Formation of ultrathin cobalt ferrite films by interdiffusion of Fe3O4/CoO bilayers

In this work an alternate pathway is demonstrated to form ultrathin cobalt ferrite (${\mathrm{Co}}_{x}{\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{O}}_{4}$) films by interdiffusion of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$/CoO bilayers. Bilayer samples with different ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$/CoO thickness ratios have been prepared by reactive molecular beam epitaxy on Nb-doped ${\mathrm{SrTiO}}_{3}$(001) substrates to obtain cobalt ferrite films of varied stoichiometry. Subsequently, oxygen-assisted postdeposition annealing experiments for consecutive temperature steps between $300{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ and $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ have been conducted monitoring the interdiffusion process by means of high-resolution x-ray reflectivity, soft and angle-resolved hard x-ray photoelectron, and x-ray absorption spectroscopy. Magnetic properties were characterized using superconducting quantum interference device magnetometry. The interdiffusion process starts from $300{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ annealing temperature and is completed for temperatures above $500{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. For completely interdiffused films with Co:Fe ratios larger than 0.84:2 a thin segregated CoO layer on top of the ferrite is formed. This CoO segregation is attributed to surface and interface effects. In addition, multiplet calculations of x-ray absorption spectra are performed to determine the occupancy of different sublattices. These results are correlated with the magnetic properties of the ferrite films. A stoichiometric ${\mathrm{CoFe}}_{2}{\mathrm{O}}_{4}$ film with partial inversion has been formed exhibiting homogeneously distributed ${\mathrm{Co}}^{2+}$ and mainly ${\mathrm{Fe}}^{3+}$ valence states if the initial Co:Fe content is 1.09:2. Thus, for the formation of stoichiometric cobalt ferrite by the proposed postdeposition annealing technique an initial Co excess has to be provided as the formation of a top CoO layer is inevitable.