Monopoles, magnetricity and the stray field from spin ice
  An analysis is presented of the behaviour of muons in the low-temperature state in spin ice. It is shown in detail how the behavior observed in some previous muon experiments on spin ice in a weak transverse field may result from the macroscopic stray field of magnetized spin ice. A model is presented which allows these macroscopic field effects to be simulated and the results agree with experiment. The persistent spin dynamics at low temperature originate from the sample and could be a muon-induced implantation effect that is operative in out-of-equilibrium systems with long relaxation times. 
S. J. Blundell
cond-mat.str-el