Slow spin relaxation in dipolar spin ice.*

Spin relaxation in dipolar spin ice Dy$_{2}$Ti$_{2}$O$_{7}$ and Ho$_{2}$Ti$_{2}$O$_{7}$ was investigated using the magnetocaloric effect and susceptibility. The magnetocaloric behavior of Dy$_{2}$Ti$_{2}$O$_{7 }$at temperatures where the orientation of spins is governed by ``ice rules`` (T $<$ T$_{ice})$ revealed thermally activated relaxation; however, the resulting temperature dependence of the relaxation time is more complicated than anticipated by a mere extrapolation of the corresponding high temperature data [1]. A susceptibility study of Ho$_{2}$Ti$_{2}$O$_{7 }$ was performed at T $>$ T$_{ice }$and in high magnetic fields, and the results suggest a slow relaxation of spins analogous to the behavior reported in a highly polarized cooperative paramagnet [2]. [1] J. Snyder et al., Phys. Rev. Lett. 91 (2003) 107201. [2] B. G. Ueland et al., Phys. Rev. Lett. 96 (2006) 027216.