Nonequilibrium processes are probed by ultrasound waves in the spin-ice material Dy${}_2$Ti${}_2$O${}_7$ at low temperatures. The sound velocity and the sound attenuation exhibit a number of anomalies versus applied magnetic field for temperatures below the “freezing” temperature of $\sim$500 mK. These robust anomalies can be seen for longitudinal and transverse acoustic modes for different field directions. The anomalies show a broad hysteresis. Most notable are peaks in the sound velocity, which exhibit two distinct regimes: an intrinsic (extrinsic) one in which the data collapse for different sweep rates when plotted as function of field strength (time). We discuss our observations in context of the emergent quasiparticles which govern the low-temperature dynamics of the spin ice.

• Received 4 August 2011

DOI:https://doi.org/10.1103/PhysRevB.84.220404