Anomalous hall effect of pyrochlore molybdate Nd₂Mo₂O₇

Hall resistivity ρ_H has been investigated for single crystals of pyrochlore molybdate Nd₂Mo₂O₇. With decreasing temperature T, the anomalous Hall effect appears at around the Curie temperature T_C (∼95K). In the relatively high temperature region (T > 60K), ρ_H can be described by the well-known equation ρ_H = R₀H + 4πR_sM, where R₀ and R_s are the ordinary and anomalous Hall coefficients, respectively, and M is the net magnetization predominantly contributed by the Mo spins. With further decreasing T, the ordering of the Nd moments, which is confirmed by neutron diffraction studies carried out by the present authors' group, induces marked change in the behavior of the anomalous part of ρ_H. The behavior in the low temperature region has been found to be simply described by the equation ρ_H = R₀H + 4πR_sM_Mo + 4πR′_sM_Nd, where M_Mo and M_Nd are the net magnetizations of the Mo- and Nd-moments, respectively. This result and the T-dependence of R_s and R′_s are quite different from those known for ordinary ferromagnets and might be related to the nonzero spin chirality observed in the present system.