Refrustration and Competing Orders in a Spin Ice Material.

Jaan Altosaar

Abstract:

Spin ices, frustrated magnetic materials analogous to common water ice, have emerged over the past fifteen
years as exemplars of high frustration in three dimensions. Recent experimental developments aimed at interrogating anew the low-temperature properties of these systems, in particular whether the predicted transition
to long-range order occurs, behoove researchers to scrutinize our current dipolar spin ice model description of
these materials. In this work we do so by combining extensive Monte Carlo simulations and mean-field theory
calculations to analyze data from previous magnetization, elastic neutron scattering and specific heat measurements on the paradigmatic Dy2Ti2O7 spin ice material. In the present work, we also reconsider the possible
importance of the nuclear specific heat, Cnuc, in Dy2Ti2O7. We find that Cnuc is not entirely negligible below a
temperature ∼ 0.5 K and must therefore be taken into account in a quantitative analysis of the calorimetric data
of this compound below that temperature. We find that in this material, small effective spin-spin exchange interactions compete with the magnetostatic dipolar interaction responsible for the main spin ice phenomenology.
This causes an unexpected “refrustration” of the long-range order that would be expected from the incompletely
self-screened dipolar interaction and which positions the material at the boundary between two competing classical long-range ordered ground states. This allows for the manifestation of new physical low-temperature
phenomena in Dy2Ti2O7, as exposed by recent specific heat measurements. We show that among the four most
likely causes for the observed upturn of the specific heat at low temperature – an exchange-induced transition to
long-range order, quantum non-Ising (transverse) terms in the effective spin Hamiltonian, the nuclear hyperfine
contribution and random disorder – only the last appears to be reasonably able to explain the calorimetric data