Quantum-Classical Reentrant Relaxation Crossover in Dy₂Ti₂O₇ Spin Ice

We have studied spin relaxation in the spin ice compound Dy₂Ti₂O₇ through measurements of the ac magnetic susceptibility. While the characteristic spin-relaxation time (τ) is thermally activated at high temperatures, it becomes almost temperature independent below T_cross∼13  K. This behavior, combined with nonmonotonic magnetic field dependence of τ, indicates that quantum tunneling dominates the relaxational process below that temperature. As the low-entropy spin ice state develops below T_ice∼4  K, τ 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.