  Low temperature measurements of the ac magnetic susceptibility along the
[110] direction of a single crystal of the dipolar spin ice material Ho2Ti2O7,
in zero static field, are presented. While behavior that is qualitatively
consistent with previous work on Ho2Ti2O7 and the related material Dy2Ti2O7 is
observed, this work extends measurements to appreciably lower temperatures and
frequencies. In the freezing regime, below 1 K, the dynamics are found to be
temperature activated, thus well described by an Arrhenius law with an
activation energy close to 6J_eff, a result that is not easily explained with
the current model of magnetic monopole excitations in dipolar spin ice. The
form and temperature dependence of the ac susceptibility spectra are found to
be nontrivial and distinct from standard glassy relaxation. Particular
attention has been paid to correcting for the demagnetization effect, which is
quite significant in these materials and has important, even qualitative,
effects on the susceptibility spectra.
