  In the ground state of Ho2Ti2O7 spin ice, the disorder of the magnetic
moments follows the same rules as the proton disorder in water ice. Excitations
take the form of magnetic monopoles that interact via a magnetic Coulomb
interaction. Muon spin rotation has been used to probe the low-temperature
magnetic behaviour in single crystal Ho2-xYxTi2O7 (x=0, 0.1, 1, 1.6 and 2). At
very low temperatures, a linear field dependence for the relaxation rate of the
muon precession lambda(B), that in some previous experiments on Dy2Ti2O7 spin
ice has been associated with monopole currents, is observed in samples with
x=0, and 0.1. A signal from the magnetic fields penetrating into the silver
sample plate due to the magnetization of the crystals is observed for all the
samples containing Ho allowing us to study the unusual magnetic dynamics of Y
doped spin ice.
