In a recent letter [Phys. Rev. Lett. 82, 1012(1999] it was found that the Tb 3+ magnetic moments in the Tb2Ti2O7 pyrochlore lattice of corner-sharing tetrahedra remain in a collective paramagnetic state down to 70mK. In this paper we present results from d.c. magnetic susceptibility, specific heat data, inelastic neutron scattering measurements, and crystal field calculations that strongly suggest that (1) the Tb 3+ ions in Tb2Ti2O7 possess a moment of approximatively 5µB, and (2) the ground state g−tensor is extremely anisotropic below a temperature of O(10 0 )K, with Ising-like Tb 3+ magnetic moments confined to point along a local cubic 111 diagonal (e.g. towards the middle of the tetrahedron). Such a very large easy-axis Ising like anisotropy along a 111 direction dramatically reduces the frustration otherwise present in a Heisenberg pyrochlore antiferromagnet. The results presented herein underpin the conceptual difficulty in understanding the microscopic mechanism(s) responsible for Tb2Ti2O7 failing to develop long-range order at a temperature of the order of the paramagnetic Curie-Weiss temperature θCW ≈ −10 1 K. We suggest that dipolar interactions and extra perturbative exchange coupling(s) beyond nearest-neighbors may be responsible for the lack of ordering of Tb2Ti2O7.

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