Local spin dynamics in the geometrically frustrated Mo pyrochlore antiferromagnet Lu2Mo2O5-yN2

The magnetic ground state of oxynitride pyrochlore Lu2Mo2O5-yN2, a candidate compound for the quantum spin liquid (S = 1/2, Mo5+), was studied by muon spin rotation/relaxation experiment. In contrast to Lu2Mo2O7 (S = 1, Mo4+) which exhibits the spin-glass behavior with a freezing temperature Tg ' 16 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 -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 a null gap.