Evidence for two distinct spin relaxation mechanisms in 'hot' spin ice Ho₂Ti₂O₇

Neutron scattering and ac-susceptibility techniques have been performed on the spin ice material Ho₂Ti₂O₇ to study the spin relaxation processes in the 'hot' paramagnetic phase (T > 1 K). Neutron spin echo (NSE) proves that above T ≅ 15 K the spin dynamics are governed by a thermally activated single-ion process. At lower temperatures (T < 15 K) this cannot account for the spin dynamics found in ac-susceptibility measurements. It is inferred that a second, slower process, with a different thermal signature dominates. We suggest that this is a quantum-mechanical tunnelling process between different spin states separated by a large energy barrier.