A Kagome ice state in the spin ice compound Dy2Ti2O7

We report on the specific heat and the entropy of single crystals of the spin ice compound Dy2Ti2O7 down to 100 mK under magnetic fields along the [111] direction in order to study detailed low temperature properties of the Kagome ice state. Previously, we reported anisotropic release of the residual entropy reflecting the Ising anisotropy. We pointed out that the anisotropic behaviour can be viewed as the difference in frustration dimensionality. In an intermediate field, the entropy for the [111] field direction is different from those of the other principal directions. This is because the frustration structure changes from that of a three-dimensional (3D) pyrochlore to that of a two-dimensional (2D) Kagome lattice with constraint from the ice rule, accompanied by a different zero-point entropy value. We call this state 'the Kagome ice state'. In the Kagome ice state, the value of the residual entropy, DeltaS(Kagome), is estimated as 0.598 J K-1/(mol Dy) by Pauling's method. In the present experiment, we observed that the specific heat at 1 T exhibits a sharp peak at 400 mK and an anomalous upturn at lower temperature. This peak indicates the entropy release from a 1-in 3-out state with no residual entropy to a 1-in 3-free state.