Within the past 20 years or so, there has occurred an explosion of interest in the magnetic behavior of pyrochlore oxides of the type A 3+ 2 B 4+ 2 O 7 where A is a rare-earth ion and B is usually a transition metal. Both the A and B sites form a network of corner-sharing tetrahedra which is the quintessential framework for a geometrically frustrated magnet. In these systems the natural tendency to form long range ordered ground states in accord with the Third Law is frustrated, resulting in some novel short range ordered alternatives such as spin glasses, spin ices and spin liquids and much new physics. This article attempts to review the myriad of properties found in pyrochlore oxides, mainly from a materials perspective, but with an appropriate theoretical context.

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Neutron scattering, muon spin relaxation, and dc susceptibility studies have been carried out on polycrystalline Tb 2 Ti 2 O 7 , a pyrochlore antiferromagnet in which the Tb 31 moments reside on a network of corner-sharing tetrahedra. Unlike other geometrically frustrated systems, Tb 2 Ti 2 O 7 remains paramagnetic down to ϳ0.07 K, rather than ordering into a conventional Néel or spin-glass-like state, despite the fact that short-range antiferromagnetic correlations (AFC) develop at ϳ50 K. At the first AFC wave vector, its low-lying, relatively flat magnetic excitation spectrum softens partially below 30 K.