Quantum-Classical Reentrant Relaxation Crossover in [Formula presented] Spin Ice

We have studied spin relaxation in the spin ice compound [Formula presented] through measurements of the ac magnetic susceptibility. While the characteristic spin-relaxation time ([Formula presented]) is thermally activated at high temperatures, it becomes almost temperature independent below [Formula presented]. This behavior, combined with nonmonotonic magnetic field dependence of [Formula presented], indicates that quantum tunneling dominates the relaxational process below that temperature. As the low-entropy spin ice state develops below [Formula presented], [Formula presented] increases sharply with decreasing temperature, suggesting the emergence of a collective degree of freedom for which thermal relaxation processes again become important as the spins become strongly correlated.