Site Determination of Zn and Mn Doping into Protein Encapsulated Fe$_{3}$O$_{4}$ Nanoparticles*

In this study, particles grown inside ferritin protein cages with 10{\%}, 15{\%}, 20{\%} and 33{\%} zinc or manganese doped into Fe$_{3}$O$_{4}$ were measured by XAS and XMCD. These spectra were compared to standards and molecular-orbital calculations to determine where in the Fe$_{3}$O$_{4}$ spinel structure the dopant is substituting. Doping into the various Fe-sites of the Fe$_{3}$O$_{4}$ spinel structure suggests that there will be a significantly different magnetic moment per iron lattice position depending on whether the A-site or B-site is favored. For Zn-doping, our results support Zn in the +2 valance state and preferentially substituting in the B-site octahedral site. For Mn-doping, we find Mn also prefers the B-site octahedral site, but may cause a structural transition to the $\gamma$-phase Fe$_{2}$O$_{3}$.