Ultrasound investigations of spin-ice materials

This thesis is devoted to ultrasound investigations of a family of rare-earth compounds known as spin ice. The crystal structure of these compounds is composed of tetrahedral units with magnetic ions in each corners. In the ground state of these materials, two spins are directed inward on each tetrahedron and two spins outward. There are a number of features that are common to the spin-ice materials Ho2Ti2O7 (HTO), Yb2Ti2O7 (YbTO), and Dy2Ti2O7 (DTO). In DTO, nonequilibrium processes have been probed by ultrasound waves at low temperatures. The sound velocity and sound attenuation exhibit a number of unusual anomalies as a function of applied magnetic field for temperatures below the freezing temperature of 500 mK. These robust anomalies can be seen for longitudinal and transverse acoustic modes for different field directions. The anomalies show broad hystereses. Most notable are peaks in the sound velocity, which exhibit two distinct regimes: an intrinsic (extrinsic) regime in which the data collapse for different sweep rates when plotted as function of field strength (time). Moreover, these quasi-periodic peaks are strongly affected by thermalcoupling conditions. We discuss our observations in context of emergent quasiparticles (magnetic monopoles) which govern the low-temperature dynamics of spin ice. I have studied spin-lattice and single-ion effects in the spin-ice materials (DTO) and (HTO) in a wide range of temperatures and magnetic fields. The sound velocity and sound attenuation of various acoustic modes experience a renormalization due to phase transformations as well as interactions with lowenergy magnetic excitations (topological defects). In particular, a sharp dip observed in the sound attenuation has been explained within the framework of the spin-ice model. In addition, crystal-electric-field effects 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 exchange-striction couplings and single-ion-type interactions. Experiments on YbTO revealed evidence of a first-order transition known as a transition from a magnetic Coulomb liquid (MCL) to Coulomb ferromagnet state at T = 0.15 K. Coupling of the sound waves to quantum fluctuations cause a sharp anomaly in the sound velocity and sound attenuation. An increase of the quantum-fluctuation frequency when lowering the temperature down to the phase transition, leads to a minimum in the sound velocity and a maximum in the sound attenuation. This behavior can be explained in frame of resonating sound waves in presence of quantum fluctuations. Below the transition temperature, the quantum fluctuation effects are less pronounced. Measurements in applied magnetic fields, revealed a transition from a fluctuating Coulomb-ferromagnet state to a state with suppressed fluctuations. The experimental data presented in this thesis, show the important role of spin-strain interactions in spin-ice materials. In addition, theoretical considerations based on exchange-striction couplings and single-ion strain interaction, strongly support most of the experimental results