Magneto-transport and magnetization studies on thermally activated flux flow in iron-based superconductors*

We study the magneto-transport properties of three iron-based high temperature superconductors, polycrystalline samples, Ba(Fe$_{0.95}$ Ni$_{0.05})_{2}$As$_{2}$ ($T_{c} =$ 20.4 K), Ba(Fe$_{0.94}$ Ni$_{0.06})_{2}$As$_{2}$ ($T_{c} =$ 18.5 K), and Ba(Fe$_{0.91}$Co$_{0.09})_{2}$As$_{2}$ ($T_{c} =$ 25.3 K) in magnetic fields of up to 18 T. The thermally activated magnetic flux behavior was retrieved by plotting ln$\rho$ vs. 1/$T$ ($\rho$ and $T$ are resistivity and temperature, respectively) and obtaining the activation energies $U_{0}$ for flux motion near $T_{c}$. We show a 3-D plot of the distribution of $U_{0}$ as a function of $T$ and magnetic field $H$. We apply the WHH model by measuring \textit{dH}$_{c2}$\textit{/dT} at $T_{c}$ to estimate the upper critical field $H_{c2}(T=$ 0); we estimate the coherence length $\xi (T=$ 0). We study the broadening of resistive transition as a function of the applied magnetic field and compare it to Tinkham's prediction for high-T$_{c}$ materials.