Comparative study of flux pinning characteristics of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and BaFe$_2$(As$_{1-x}$P$_x)_2$ single crystals

We have studied the magnetization behavior of iron-pnictide superconductor, Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ (Co-doped Fe122) with various Co doping and BaFe$_2$(As$_{0.65}$P$_{0.35})_2$ (P-doped Fe122) single crystals. All of the Co-doped Fe122 crystals showed a very pronounced ``peak effect'' in all the temperature range, irrespective of doping state. It is important to mention that a similar peak effect was previously reported for REBa$_2$Cu$_3$O$_y$. In order to get further insight into the pinning mechanism of the present system, we analyzed the pinning force density $F_p=J_cB$. A good scaling of the $F_p$ versus the reduced field, $b=B/B_{irr}$, was established for all the Co-doped Fe122 crystals and the scaling curves were well fitted with the function given by $F_p/F_{p,max} =Ab^p(1-b)^q$, where $A$ is a numerical parameter, $p$ and $q$ are describing the actual pinning mechanism. It was found that $p$ value monotonically increases with x, while $q$ value decreases with x. On the other hand, P-doped Fe122 did not show ``peak effect''. We will discuss about the possible pinning mechanism causing the peak effect.