MUON SPIN RELAXATION INVESTIGATION OF THE SPIN DYNAMICS OF GEOMETRICALLY FRUSTRATED ANTIFERROMAGNETS Y2MO2O7 AND TB2MO2O7

The spin dynamics of geometrically frustrated pyrochlore antiferromagn ets Y2Mo2O7 and Tb2Mo2O7 have been investigated using muon spin relaxa tion. A dramatic slowing down of the moment fluctuations occurs as one approaches the spin freezing temperatures (T-F=22 and 25 K, respectiv ely) from above. Below T-F there is a disordered magnetic state simila r to that found in a spin glass but with a residual muon spin relaxati on rate at low temperatures. These results show that there is a large density of states for magnetic excitations in these systems near zero energy.