  The Meissner effect has been studied in Ba(Fe0.926Co0.074)2As2 and
Ba0.6K0.4Fe2As2 single crystals and compared to well known, type-II
superconductors LuNi2B2C and V3Si. Whereas flux penetration is mostly
determined by the bulk pinning (and, perhaps, surface barrier) resulting in a
large negative magnetization, the flux expulsion upon cooling in a magnetic
field is very small, which could also be due to pinning and/or surface barrier
effects. However, in stark contrast with the expected behavior, the amount of
the expelled flux increases almost linearly with the applied magnetic field, at
least up to our maximum field of 5.5 T, which far exceeds the upper limit for
the surface barrier. One interpretation of the observed behavior is that there
is a field-driven suppression of magnetic pair-breaking.
