F.C Voogt

In situ RHEED and XPS studies of epitaxial thin α-Fe2O3(0001) films on sapphire

In situ RHEED and XPS measurements of epitaxial alpha-Fe2O3(0001) films are reported as a function of the number of deposited monolayers. The films were prepared on alpha-Al2O3(0001) substrates by MBE. The RHEED patterns suggest that layer-by-layer growth of alpha-Fe2O3(0001) occurs for the first few monolayers. Subsequently, the growth mode changes to three-dimensional growth. The in-plane lattice constant of the first monolayer of alpha-Fe2O3(0001) is expanded relative to that of the bulk, although in the case of lattice matching between alpha-Al2O3 and alpha-Fe2O3 a contraction would be expected. This can be explained by assuming a basic hexagonal structure for the first monolayer with a random distribution of ferric ions over the octahedral sites between the close-packed oxygen layers. Beyond the first monolayer, the ordered corundum structure is formed. The lineshapes of the XPS Fe 2p core level spectra are also found to be thickness-dependent. The deviation of the Madelung potential at the surface shifts the positions of the Fe 2p peaks to lower binding energies. For the first few monolayers, the satellite intensity is reduced because the interplanar contraction leads to a shorter Fe-O distance.

Growth and characterization of non-stoichiometric magnetite Fe3−δO4 thin films

Abstract Epitaxial thin films of magnetite, Fe 3 − δ O 4 , were produced by evaporating natural Fe or the Mossbauer active isotope 57 Fe onto MgO(100) substrates, with simultaneous oxidization of the iron film by a controlled NO 2 flux. RHEED intensity oscillations show that the growth proceeds in a layer-by-layer fashion. Mossbauer spectroscopy was used to determine the conditions under which stoichiometric and non-stoichiometric magnetite thin films can be obtained. Furthermore, we report on a p(1 × 1) reconstruction of the Fe 3 O 4 (001) surface, which has not been observed before for epitaxial thin films of magnetite.

The role of interfacial structure in determining magnetic behaviour in MBE-grown Fe3O4/MgO multilayers on MgO(001)

Abstract The atomic structure at the Fe 3 O 4 –MgO interface and its role in defining magnetic behaviour in Fe 3 O 4 –MgO multilayers, grown by MBE on MgO(001), has recently been a subject of much interest and controversy: the existence of a “dead” magnetic layer at the interface has been proposed to explain why ultra-thin magnetite layers become nonmagnetic. By using Mossbauer probe layers it was shown that this model is incorrect. In this paper we present additional structural evidence, from X-ray reflectivity and RHEED measurements, showing that the magnetite layers have the same structure and composition at the interface and in the bulk in agreement with a uniform magnetic structure throughout the entire layer.

GROWTH AND PROPERTIES OF EPITAXIAL IRON OXIDE LAYERS

Epitaxial layers of iron oxides have been grown on a MgO(001) substrate by evaporating natural Fe or57Fe from Knudsen cells in the presence of a NO2 flow directed to the substrate. The resulting layers have been investigated in situ with LEED, RHEED, AES and XPS and ex situ with CEMS and ion beam analysis. For substrate temperatures between 200 and 400°C we observe RHEED oscillations during deposition, indicative of layer-by-layer growth. By adjusting the flux of NO2 at the surface, all stable and metastable cubic phases in the Fe-O system could be grown: FeO (wustite), Fe3O4 (magnetite), γ-Fe2O3 (maghemite) and solid solutions between the latter two phases. Rutherford backscattering spectra show a relatively high minimum yield in the channel directions.

MBE-growth of iron and iron oxide thin films on MgO(100), using NO2, NO, and N2O as oxidising agents

We have made a study of the use of NO2 as the source of oxygen in the MBE-growth of iron oxides thin films. It is found that NO2 is a much more efficient oxidising agent than molecular O-2. As indicated by Mossbauer spectroscopy, performed on Fe-57 probe layers, NO2 is not only capable of forming stoichiometric magnetite Fe3O4, but also ail non-stoichiometric Fe3-deltaO4 phases. Even the metastable maghemite phase gamma-Fe2O3 (Fe3-deltaO4 with delta = 1/3) can be formed. All iron oxides grow layer-by-layer-like, as indicated by strong RHEED intensity oscillations. When small doses of NO2 are used, new wustite Fe1-xO and Fe3O4 phases are formed. In contrast to the Fe3-deltaO4 films, these phases have nitrogen incorporated into the crystal lattice. Similar compounds are obtained when NO is used as the source of oxygen. The use of N2O does not lead to the formation of iron oxides. It does, however, alter the growth mode of Fe on MgO(100). Whereas Fe deposited under UHV conditions forms 3D islands, the N2O acts as a surfactant and induces 2D layer-by-layer growth.