Spin ice is an example of a system in which the geometry of the lattice produces frustration, i.e. if two neighbouring spins are mutually arranged a third neighbouring spin cannot be. 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 so-called ‘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.
Diffuse scattering from the Kagomé ice phase of Ho2Ti2O7, showing distinctive pinch point scattering at x = 0, y = 0.667 and 1.333.
Research authors: T Fennell, ST Bramwell, DF McMorrow (University College London), P Manuel (ISIS), AR Wildes (ILL, France)
Further information: T Fennell et al., Nature Physics 3, (2007) 566
Notes for Editors:
1. About the London Centre for Nanotechnology
The London Centre for Nanotechnology is an interdisciplinary joint enterprise between University College London and Imperial College London. In bringing together world-class infrastructure and leading nanotechnology research activities, the Centre aims to attain the critical mass to compete with the best facilities abroad. Research programmes are aligned to three key areas, namely Planet Care, Healthcare and Information Technology and bridge together biomedical, physical and engineering sciences. Website: www.london-nano.com
2. About ISIS
ISIS is a world-leading centre for research in physical and life sciences operated by the Science and Technology Facilities Council at the Rutherford Appleton Laboratory, Oxfordshire, UK.
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