Thermodynamic and single-ion properties of Tb3+ within the collective paramagnetic-spin liquid state of the frustrated pyrochlore antiferromagnet Tb2Ti2O7

Abstract

In a recent paper [J. S. Gardner et al., Phys. Rev. Lett. 82, 1012 (1999)] it was found that the ${\mathrm{Tb}}^{3+}$ magnetic moments in the ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ antiferromagnetic pyrochlore lattice of corner-sharing tetrahedra remain in a collective paramagnetic state down to 70 mK. In this paper we present results from dc magnetic susceptibility, specific-heat data, inelastic neutron-scattering measurements, and crystal-field calculations that strongly suggest that (i) the ${\mathrm{Tb}}^{3+}$ ions in ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ possess a moment of approximatively $5{\ensuremath{\mu}}_{\mathrm{B}},$ and (ii) the ground state g-tensor is extremely anisotropic below a temperature of ${O(10}^{0}) \mathrm{K},$ with Ising-like ${\mathrm{Tb}}^{3+}$ 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 ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ failing to develop long-range order at a temperature of the order of the paramagnetic Curie-Weiss temperature ${\ensuremath{\theta}}_{\mathrm{CW}}\ensuremath{\approx}\ensuremath{-}{10}^{1} \mathrm{K}.$ We suggest that dipolar interactions and extra perturbative exchange coupling(s) beyond nearest neighbors may be responsible for the lack of ordering of ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}.$