Two-Dimensional Monopole Dynamics in the Dipolar Spin Ice Dy2Ti2O7

The spin dynamics of the dipolar spin-ice Dy2Ti2O7 under a dc magnetic field (B-dc) along the [111] direction has been investigated using ac susceptibility (chi(ac)), which enables us to study two-dimensional monopole motion in an ac magnetic field perpendicular to Bdc. We have observed the slowing down of monopole dynamics in the kagome spin-ice regime (B-dc similar to 0.4 T) and the extreme speeding up under B-dc close to the critical point (B-dc similar to 0.93 T). An analysis using a Monte Carlo simulation suggests that the dynamics changes from a random walk of monopoles to a collective motion as B-dc increases.