The influence of fast neutron irradiation on flux pinning in ${\mathrm{HgBa}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{8+x}$ single crystals ${(T}_c=120\mathrm{}\mathrm{K})$ subjected to a fluence of $5\times {10}^{17}{\mathrm{cm}}^{\mathrm{-}2}$ was studied. Magnetic measurements were performed using a commercial superconducting quantum interference device magnetometer and a miniaturized torque magnetometer. In the unirradiated state, the irreversibility line (IL), plotted as $\ln {(H}_{\mathrm{irr}})$ vs $\ln \left(1\mathrm{}-T_{\mathrm{irr}}{/T}_c\right),$ shows two slopes. At higher temperatures (85–100 K) the IL is described by a power-law dependence $H_{\mathrm{irr}}{\mathrm{}\left(T\right)=H}_{\mathrm{irr}}\mathrm{}\left(0\right)\mathrm{}$ $(1\mathrm{}-T_{\mathrm{irr}}{/T}_c{)}^{\alpha }$ with $\alpha \approx \mathrm{2.1.}$ At lower temperatures (25–60 K), a more rapid change of $H_{\mathrm{irr}}$ with temperature is observed, with the exponent $\alpha \approx \mathrm{4.8.}$ Irradiation shifts the IL to significantly higher magnetic fields/temperatures, where it is rather well described by a single power-law dependence with the exponent $\alpha \approx \mathrm{2.3.}$ The effective mass anisotropy $\gamma {=(m}_c{/m}_{\mathrm{ab}}{)}^{1/2},$ as determined 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 $j_s{\mathrm{}\left(T\right)=j}_s\mathrm{}\left(0\right)\mathrm{}\exp \left(-{T/T}_0\right).\mathrm{}$ Irradiation increases the characteristic temperature $T_0$ from about 5.9 K (in the as-prepared crystal) to $T_0=9.4\mathrm{}\mathrm{K},$ 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 field), which was present for the unirradiated crystal.

• Received 7 December 1998

DOI:https://doi.org/10.1103/PhysRevB.61.791