Nature of spin freezing transition of geometrically frustrated pyrochlore system R2Ru2O7 (R = rare earth elements and Y)

At the spin-glass-like transition of yttrium ruthenium pyrochlore system Y2 Ru2O7, a significant jump of the specific heat has been found, suggesting t he second order-like nature of the transition. It is commonly observed for other pyrochlore systems R2Ru2O7 (R = various rare earth elements RE). For all REs studied here, the transition temperature TG is larger for the large r R ions, which may be due to the increase of the transfer energies of the 4d electrons between the neighboring Ru atoms with increasing RE ionic radi us. Neutron Rietveld analyses on Y2Ru2O7 and Nd2Ru2O7 have revealed that be low T-G, the Ru spins are in an almost long-range ordered antiferromagnetic state, though it is magnetically glassy in a macroscopic sense, and that t he lattice system does not participate in the transition. It is concluded t hat the specific heat jump is created by the purely magnetic origin. (C) 20 00 Elsevier Science Ltd. All rights reserved.