Magnetic density of states at low energy in geometrically frustrated   systems
  Using muon-spin-relaxation measurements we show that the pyrochlore compound Gd2Ti2O7, in its magnetically orderered phase below ~ 1 K, displays persistent spin dynamics down to temperatures as low as 20 mK. The characteristics of the induced muon relaxation can be accounted for by a scattering process involving two magnetic excitations, with a density of states characterized by an upturn at low energy and a small gap depending linearly on the temperature. We propose that such a density of states is a generic feature of geometrically frustrated magnetic materials. 
A. Yaouanc, P. Dalmas de Reotier, V. Glazkov, C. Marin, P. Bonville,   J.A. Hodges, P.C.M. Gubbens, S. Sakarya, C. Baines