At low temperature most systems in condensed matter physics approach a ground state that is either unique or is one of small, symmetry-related set. Geometrically frustrated magnets stand out as exceptions to this general rule.
Within a classical approximation, they have macroscopically many ground states which are unrelated by symmetry, and at low temperatures they show both strong correlations and large fluctuations. Nearest neighbour antiferromagnets on the pyrochlore lattice have become standard examples of such systems. The chromium spinels ZnCr2O4 and MgCr2O4are interesting for being approximate realisations of the Heisenberg pyrochlore antiferromagnet, and provide a contrast to the Ising spin-ice materials. In this talk I will review current theoretical and experimental understanding of the behaviour of geometrically frustrated magnets at low temperature and describe efforts to understand neutron scattering from these chromium spinels.
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