  The magnetic ground state of oxynitride pyrochlore
Lu$_2$Mo$_2$O$_{5-y}$N$_2$, a candidate compound for the quantum spin liquid
($S=1/2$, Mo$^{5+}$), was studied by muon spin rotation/relaxation experiment.
In contrast to Lu$_2$Mo$_2$O$_7$ ($S=1$, Mo$^{4+}$) which exhibits the
spin-glass behavior with a freezing temperature $T_g\simeq16$ K, no such spin
freezing or long range magnetic order was observed down to 0.3 K. Moreover, two
separate magnetic domains were detected below $\sim$13 K, which were
characterized by differences in spin dynamics. The first is the "sporadic" spin
fluctuation seen in frustrated antiferromagnets, where the amplitude of the
hyperfine fields suggests that the excitation comprises a local cluster of
unpaired spins. The other is rapid paramagnetic fluctuation, which is only
weakly suppressed at low temperatures. In place of the paramagnetic
fluctuation, the volume fraction for the sporadic fluctuation steadily
increases with decreasing temperature, indicating the formation of an
excitation gap with a broad distribution of the gap energy involving null gap.
