Neutron diffraction study of pyrochlore compound R2Ru2O7 (R=Y, Nd) above and below the spin freezing temperature

Neutron Rietveld analyses have been carried out on Y2Ru2O7 and Nd2Ru2O7 to clarify the primary mechanism of the transition at T-G to the spin-glass-li ke state which is accompanied by the specific heat jump. The results indica te that the lattice system does not seem to participate in the transition a nd changes of the neutron diffraction data with decreasing temperature thro ugh T-G can consistently be understood by the ordering of magnetic moments at Ru sites to an almost long range ordered state, even though the low temp erature phase is glassy in the macroscopic sense.