Increment of the collective pinning energy in Na1 − xCa x Fe2As2 single crystals with random point defects introduced by proton irradiation

Abstract

We study the influence of random point defects introduced by 3 MeV proton irradiation (doses 1 × 1016 and 2 × 1016 cm2) on the vortex dynamics of Na x Ca1 − 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 μ close to the one expected in the so-called large bundle regime (μ ≈ 7/9) for random point defects.