Flux pinning in neutron irradiated YBa2Cu3O 7-x

Identical polycrystalline samples of YBa₂Cu₃O_7-x have been irradiated with fast neutrons (E>0.1 MeV) in eight steps between 0 and 2.1×10 ¹⁸ n/cm². Notable irradiation effects include a T_c depression of nearly 2 K at the highest fluence and large improvements in the critical current density for fields from 0-9 T and temperatures from 4-80 K. Critical currents approaching 2×10⁷ A/cm² are observed for optimally irradiated materials at 5 K (in zero field), while at 77 K J _c(0 kOe) approaches 5×10⁵ A/cm². Irradiation is seen to take a nearly equilibrium magnetization curve at 77 K and broaden it to a significantly hysteretic curve. A substantial shift in the effective pinning potential as a function of current density is inferred from magnetic relaxation measurement at H=1 T. This is the first such measurement in which systematically increased activation energies for flux creep (as a function of current density) are noted