Low temperature spin dynamics of geometrically frustrated antiferromagnets Y2Mo2O7 and Y2Mo1.6Ti0.4O7 studied by muon spin relaxation

The spin dynamics of geometrically frustrated pyrochlore antiferromagnets Y2Mo2O7 and Y2Mo1.6Ti0.4O7 have been investigated using muon spin relaxation. In Y2Mo2O7 a dramatic slowing down of the Mo4+ spin fluctuations occurs as one approaches the spin freezing temperature TF of 22 K from above. At lower temperatures the spins freeze into a disordered magnetic state similar to that found in a spin glass but with a small residual muon spin relaxation rate at low temperatures. This residual relaxation rate is larger in Y2Mo1.6Ti0.4O7 where TF=15 K. These results suggest that there is a nonzero density of states for magnetic excitations in these systems near zero energy.