Dynamics of Vortices in Heavy-ion Irradiated Co-doped BaFe$_2$As$_2$*

Effects of heavy-ion irradiation on the critical current density, $J_c$, and vortex dynamics are investigated in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ irradiated by heavy- ions of various kinds. Irradiations induce a large enhancement of $J_c$ in the case of 200 MeV Au creating nearly continuous columnar tracks. On the other hand, in the case of 800 MeV Xe irradiation, despite the enhancement of $J_c$, clear columnar defects are not observed. In the case of 2.6 GeV U irradiation, new types of structure appears in the $M$-$H$ loop at high matching field. We also discuss the behavior of vortex dynamics in the irradiated Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$.