  The idea of magnetic monopoles in spin ice has enjoyed much success at
intermediate temperatures, but at low temperatures a description in terms of
monopole dynamics alone is insufficient. Recently, numerical simulations were
used to argue that magnetic impurities account for this discrepancy by
introducing a magnetic equivalent of residual resistance in the system. Here we
propose that oxygen deficiency is the leading cause of magnetic impurities in
as-grown samples, and we determine the defect structure and magnetism in
Y2Ti2O(7-\delta) using diffuse neutron scattering and magnetization
measurements. These defects are eliminated by oxygen annealing. The
introduction of oxygen vacancies causes Ti4+ to transform to magnetic Ti3+ with
quenched orbital magnetism, but the concentration is anomalously low. In the
spin-ice material Dy2Ti2O7 we find that the same oxygen-vacancy defects
suppress moments on neighbouring rare-earth sites, and that these magnetic
distortions dramatically slow down the long-time monopole dynamics at
sub-Kelvin temperatures.
