  The most effective way to enhance the dissipation-free supercurrent in
presence of magnetic field for type II superconductors is the introduction of
defects that acts as artificial pinning centres (APCs) for the vortices. For
instance, the in-field critical current density of doped BaFe2As2 (Ba122), one
of the most technologically important Fe-based superconductors, has been
improved over the last decade by APCs created by ion-irradiation. The technique
of ion-irradiation has been commonly implemented to determine the ultimate
superconducting properties. However, this method is rather complicated and
expensive. Here, we report on a surprisingly high critical current density and
strong pinning efficiency close to the crystallographic c-axis for a K-doped
Ba122 epitaxial thin film without APCs, achieving performance comparable to
ion-irradiated K-doped Ba122 single crystals. Microstructural analysis reveals
that the film is composed of columnar grains having width around 30-60 nm. The
grains are rotated around the b- (or a-) axis by 1.5 degree and around the
c-axis by -1 degree, resulting in the formation of low-angle grain boundary
networks. This study demonstrates that the upper limit of in-field properties
reached in ion-irradiated K-doped Ba122 is achievable by grain boundary
engineering, which is a simple and industrially scalable manner.
