An electron hopping on non-coplanar spin sites with spin chirality obtains a complex phase factor (Berry phase) in its quantum mechanical amplitude that acts as an internal magnetic field, and is predicted to manifest itself in the Hall effect when it is not cancelled. The present combined work of transport measurement, neutron scattering, and theoretical calculation provides evidence that the gigantic anomalous Hall effect observed in Nd2Mo2O7, a pyrochlore ferromagnet with geometrically frustrated lattice structure, is mostly due to the spin chirality and the associated Berry phase originating from the Mo spin tilting.

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The stripe order in La 2Ϫx Sr x NiO 4ϩ␦ with 0.289ՇxՇ0.5 was studied with neutron-scattering technique. At low temperatures, all samples exhibit hole stripe order. Incommensurability ⑀ of the stripe order is approximately linear in the hole concentration n h ϭxϩ2␦ up to xϭ1/2, where ␦ denotes the off stoichiometry of oxygen atoms. The charge and spin ordering temperatures exhibit maxima at n h ϭ