Transport and magnetotransport properties across the two-step Verwey transition in BaGdFe₂O_5+w

Magnetoresistance, electrical transport, and magnetic properties associated with a two-step Verwey-type transition in BaGdFe2 O5+w (-0.015<w<0.181) are studied as a function of temperature and oxygen nonstoichiometry (w). A disproportionation of Fe2.5+ into Fe2.5- and Fe2.5+ upon cooling through the transition temperature Tp and charge ordering into Fe2+ and Fe3+ at TV are manifested in electrical-conductivity and Seebeck-coefficient data. Above TV, electrical conductivity shows an activated hopping behavior with activation energy of ∼0.13 eV. Seebeck measurements identify holes as charge carriers below TV. Above TV, both holes and valence-mixing electrons need to be considered, although the Seebeck coefficient remains positive up to room temperature. This suggests that the activation energy for electrons is higher than that for holes, and the actual value is close to that obtained from conductivity data. Increasing w increases electrical conductivity and decreases Seebeck coefficient in the charge-ordered state. In the valence-mixed state, increasing w increases Seebeck coefficient, but conductivity increases only up to w≈0.1, from which the decay of the valence mixing takes over and conductivity begins to approach values extrapolated from the charge-ordered state. Magnetoresistance peaks with negative ratio up to ∼2% are observed, corresponding to a small magnetic-susceptibility change at TV.