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Magnetic and transport properties of the pyrochlore molybdate R 2- x R x ′ Mo 2 O 7 (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 Nd 2 Mo 2 O 7 , 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, 4π R s M Mo and 4π R s ′ M Nd , the contributions from the uniform magnetizations M M...

Hall resistivity ρ H has been investigated for single crystals of pyrochlore molybdate Nd 2 Mo 2 O 7 . With decreasing temperature T , the anomalous Hall effect appears at around the Curie temperature T C (∼95 K). In the relatively high temperature region ( T > 60 K), ρ H can be described by the well-known equation ρ H = R 0 H +4 π R s M , where R 0 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 = R 0 H +4 π R s M Mo +4π R s ' M 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 f...

Neutron Rietveld analyses have been carried out on Y 2 Ru 2 O 7 and Nd 2 Ru 2 O 7 to clarify the primary mechanism of the transition at T G to the spin-glass-like state which is accompanied by the specific heat jump. The results indicate that the lattice system does not seem to participate in the transition and changes of the neutron diffraction data with decreasing temperature through T G can consistently be understood by the ordering of magnetic moments at Ru sites to an almost long range ordered state, even though the low temperature phase is glassy in the macroscopic sense.

Transport and magnetic properties of pyrochlore ruthenates R 2- x Bi x Ru 2 O 7 (R=Sm and Eu), which exhibit the Mott insulator to metal transition with increasing x , have been studied. The electr...

Light-induced ESR (LESR) measurements have been performed on undoped poly(parapheny- lene vinylene) (PPV) down to 4 K. The ESR signal increases significantly for the excitation energy above 3.1–3.2 eV, as in the case of the excitation spectra of photocarriers reported in PPV derivatives. The anisotropic light-induced ESR spectra in oriented samples are well reproduced by the spectra calculated using a theoretical polaron spin distribution in the case of finite electron-electron interaction. These spectral features indicate that the observed spins are photogenerated polarons.

Electron nuclear double resonance (ENDOR) measurements of paramagnetic species existing in the dark state of undoped poly (paraphenylene vinylene) have been performed using stretch-oriented samples. The spectra have shown a clear anisotropy of a π-electron on the conjugated chain. The maximum value of the spin density reaches 0.09, which corresponds to the half-width of the spatial extension of the spin over several phenyl rings. Three distinct spectral turning points have been resolved for the spectra of stretch direction, showing that the variation of the spin density is not monotonic. The resemblance between the ENDOR-induced ESR spectra and the photo-induced ESR spectra at low temperatures strongly suggests that the observed spins are trapped polarons.