Magnetization and susceptibility studies on single crystals of the pyrochlore Ho$_{ 2}$Ti$_{ 2}$O$_{ 7}$ are reported for the first time. Magnetization isotherms are shown to be qualitatively similar to that predicted by the nearest neighbor spin-ice model. Below the lock-in temperature, $T^{ \ast }\simeq 1.97$ K, magnetization is consistent with the locking of spins along [111] directions in a specific two-spins-in, two-spins-out arrangement. Below $T^{ \ast}$ the magnetization for $B||[111]$ displays a two step behavior signalling the breaking of the ice rules.
Categories: cond-mat.dis-nn