Neutron Studies of Tb2Mo2O7

We have used the new high energy resolution spectrometer (BaSiS), at the Spallation Neutron Source in Oak Ridge, to conclusively prove the existence of a low energy mode at 0.34(1) meV in the spin glass Tb2Mo2O7. This mode is reminiscent of the excitation observed in the ordered phases of both Tb2Ti2O7 and Tb2Sn2O7. The dynamical nature of the transition seen in the magnetization at ~ 25 K suggests that this frustrated magnet shows a dynamic crossover between a high-temperature phase of poorly correlated, quickly relaxing spins to a low-temperature regime with much slower, short ranged spin correlations extending no further than to the next nearest neighbor. Existing theories explain the spin glass transition in terms of a phase transition and order parameters, and assume the existence of a distinct spin glass phase. There is no evidence for such a phase in Tb2Mo2O7.