Ultrasound investigations of intrinsic and extrinsic nonstationary field-driven processes in spin ice

The elastic properties of spin-ice material Dy₂Ti₂O₇ has 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. The sound velocity and the sound attenuation exhibit a number of anomalies versus magnetic field at temperatures below the ”freezing” temperature of approximately 500 mK. 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. Additionally a sharp drop in the sound velocity can be seen at B = 1.25 T. This can indicate a 1st-order phase transition from the low-density to the high-density monopole state. We discuss our observations in context of the emergent quasiparticles which govern the low-temperature dynamics of the spin-ice.