A new macroscopically degenerate ground state in the spin ice compound Dy2Ti2O7 under a magnetic field

The low-temperature magnetic properties of the pyrochlore compound Dy2Ti2O7 in magnetic fields applied along the [111] direction are reported. Below 1 K, a clear plateau has been observed in the magnetization process in the field range 2-9 kOe, followed by a sharp moment jump at around 10 kOe that corresponds to a breaking of the spin ice state. We found that the plateau state is disordered, with a residual entropy of nearly half the value for the zero-field state, whose macroscopic degeneracy comes from a frustration of the spins on the Kagome layers perpendicular to the magnetic field.