Influence of neutron-irradiation-induced defects on the flux pinning inHgBa2Ca2Cu3O8+xsingle crystals

Abstract

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 ${(T}_{c}=120\mathrm{K})$ subjected to a fluence of $5\ifmmode\times\else\texttimes\fi{}{10}^{17}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ 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 $\mathrm{ln}{(H}_{\mathrm{irr}})$ vs $\mathrm{ln}(1\ensuremath{-}{T}_{\mathrm{irr}}{/T}_{c}),$ shows two slopes. At higher temperatures (85--100 K) the IL is described by a power-law dependence ${H}_{\mathrm{irr}}{(T)=H}_{\mathrm{irr}}(0)$ $(1\ensuremath{-}{T}_{\mathrm{irr}}{/T}_{c}{)}^{\ensuremath{\alpha}}$ with $\ensuremath{\alpha}\ensuremath{\approx}2.1.$ At lower temperatures (25--60 K), a more rapid change of ${H}_{\mathrm{irr}}$ with temperature is observed, with the exponent $\ensuremath{\alpha}\ensuremath{\approx}4.8.$ 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 $\ensuremath{\alpha}\ensuremath{\approx}2.3.$ The effective mass anisotropy $\ensuremath{\gamma}{=(m}_{c}{/m}_{\mathrm{ab}}{)}^{1/2},$ 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 ${j}_{s}{(T)=j}_{s}(0)\mathrm{exp}(\ensuremath{-}{T/T}_{0}).$ Irradiation increases the characteristic temperature ${T}_{0}$ from about 5.9 K (in the as-prepared crystal) to ${T}_{0}=9.4\mathrm{K},$ 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.