Effects of Post-deposition Annealing on Epitaxial CoO/Fe₃O₄ Bilayers on SrTiO₃(001) and Formation of Thin High-Quality Cobalt Ferrite-like Films

Abstract

In order to explore an alternative pathway to prepare ultrathin CoFe2O4 films, epitaxial CoO/Fe3O4 bilayers with varying film thickness of the CoO film were grown on Nb-doped SrTiO3(001) substrates via reactive molecular beam epitaxy. Thereafter, cobalt ferrite films with varying stoichiometry were prepared by post-deposition annealing at different temperatures. The thermally mediated interdiffusion resulted in the formation of vertical compressive and lateral tensile strained Co x Fe3 – x O4 films (x = 0.6 – 1.4) with homogeneous distribution of Fe and Co cations for each film. The chemical and electronic variations after each annealing step were studied by means of soft and hard X-ray photoelectron spectroscopy. The homogeneity of the cation distributions in the films were additionally verified after the last annealing step by angle-resolved hard X-ray photoelectron spectroscopy. For the cobalt ferrite film with x = 1.4, an additional crystallographic phase of Co1 – y Fe y O was observed by (grazing incidence) X-ray diffraction measurements after annealing at 600 °C. X-ray reflectivity measurements were performed to determine the film thickness of the formed Co x Fe3 – x O4 films.

Cationic Ordering and Its Influence on the Magnetic Properties of Co-Rich Cobalt Ferrite Thin Films Prepared by Reactive Solid Phase Epitaxy on Nb-Doped SrTiO3(001)

“…In order to gain information on the chemical composition and the homogeneity of the Co and Fe cation distribution of the cobalt ferrite films, a quantitative analysis was performed for each incidence angle mentioned in the experimental details section [ 18 ]. We found for each cobalt ferrite film a homogeneous depth distribution of the Co and Fe cations throughout the whole film.…”

“…Two films with stoichiometries of and and total film thicknesses of and , respectively, were prepared by RSPE from epitaxial CoO/Fe O bilayers grown on Nb-doped SrTiO (001) substrates (cf. Figure 1 ) at the beamline BM25-SpLine of the European Synchrotron Radiation Facility (ESRF) [ 18 ]. The sample preparation was extensively monitored by several chemical and structural characterization techniques such as soft and hard X-ray photoelectron spectroscopy and synchrotron radiation based X-ray diffraction including surface sensitive grazing incidence X-ray diffraction to follow the evolution from the CoO/Fe O bilayer stacks to completely reacted Co ferrite films.…”

“…The sample preparation was extensively monitored by several chemical and structural characterization techniques such as soft and hard X-ray photoelectron spectroscopy and synchrotron radiation based X-ray diffraction including surface sensitive grazing incidence X-ray diffraction to follow the evolution from the CoO/Fe O bilayer stacks to completely reacted Co ferrite films. A detailed description of the sample preparation can be found in reference [ 18 ].…”

“…Whereas Mössbauer spectroscopy studies suggest cations in octahedral coordination, which are in the low-spin state [ 16 ], more recent work found cations in tetrahedral coordination and in high-spin state [ 17 ]. We wanted to go a step further and performed a comprehensive analysis of the electronic and magnetic properties of ultrathin Co ferrite films, which we produced very recently by intermixing epitaxial CoO/Fe O bilayers using a post-deposition annealing (PDA) approach [ 18 ].…”

“…Generally, cobalt ferrite thin films are often prepared by deposition methods such as molecular beam epitaxy [ 19 ], sputter deposition [ 5 ] and pulsed laser deposition [ 20 ]. Quite recently, we demonstrated that thin cobalt ferrite films can be prepared by reactive solid phase epitaxy (RSPE) due to the intermixing of epitaxial Fe O /CoO and CoO/Fe O bilayers [ 18 , 21 ]. For both bilayer systems, the intermixing of the oxide layers was caused by PDA on Nb-doped SrTiO (001) substrates (lattice constant Å).…”