Revisiting static and dynamic spin-ice correlations in Ho₂ Ti₂ O₇ with neutron scattering

Elastic and inelastic neutron-scattering studies have been carried out on the pyrochlore magnet Ho₂ Ti₂ O₇. Measurements in zero applied magnetic field show that the disordered spin-ice ground state of Ho₂ Ti₂ O₇ is characterized by a pattern of rectangular diffuse elastic scattering within the [HHL] plane of reciprocal space, which closely resembles the zone-boundary scattering seen in its sister compound Dy₂ Ti₂ O₇. Well-defined peaks in the zone-boundary scattering develop only within the spin-ice ground state below ∼2 K. In contrast, the overall diffuse-scattering pattern evolves on a much higher-temperature scale of ∼17 K. The diffuse scattering at small wave vectors below [001] is found to vanish on going to Q=0, an explicit signature of expectations for dipolar spin ice. Very high energy-resolution inelastic measurements reveal that the spin-ice ground state below ∼2 K is also characterized by a transition from dynamic to static spin correlations on the time scale of 10^-9 s. Measurements in a magnetic field applied along the [11̄ 0] direction in zero-field-cooled conditions show that the system can be broken up into orthogonal sets of polarized α chains along [11̄ 0] and quasi-one-dimensional β chains along [110]. Three-dimensional correlations between β chains are shown to be very sensitive to the precise alignment of the [11̄ 0] externally applied magnetic field.