Ultrasound investigations of spin-ice materials

The elastic properties of spin-ice materials Dy₂Ti₂O₇ and Ho₂Ti₂O₇ have been studied for different longitudinal and transverse acoustic modes in a temperature range from 20 mK to 300 K and magnetic fields applied along various crystallographic directions up to 17.5 T. The sound velocity and the sound attenuation exhibit a number of anomalies versus applied magnetic field at temperatures below the “freezing” temperature. In Dy₂Ti₂O₇ compound, most notable are peaks in the sound velocity, which exhibit two distinct regimes: an intrinsic (extrinsic) one in which the data collapse for different field sweep rates when plotted as function of field strength (time). The intrinsic regime involves the release of Zeeman energy from spins, the extrinsic one, transfer of energy out of the sample. At B = 1.25 T additionally a sharp drop in the sound velocity can be seen. This can indicate a 1st-order phase transition from a low-density monopole state to the highdensity monopole state. We discuss our observations in context of the emergent quasiparticles which govern the low-temperature dynamics of the spin ice.