  Measurements of the critical current density (Jc) by magnetization and the
upper critical field (Hc2) by magnetoresistance have been performed for
hafnium-doped MgB2. There has been a remarkable enhancement of Jc as compared
to that by ion irradiation without any appreciable decrease in Tc, which is
beneficial from the point of view of applications. The irreversibility line
extracted from Jc shows an upward shift. In addition, there has been an
increase in the upper critical field which indicates that Hf partially
substitutes for Mg. Hyperfine interaction parameters obtained from time
differential perturbed angular correlation (TDPAC) measurements revealed the
formation of HfB and HfB2 phases along with the substitution of Hf. A possible
explanation is given for the role of these species in the enhancement of Jc in
MgB2 superconductor.
