Temperature dependence of the vortex pinning enhancement by γ irradiation of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀ polycrystals

The saturation remanent magnetization M_RS, taken to represent the vortex pinning strength, was measured at various temperatures from 4.2 K up to near T_c on two similarly prepared polycrystalline samples of B_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀ (BPSCCO), one of which had been subsequently γ-irradiated. For both samples, the decrease of M_RS with rising temperature (T) is found to consist of two distinct components, one of which varies linearly with t (= 1-T/T_c) up to T_c, and the other decays very rapidly, essentially as t^β with β just under 10. The γ irradiation is seen to produce a threefold enhancement of the linear-t component, but its major effect is a 10-fold enhancement of the exponential-t^beta component. The direct connection between M_RS and the vortex pinning strength is tested and confirmed by sample-rotational magnetization measurements. Hence, the M_RS component whose temperature decay is exponentially rapid can be attributed to a thermal decoupling of vortices from localized pinning sites. The pronounced irradiation-enhancement of this component thus implies that the defects produced by the γ-rays are acting primarily as vortex pinning sites that are effectively very restricted in size.