Bruno recently posted the first paper resulting from his PhD thesis to the preprint archive. In it we take the first steps towards a microscopic view of the magnetic rare earth ions in spin ice, specifically taking into account their quantum-mechanical nature and the effect of crystal electric and magnetic fields. We thus depart from the simplified description of spin ice as a classical Ising system which has served the field so well in the past. This departure is an essential ingredient in the description of spin-flip dynamics and, therefore, of monopole propagation in these systems. Interestingly, the richer description has other consequences, including some curious magnetic anisotropies that might be experimentally observable.

Single-ion anisotropy and magnetic field response in spin ice materials Ho2Ti2O7 and Dy2Ti2O7

Motivated by its role as a central pillar of current theories of dynamics of spin ice in and out of equilibrium, we study the single-ion dynamics of the magnetic rare earth ions in their local environments, subject to the effective fields set up by the magnetic moments they interact with. This effective field has a transverse component with respect to the local easy-axis of the crystal electric field, which can induce quantum tunnelling. We go beyond the projective spin-1/2 picture and use instead the full crystal-field Hamiltonian. We find that the Kramers vs non-Kramers nature, as well as the symmetries of the crystal-field Hamiltonian, result in different perturbative behaviour at small fields (≲1 T), with transverse field effects being more pronounced in Ho2Ti2O7 than in Dy2Ti2O7. Remarkably, the energy splitting range we find is consistent with time scales extracted from experiments. We also present a study of the static magnetic response which highlights the anisotropy of the system in the form of an off-diagonal g tensor and we investigate the effects of thermal fluctuations in the temperature regime of relevance to experiments. We show that there is a narrow yet accessible window of experimental parameters where the anisotropic response can be observed.