Ultrasonic investigations of spin-ices Dy$_2$Ti$_2$O$_7$ and Ho$_2$Ti$_2$O$_7$ in and out of equilibrium

TL;DR

This study uses ultrasound techniques to explore the magnetic and lattice properties of spin-ice materials Dy2Ti2O7 and Ho2Ti2O7 under high magnetic fields, revealing phase transitions, monopole dynamics, and thermal effects.

Contribution

It provides new insights into spin-ice behavior, including the impact of non-equilibrium processes and thermal coupling on acoustic properties under extreme magnetic fields.

Findings

Detection of a dip in sound attenuation at the monopole gas-liquid transition in DTO.

Observation of macroscopic thermal-coupling effects suppressing thermal runaway.

Renormalization of sound velocity due to crystal-electric-field effects at high fields.

Abstract

We report ultrasound studies of spin-lattice and single-ion effects in the spin-ice materials Dy$_2$Ti$_2$O$_7$ (DTO) and Ho$_2$Ti$_2$O$_7$ (HTO) across a broad field range up to 60 T, covering phase transformations, interactions with low-energy magnetic excitations, as well as 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, quasi-periodic peaks in the acoustic properties of DTO due to non-equilibrium processes are found to be strongly affected by {\em 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 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.