Effects of strong magnetic fluctuations on the transport behavior of pyrochlore ruthenates near the metal-insulator phase boundary

Transport and magnetic properties of pyrochlore ruthenates R_2-xBi_xRu₂O₇ (R=Sm and Eu), which exhibit the Mott insulator to metal transition with increasing x, have been studied. The electrical resistivity p and the Hall coefficient R_H are found to strongly couple to the change of the magnetic state, or the transition to the spin-glass state which takes place at temperature T = T_G. Near the metal-insulator phase boundary, R_H above T_G increases rather rapidly with decreasing T even in the metallic phase, and the Hall angle θ_H follows a simple relation, cot θ_H ∝ a + bT², over the wide T-range. At T = T_G, R_H has a peak and it decreases with decreasing T. These results indicate that strong spin fluctuations above T_G induce the anomalous T-dependence of R_H in the present magnetically frustrated three-dimensional system and R_H decreases as the spin freezing proceeds.