Challenges in the collective behaviour of spin ice materials

The opportunity to observe magnetic monopoles in spin ice materials has driven a significant theoretical and experimental research effort over the past few years. While a broad class of experimental results have confirmed the monopole picture, some experiments continue to present tantalising puzzles which have not yet been possible to resolve via straightforward application of Coulomb liquid theories a la Debye-H\"{u}ckel. This is illustrated perhaps most strikingly by the departure from the expected asymptotic Arrhenius behaviour of the characteristic relaxation time scales observed at very low temperatures in magnetisation and magnetic susceptibility measurements. Here we investigate some of these phenomena and attempt to identify the necessary extensions of existing theories.