Dynamics of diluted Ho spin ice Ho2-xYxTi2O7 studied by neutron spin echo spectroscopy and ac susceptibility

We have studied the spin relaxation in diluted spin ice Ho2-xYxTi2O7 by means of neutron spin echo spectroscopy and ac susceptibility measurements. Remarkably, the geometrical frustration is not relieved by doping with nonmagnetic Y, and the dynamics of the freezing is unaltered in the spin echo time window up to x >= 1.6. At higher doping with nonmagnetic Y (x similar or equal to 1.6) a new relaxation process at relatively high temperature (up to at least T similar or equal to 55 K) appears which is more than 10 times faster than the thermally activated main relaxation process. We find evidence that over the whole range of composition all Ho spins participate in the dynamics. These results are compared to ac susceptibility measurements of the diluted Ho and Dy spin ice systems, which show that at low temperature the Arrhenius behavior is masked by another dynamical process with little temperature dependence. X-ray absorption fine structure (EXAFS) spectra and x-ray diffraction show that the samples are structurally well ordered.