Muon spin relaxation investigation of the spin dynamics of geometrically frustrated antiferromagnets ${\mathrm{Y}}_2$${\mathrm{Mo}}_2$${\mathrm{O}}_7$ and ${\mathrm{Tb}}_2$${\mathrm{Mo}}_2$${\mathrm{O}}_7$

The spin dynamics of geometrically frustrated pyrochlore antiferromagnets ${\mathrm{Y}}_2$ ${\mathrm{Mo}}_2$ ${\mathrm{O}}_7$ and ${\mathrm{Tb}}_2$ ${\mathrm{Mo}}_2$ ${\mathrm{O}}_7$ have been investigated using muon spin relaxation. A dramatic slowing down of the moment fluctuations occurs as one approaches the spin freezing temperatures (${\mathit{T}}_{\mathit{F}}$=22 and 25 K, respectively) from above. Below ${\mathit{T}}_{\mathit{F}}$ there is a disordered magnetic state similar to that found in a spin glass but with a residual muon spin relaxation rate at low temperatures. These results show that there is a large density of states for magnetic excitations in these systems near zero energy. © 1996 The American Physical Society.