Intrinsic spin-disordered ground state of the Ising garnet Ho ₃Ga₅O₁₂

Motivated by predictions of a spin-ice state on the Ising garnet lattice, we have completed neutron-scattering experiments on single crystals of Ho ₃Ga₅O₁₂. Our results show that although the Ho³⁺ spins have Ising-type character with an easy-axis anisotropy, the low-temperature ground state has coexisting long-range- and short-range-ordered spins and is therefore not an ice state. Inelastic neutron-scattering measurements reveal the presence of low-lying crystal-field states that develop a softening at the onset of short-range magnetic ordering. We suggest that the specific tuning of the exchange and dipolar interactions, along with the accessibility of these low-lying excitations, conspire to drive the system to a disordered state.