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.
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1996
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