This leads not to long range order as found in many magnetic systems at low temperatures, but to a multitude of degenerate states. In such a system unusual transitions are expected in which the entropy decreases to zero while the internal energy is unchanged. Such socalled ‘Kasteleyn transitions’ have previously been observed in lipid bilayer systems: spin ice affords the first magnetic example. Ho2Ti2O7 is an example of a spin ice and in applied magnetic field the quasi-two dimensional version can be obtained (so-called Kagomé ice). Using PRISMA we were able to verify that the neutron scattering in the Kagomé ice phase changes as expected close to a Kasteleyn transition. Simultaneously we observed the highly anisotropic 'pinch point' scattering, seemingly indicative of simultaneous long and short range order, being sharp in one direction and diffuse in all others. In fact such features are the key signature of any type of topological constraint in a frustrated system which can be mapped to the ice rules. These observations open the way to the observation of similar effects in other related systems, such as some hydrogen bonded networks, where it is also expected that ice rule constraints operate.
T Fennell, ST Bramwell, DF McMorrow (University College London), P Manuel (ISIS), AR Wildes (ILL, France)
Research date: December 2007
T Fennell et al., Nature Physics 3, (2007) 566