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We have performed AC susceptibility and DC magnetic relaxation measurements on the spin ice system Dy 2 Ti 2 O 7 down to 0.08 K. The relaxation time of the magnetization has been estimated below 2 K down to 0.08 K. The spin dynamics of Dy 2 Ti 2 O 7 is well described by using two relaxation times (τ S (short time) and τ L (long time)). Both τ S and τ L increase on cooling. Assuming the Arrhenius law in the temperature range 0.5-1 K, we obtained an energy barrier of 9 K. Below 0.5 K, both τ S and τ L show a clear deviation from the thermal activated dynamics toward temperature independent relaxation, suggesting a quantum dynamics.

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