Observation of dynamic flux-line relaxation in ion-irradiated ${\mathrm{Bi}}_2{\mathrm{Sr}}_{1.8}\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_x$ by Lorentz microscopy

The dynamical behavior of individual flux lines in ${\mathrm{Bi}}_2{\mathrm{Sr}}_{1.8}\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_x$ thin film irradiated with 240-MeV ${\mathrm{Au}}^{14+}$ ions was investigated by Lorentz microscopy. To evaluate the pinning effect of columnar defects, the irradiation to a single crystal was partially blocked by using a mask. The flux-line relaxation after changing the equilibrium conditions in the two regions, one irradiated and the other without irradiation, was observed simultaneously. Just after switching off the magnetic field at 4.5 K, the flux-line density in both regions decayed logarithmically with time from uniform configuration. However, the relaxation rate in the irradiated region was less than that in the nonirradiated region. The flux-line density in the equilibrium state in the irradiated region was much higher than that in the nonirradiated region. These results revealed that the pinning force was enhanced by ion irradiation. Also it was observed that reversely polarized flux lines are generated and pair annihilation occurs between opposite flux lines during the relaxation, which has never been revealed by conventional macroscopic techniques.