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

DOI: 10.1103/PhysRevB.100.155418 DOI Help

Authors: J. Rodewald (Osnabrück University) , J. Thien (Osnabrück University) , T. Pohlmann (Osnabrück University; DESY) , M. Hoppe (Osnabrück University; DESY) , F. Timmer (Osnabrück University) , F. Bertram (DESY) , K. Kuepper (Osnabrück University) , J. Wollschlager (Osnabrück University)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Physical Review B , VOL 100

State: Published (Approved)
Published: October 2019
Diamond Proposal Number(s): 15205 , 15334

Abstract: n this work an alternate pathway is demonstrated to form ultrathin cobalt ferrite ( Co x Fe 3 − x O 4 ) films by interdiffusion of Fe 3 O 4 /CoO bilayers. Bilayer samples with different Fe 3 O 4 /CoO thickness ratios have been prepared by reactive molecular beam epitaxy on Nb-doped SrTiO 3 (001) substrates to obtain cobalt ferrite films of varied stoichiometry. Subsequently, oxygen-assisted postdeposition annealing experiments for consecutive temperature steps between 300 ∘ C and 600 ∘ 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 ∘ C annealing temperature and is completed for temperatures above 500 ∘ 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 CoFe 2 O 4 film with partial inversion has been formed exhibiting homogeneously distributed Co 2 + and mainly 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.

Journal Keywords: Composition; Interfaces; Surfaces; Thin films; Molecular beam epitaxy; Photoemission spectroscopy; X-ray absorption spectroscopy; X-ray reflectivity

Subject Areas: Physics, Materials

Instruments: I07-Surface & interface diffraction , I09-Surface and Interface Structural Analysis

Other Facilities: PETRA III