Upper critical and irreversible fields of polycrystalline CeFeAsO$_{1-x}$F$_{x}$ superconductors

We have investigated the upper critical ($H_{c2})$ and irreversible ($H_{irr})$ fields of polycrystalline samples of Ce oxypnictide at different doping levels. $H_{c2}$ was obtained from temperature dependent resistivity measurements with increasing applied magnetic field. Critical field values as high as 150 Tesla were observed with a decreasing trend as the doping level shifts from a slightly under-doped state to the highly over-doped region. The irreversible fields were lower in this superconductor compared with other rare-earth oxypnictides, with values below 3 Tesla at 20 K. However, $H_{irr}$ was found to increase with increasing doping, opposite to that of $H_{c2}$. The origin of $H_{irr}$ was studied by determining the exponent `n' extracted from plots of log$_{10}(H_{irr})$ versus log$_{10}$(1-$T$/$T_{c})^{n}$. We found that $H_{irr}$ follows a 3D vortex lattice-melting model similar to the other low anisotropic iron-based superconductors.