Cooperative Paramagnetism in the Geometrically Frustrated Pyrochlore Antiferromagnet Tb2Ti2O7

Neutron scattering, muon spin relaxation, and dc susceptibility studies have been carried out on polycrystalline Tb2Ti2O7, a pyrochlore antiferromagnet in which the Tb3+ moments reside on a network of corner-sharing tetrahedra. Unlike other geometrically frustrated systems, Tb2Ti2O7 remains paramagnetic down to ∼0.07 K, rather than ordering into a conventional Néel or spin-glass-like state, despite the fact that short-range antiferromagnetic correlations (AFC) develop at ∼50 K. At the first AFC wave vector, its low-lying, relatively flat magnetic excitation spectrum softens partially below 30 K.