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A comprehensive study of upper critical and irreversibility magnetic fields in Ba(Fe0.92Co0.08)2As2 and Ba(Fe0.91Co0.09)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. Possible multi-band structure of these materials is lumped into effective parameters of the single-band model. The higher temperature, upper critical field H c2 data is well described by the one-band Werthamer–Helfand–Hohenberg (WHH) model. The large values of the Maki parameter (6.5 and 4.4) indicate that the Zeeman pair breaking dominates over the orbital pair breaking and spin-paramagnetic pair-breaking effect is significant in these materials. The fitted WHH curve departs from the experimental data at low temperatures, particularly for the Ba(Fe0.91Co0.09)2As2 sample, suggesting an emergence of a new phase that can be attributed to the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state. The H c2 temperature behavior can be described within a single-band model used in WHH with added finite-Q dependence to introduce FFLO instability. The anisotropy in the behavior of the upper critical field could also explain the data.

Vortex dynamics and nonlinear ac response are studied in a Ba(Fe0.94 Ni0.06)2As2(T c = 18.5 K) bulk superconductor in magnetic fields up to 12 T via ac susceptibility measurements of the first ten harmonics. A comprehensive study of the ac magnetic susceptibility and its first ten harmonics finds shifts to higher temperatures with increasing ac measurement frequencies (10 to 10,000 Hz) for a wide range of ac (1, 5, and 10 Oe) and dc fields (0 to 12 T). The characteristic measurement time constant t 1 is extracted from the exponential fit of the data and linked to vortex relaxation. The Anderson-Kim Arrhenius law is applied to determine flux activation energy E a /k as a function dc magnetic field. The de-pinning, or irreversibility lines, were determined by a variety of methods and extensively mapped. The ac response shows surprisingly weak higher harmonic components, suggesting weak nonlinear behavior. Our data does not support the Fisher model; we do not see an abrupt vortex glass to vortex liquid transition and the resistivity does not drop to zero, although it appears to approach zero exponentially.

Thermally assisted flux flow (TAFF) based on magneto-resistivity and ac susceptibility measurements is studied in a Ba(Fe0.91Co0.09)2As2(T c=25.3 K) sample in magnetic fields up to 18 T. In addition to the upper critical field μ 0 H c2 and the coherence length ξ(0), the flux flow activation energy U(T,H) has also been determined. The resistive transition width is proportional to μ 0 H, in contrast to Tinkham’s theoretical prediction. By applying Fisher’s model, the glass melting transition temperature T g, which occurs in the upper TAFF state and not in the zero resistivity vortex solid regime, is calculated. The onset of TAFF temperature and the crossover temperature T x from TAFF to flux flow are determined. By contrasting the ac susceptibility data with the resistivity data, considerable flux penetration appears even in the zero resistivity state, in addition to ac losses. The H-T phase diagram is drawn and shows weak pinning regime as the field approaches μ 0 H c2, and the strength of the weak pinning decreases to 0 with increasing magnetic field from 0 to 18 T.

Thermally assisted flux flow (TAFF) is studied in bulk Ba(Fe0.95 Ni0.05)2As2 (T c = 20.4 K) and Ba(Fe0.94 Ni0.06)2As2 (T c = 18.5 K) superconductors by transport measurements in magnetic fields up to 18 T. In addition, the upper critical field μ 0 H c2(0) and the coherence length ξ(0) are determined. The data is analyzed in the context of the widely accepted Anderson-Kim model and Fischer model. The onset TAFF temperature and the crossover temperature T x from TAFF to flux flow are determined. The flux pinning activation energy U is modeled as U(T,H) = U 0(H) f(t) where f(t) is some temperature function and the modified Anderson-Kim model is used to extract U 0, which is graphed as a function of magnetic field μ 0 H near T c. The resistive regime is observed, which is caused by fluctuations. Fisher’s model is applied to determine the glass melting transition temperature; it occurs in the upper TAFF state and not in the expected zero-resistivity vortex solid regime. Furthermore, the resistive transition width is proportional to μ 0 H, in contrast to Tinkham’s prediction. The H-T phase diagram is drawn.

The de-pinning or irreversibility lines were determined by ac susceptibility, magnetization, radio-frequency proximity detector oscillator (PDO), and resistivity methods in Ba(Fe0.92Co0.08)2As2 ( T c = 23.2 K), Ba(Fe0.95Ni0.05)2As2 ( T c = 20.4 K), and Ba(Fe0.94Ni0.06)2As2 ( T c = 18.5 K) bulk superconductors in ac, dc, and pulsed magnetic fields up to 65 T. A new method of extracting the irreversibility fields from the radio-frequency proximity detector oscillator induction technique is described. Wide temperature broadening of the irreversibility lines, for any given combination of ac and dc fields, is dependent on the time frame of measurement. Increasing the magnetic field sweep rate (dH/dt) shifts the irreversibility lines to higher temperatures up to about dH/d t = 40,000 Oe/s; for higher dH/dt, there is little impact on the irreversibility line. There is an excellent data match between the irreversibility fields obtained from magnetization hysteresis loops, PDO, and ac susceptibility measurements, but not from resistivity measurements in these materials. Lower critical field vs. temperature phase diagrams are measured. Their very low values near 0 T indicate that these materials are in mixed state in nonzero magnetic fields, and yet the strength of the vortex pinning enables very high irreversibility fields, as high as 51 T at 1.5 K for the Ba(Fe0.92Co0.08)2As2 polycrystalline sample, showing a promise for liquid helium temperature applications.