How 'spin ice' freezes

The large degeneracy of states resulting from the geometrical frustration o f competing interactions is an essential ingredient of important problems i n fields as diverse as magnetism(1), protein folding(2) and neural networks (3). As first explained by Pauling(4), geometrical frustration of proton po sitions is also responsible for the unusual low-temperature thermodynamics of ice and its measured 'ground state' entropy(5). Recent work has shown th at the geometrical frustration of ice is mimicked by Dy2Ti2O7, a site-order ed magnetic material in which the spins reside on a lattice of corner-shari ng tetrahedra where they form an unusual magnetic ground state known as 'sp in ice'(6-13). Here we identify a cooperative spin-freezing transition lead ing to the spin-ice ground state in Dy2Ti2O7. This transition is associated with a very narrow range of relaxation times, and represents a new form of spin-freezing. The dynamics are analogous to those associated with the fre ezing of protons in ice, and they provide a means through which to study gl ass-like behaviour and the consequences of frustration in the limit of low disorder.