Magnetic and transport properties of phyrochlore molybdates

Magnetic and transport properties of the pyrochlore molybdate R2-xRx'Mo2O7 (R, R' = rare earth, Y), which exhibits the metal-insulator (M-I) transition with varying x, have been studied. The transition from an antiferromagnetic spin-glass state to a ferromagnetic one occurs at x far from the M-I phase boundary, indicating that the intimate correlation of the magnetic state with the conducting nature is not so obvious. In Nd2Mo2O7, the ordering of Mo spins takes place at about 95 K, where the anomalous Hall resistivity appears with decreasing temperature T along with the spontaneous magnetization. With the ordering of Nd moments which becomes significant below similar to 20K with decreasing T, the behavior of the anomalous Hall resistivity markedly changes, while the Mo-magnetization is partially cancelled by the Nd magnetization. It is shown that the anomalous Hall resistivity can be described by two components, 4 piR(s)M(Mo), and 4 piR(s)'M-Nd, the contributions from the uniform magnetizations M-Mo and M-Nd of Mo and Nd moments, respectively. The sign of R-s' is opposite to that of R-s. Brief arguments are given if the unusual behavior of the Hall resistivity is related with the nonzero chirality of the magnetic moment system.