Direct evidence for a dynamical ground state in the highly frustrated   Tb$_2$Sn$_2$O$_7$ pyrochlore
  MuSR experiments have been performed on powder sample of the "ordered spin ice" Tb$_2$Sn$_2$O$_7$ pyrochlore compound. At base temperature (T=35mK) the muon relaxation is found to be of dynamical nature which demonstrates that strong fluctuations persist below the ferromagnetic transition (T_C=0.87K). Hints of long range order appear as oscillations of the muon polarization when an external field is applied and also as a hysteretic behavior below T_C. We propose a dynamical and strongly correlated scenario where dynamics results from fluctuation of large spin clusters with the "ordered spin ice" structure. 
F. Bert, P. Mendels, A. Olariu, N. Blanchard, G. Collin, A. Amato, C.   Baines and A.D. Hillier