Electronic and Magnetic Properties of Co$_{1-x}$Fe$_{2+x}$O$_{4}$*

New materials having both resistivities similar to those of semiconductors and high room temperature spin polarizations would greatly benefit spintronic devices. Cobalt ferrite (CoFe$_{2}$O$_{4})$ is predicted to have a high spin polarization, but is an insulator; however, it begins to conduct when doped with iron. In this work, epitaxial Co$_{1-x}$Fe$_{2+x}$O$_{4}$ thin films (0 $\le x\le $ 0.5) are grown by MBE on MgO (001). XPS and XMCD are used to determine the stoichiometry and cation valence states, and UPS measures the density-of-states near the Fermi energy. Transport measurements demonstrate the ability to tailor the conductivity by varying the value of $x$. Bulk magnetic moments are obtained with MOKE and SQUID magnetometry, while site specific magnetic moments are obtained with XMCD. These measurements enable us to determine which stoichiometry gives a material that is best suited for use as a spin-polarized source or detector in spintronic devices.