Zero Point Entropy in Spin Ice - Dy sub 2 TiO sub 2 O sub 7

Common water ice (I sub n) is an unusual solid - the oxygen atoms form a periodic structure but the hydrogen atoms are highly disordered due to the presence of two inequivalent O-H bond lengths. Pauling showed that this leads to a macroscopic degeneracy of possible ground states. The dynamics associated with this degeneracy are inaccessible, however, because the energy barriers for H reorganization are large - ice melts and the H-dynamics can not be studied independently of O-motion. An analogous system in which this degeneracy can be studied is a magnet with the pyrochlore structure, where spin orientation is associated with H position in ice (I sub n). We present specific heat data for such a system, Dy sub 2 Ti sub 2 O sub 7, and find a total spin entropy of 0.67RIn2, similar to ice (I sub n) which has 0.71RIn2, thus demonstrating the validity of the correspondence between ice and this magnetic analogue. By applying a magnetic field we find behavior not seen in pure ice - restoration of much of the ground state entropy and new transitions involving transverse spin degrees of freedom.