Abstract
A comprehensive study of upper critical and irreversibility magnetic fields in Ba(Fe0.95Ni0.05)2As2(large grain and small grain samples) and Ba(Fe0.94Ni0.06)2As2, polycrystalline bulk pnictide superconductors was made in pulsed fields of up to 65 T. The full magnetic field-temperature (H − T) phase diagrams, starting at 1.5 K, were obtained. The higher temperature, upper critical field H c2 data are well described by the one-band Werthamer, Helfand, and Hohenberg (WHH) model. The large values of the Maki parameter α indicate that the Zeeman pair breaking dominates over the orbital pair breaking and spin-paramagnetic pair-breaking effect is significant in these materials. At low temperatures, the experimental data depart from the fitted WHH curves, suggesting an emergence of a new phase that could be attributed to the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. Possible multi-band structure of these materials is lumped into effective parameters of the single-band model.
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