Specific Heat and ac Susceptibility Studies on Ruthenium Pyrochlores R2Ru2O7 (R=Rare Earths)

Specific heat and ac susceptibility measurements were performed for R2Ru2O7 (R=Y, Pr, Nd, Sm, Eu, and Lu). For Y2Ru2O7 and Lu2Ru2O7, a -type specific heat anomaly has been observed at 77 and 78 K, respectively, which is consistent with the magnetic transition found in their magnetic susceptibility vs temperature curves. In the real part of the ac susceptibilities, small cusps are observed at 78 K for Y2Ru2O7 and at 80 K for Lu2Ru2O7. Both the real part and the imaginary part of the ac susceptibilities are independent of frequency, indicating that these compounds are not in the ordinary spin-glass. Similar large jumps of specific heat are observed at 162, 143, 124, and 118 K for R2Ru2O7 (R=Pr, Nd, Sm, and Eu). For Sm2Ru2O7 and Eu2Ru2O7, no specific heat anomaly has been found at lower temperatures, although magnetic anomaly was observed at ca. 20 K in their dc susceptibility vs temperature curves. ac susceptibility measurements show that both the real and the imaginary susceptibilities show anomaly at ca. 20 K and this temperature decreases with decreasing frequency. These results strongly indicate that the transition observed at ca. 20 K is the spin-glass one. The experimental results on Nd2Ru2O7 show that magnetic properties of this compound are complicated.