equilibrium

We report ultrasound studies of spin-lattice and single-ion effects in the spin-ice materials Dy2Ti2O7 (DTO) and Ho2Ti2O7 (HTO) across a broad field range up to 60 T, covering phase transformations, interactions with low-energy magnetic excitations, and single-ion effects. In particular, a sharp dip observed in the sound attenuation in DTO at the gas-liquid transition of the magnetic monopoles is explained based on an approach involving negative relaxation processes. Furthermore, quasiperiodic peaks in the acoustic properties of DTO due to nonequilibrium processes are found to be strongly affected by macroscopic thermal-coupling conditions: the thermal runaway observed in previous studies in DTO can be suppressed altogether by immersing the sample in liquid helium. Crystal-electric-field effects having a higher energy scale lead to a renormalization of the sound velocity and sound attenuation at very high magnetic fields. We analyze our observations using an approach based on an analysis of exchange-striction couplings and single-ion effects