Magnetic and transport properties of phyrochlore molybdates

Magnetic and transport properties of the pyrochlore molybdate R_{2-cursive Greek chi}R′_{cursive Greek chi}-Mo₂O₇ (R, R′ = rare earth, Y), which exhibits the metal-insulator (M-I) transition with varying cursive Greek chi, have been studied. The transition from an antiferromagnetic spin-glass state to a ferromagnetic one occurs at cursive Greek chi 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 ∼20 K 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, 4πR_sM_Mo, and 4πR′_sMNd, 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.