Influence of neutron-irradiation-induced defects on the flux pinning in HgBa2Ca2Cu3O8+x single crystals

The influence of fast neutron irradiation on flux pinning in HgBa2Ca2Cu3O8x single crystals (T-c = 120 K) subjected to a fluence of 5 x 10(17) cm(-2) was studied. Magnetic measurements were performed using a commercial super conducting quantum interference device magnetometer and a miniaturized torq ue magnetometer. In the unirradiated state, the irreversibility line (IL), plotted as In(H-irr) vs In(1-T-irr/T-c), shows two slopes. At higher temper atures (85-100 K) the IL is described by a power-law dependence H-irr(T) =H -irr(0) (1 - T-irr/T-c)(alpha) with alpha approximate to 2.1. At lower temp eratures (25-60 K), a more rapid change of H-irr with temperature is observ ed, with the exponent alpha approximate to 4.8. Irradiation shifts the IL t o significantly higher magnetic fields/ temperatures, where it is rather we ll described by a single power-law dependence with the exponent alpha appro ximate to 2.3. The effective mass anisotropy gamma=(m(c)/m(ab))(1/2), as de termined from torque measurements, decreases after neutron irradiation. The shielding current density as a function of temperature up to 60 K is well approximated by the exponential dependence j(s)(T) =j(s)(0)exp(- T/T-0). Ir radiation increases the characteristic temperature T-0 from about 5.9 K (in the as-prepared crystal) to T-0=9.4K, clearly reflecting a slower decay of j(s) with temperature. Neutron-generated defects significantly increase j( s) and suppress the "fishtail effect'' (an increase of j(s) with magnetic f ield), which was present for the unirradiated crystal.