Upper critical and irreversibility fields in Ni- and Co-doped pnictide bulk superconductors

Abstract A comprehensive study of upper critical and irreversibility magnetic fields in Ba(Fe 0.95 Ni 0.05 ) 2 As 2 (large grain and small grain samples), Ba(Fe 0.94 Ni 0.06 ) 2 As 2 , Ba(Fe 0.92 Co 0.08 ) 2 As 2 , and Ba(Fe 0.92 Co 0.09 ) 2 As 2 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 measured. The higher temperature, upper critical field H c2 data are well described by the one-band Werthamer, Helfand, and Hohenberg (WHH) model. 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. The large values of the Maki fitting 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. Possible multi-band structure of these materials is lumped into effective parameters of the single-band model. Table of measured physical parameters allows us to compare these pnictide superconductors for different Co- and Ni- doping levels and granularity.