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

J. Rodewald, J. Thien, T. Pohlmann, M. Hoppe, F. Timmer, F. Bertram, K. Kuepper, and J. Wollschläger
Phys. Rev. B 100, 155418 – Published 16 October 2019
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Abstract

In this work an alternate pathway is demonstrated to form ultrathin cobalt ferrite (CoxFe3xO4) films by interdiffusion of Fe3O4/CoO bilayers. Bilayer samples with different Fe3O4/CoO thickness ratios have been prepared by reactive molecular beam epitaxy on Nb-doped SrTiO3(001) substrates to obtain cobalt ferrite films of varied stoichiometry. Subsequently, oxygen-assisted postdeposition annealing experiments for consecutive temperature steps between 300C and 600C 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 300C annealing temperature and is completed for temperatures above 500C. 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 CoFe2O4 film with partial inversion has been formed exhibiting homogeneously distributed Co2+ and mainly Fe3+ 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.

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  • Received 18 April 2019
  • Revised 6 September 2019

DOI:https://doi.org/10.1103/PhysRevB.100.155418

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

©2019 American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

J. Rodewald1, J. Thien1, T. Pohlmann1,2, M. Hoppe1,2, F. Timmer1, F. Bertram2, K. Kuepper1, and J. Wollschläger1

  • 1Department of Physics, Osnabrück University, Barbarastraße 7, D-49076 Osnabrück, Germany
  • 2DESY, Photon Science, Notkestraße 85, D-22607 Hamburg, Germany

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Issue

Vol. 100, Iss. 15 — 15 October 2019

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