Site determination of Zn doping in protein encapsulated ZnxFe3−xO4 nanoparticles

The x-ray absorption spectra of the Fe and Zn L edges for 6.7nm Fe3O4 nanoparticles grown inside 12nm ferritin protein cages with 10%, 15%, 20%, and 33% zinc doping show that Zn is substitutional as Zn2+ within the iron oxide host structure. A Neel–Arrhenius plot of the blocking temperature in frequency dependent ac-susceptibility measurements shows that the particles are noninteracting and that the anisotropy energy barrier is reduced with Zn loading. X-ray magnetic circular dichroism of the Fe displays a linear decrease with Zn doping in sharp contrast to the initial increase present in the bulk system. The most plausible explanation for the decrease in moment is that Zn substitutes preferentially into the tetrahedral A site as a Zn2+ cation, generating a mixed spinel.