Increment of the collective pinning energy in Na1 −xCaxFe₂As₂single crystals with random point defects introduced by proton irradiation

We study the influence of random point defects introduced by 3 MeV proton irradiation (doses 1 x 10(16) and 2 x 10(16) cm(2)) on the vortex dynamics of NaxCa1 - xFe2As2 (x = 0.5 and x = 0.75) single crystals. Our results indicate that the irradiation produces an enhancement of the critical current density and a reduction of the creep rate in vortex relaxation. The plateau in the temperature dependence of vortex creep rate initially present in as-grown single crystals disappears after irradiation. This fact can be associated with a large increment of the collective pinning energy (from <100 to 350-400 K). On the other hand, Maley analysis indicates that after irradiation both samples present a glassy exponent close to the one expected in the so-called large bundle regime (mu approximate to 7/9) for random point defects.