  Nanoscale defects in superconductors play a dominant role in enhancing
superconducting properties through electron scattering, modulation of coherence
length, and correlation with quantized magnetic flux. For iron-based
superconductors (IBSCs) that are expected to be employed in high-field magnetic
applications, a fundamental question is whether such defects develop an upper
critical field (Hc2) similar to that of conventional BCS-type superconductors.
Herein, we report the first demonstration of a significantly improved Hc2 in a
122-phase IBSC by introducing defects through high-energy milling. Co-doped
Ba122 polycrystalline bulk samples (Ba(Fe,Co)2As2) were prepared by sintering
powder which was partially mechanically alloyed through high-energy milling. A
remarkable increase in full-width at half maximum of X-ray powder diffraction
peaks, anomalous shrinkage in the a-axis, and elongation in the c-axis were
observed. When lattice defects are introduced into the grains, semiconductor
behavior of the electric resistivity at low temperature (T < 100 K), slight
decrease in transition temperature (Tc), upturn of Hc2(T) near Tc, and a large
increase in Hc2(T) slope were observed. The slope of Hc2(T) increased
approximately by 50%, i.e., from 4 to 6 T/K, and exceeded that of single
crystals and thin films. Defect engineering through high-energy milling is
expected to facilitate new methods for the designing and tuning of Hc2 in
122-phase IBSCs.
