Temperature and magnetic field dependence of spin-ice correlations in the pyrochlore magnet Tb2Ti2O7

We present a parametric study of the diffuse magnetic scattering at (1/2, 1/2, 1/2) positions in reciprocal space, ascribed to a frozen antiferromagnetic spin ice state in single-crystalline Tb2Ti2O7. Our high-resolution neutron scattering measurements show that the elastic (-0.02 meV < E < 0.02 meV) (1/2, 1/2, 1/2) scattering develops strongly below approximate to 275 mK, and correlates with the opening of a spin gap of approximate to 0.06 to 0.08 meV over most of the Brillouin zone. The nature of the transition at 275 mK has many characteristics of spin glass behavior, consistent with ac-susceptibility measurements. The application of a magnetic field of 0.075 T applied along the [1 (1) over bar0] direction destroys the (1/2, 1/2, 1/2) elastic scattering, revealing the fragility of this short-range ordered ground state. We construct a refined H-T phase diagram for Tb2Ti2O7 and [1 (1) over bar0] fields which incorporates this frozen spin ice regime and the antiferromagnetic long-range order previously known to be induced in relatively large fields. Specific heat measurements on the same crystal reveal a sharp anomaly at T-c approximate to 450 mK and no indication of a transition near approximate to 275 mK. We conclude that the higher temperature specific heat peak is not related to the magnetic ordering but is likely a signal of other, nonmagnetic dipole correlations.