Abstract
In this work an alternate pathway is demonstrated to form ultrathin cobalt ferrite () films by interdiffusion of /CoO bilayers. Bilayer samples with different /CoO thickness ratios have been prepared by reactive molecular beam epitaxy on Nb-doped (001) substrates to obtain cobalt ferrite films of varied stoichiometry. Subsequently, oxygen-assisted postdeposition annealing experiments for consecutive temperature steps between and 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 annealing temperature and is completed for temperatures above . 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 film with partial inversion has been formed exhibiting homogeneously distributed and mainly 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.
4 More- Received 18 April 2019
- Revised 6 September 2019
DOI:https://doi.org/10.1103/PhysRevB.100.155418
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