Structure, composition and crystallinity of epitaxial magnetite thin films N.-T.H. Kim-Ngan a, * , A.G. Balogh b , J.D. Meyer c , J. Brötz b , S. Hummelt b , M. Zaja ˛c d , T. S ´ le ˛zak d , J. Korecki d,e a Institute of Physics, Pedagogical University, 30 084 Kraków, Poland b Institute of Materials Science, Darmstadt University of Technology, Petersenstrasse 23, 64287 Darmstadt, Germany c Institute for Nuclear Physics, J.W. Goethe-University, Max-von-Laue-Str. 1, 60438 Frankfurt/Main, Germany d Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Kraków, Poland e Institute of Catalysis & Surface Chemistry, Polish Academy of Sciences, 30-239 Kraków, Poland article info Article history: Received 12 February 2008 Accepted for publication 23 April 2008 Available online 10 May 2008 Keywords: RBS Channeling MBE CEMS Magnetite Fe 3 O 4 X-ray reflectometry abstract Epitaxially-grown Fe 3 O 4 (0 0 1) thin films by reactive deposition on MgO(1 0 0) substrates were studied using low-energy electron diffraction (LEED), conversion electron Mössbauer spectroscopy (CEMS), Ruth- erford backscattering spectrometry (RBS), channeling (RBS-C) experiments and X-ray reflectometry (XRR). No visible influence from the ion irradiation of the samples on the CEMS spectra was found, while surface oxidation of the samples was observed after exposure to the atmospheric pressure. RBS analysis indicated the presence of magnesium with an average amount of 3% in the films. RBS-C experiments yielded a value of 22% for the minimum yield of Fe and a value of 0.62° for the half-angle for Fe in the film indicating a good crystal quality of the films. The value for film-thickness obtained from XRR is in a good agreement with that from RBS and the nominal value. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Magnetite (Fe 3 O 4 ), a ferrimagnet with a net magnetic moment of 4.1l B and a Néel temperature of 858 K, is the oldest known mag- netic material and has been investigated extensively due to its high technological applications in recording media, corrosion and catal- ysis [1–3]. In recent years, magnetite has attracted again theoreti- cal, experimental and technological interest. It is referred as one of the semi-metallic materials having a full spin polarization on the Fermi level E F [4]. Thus it is viewed as an ideal candidate for room temperature spintronic applications [5]. Recently, studies of high- quality Fe 3 O 4 thin films using spin-resolved photoelectron spec- troscopy have indicated that it was not possible to obtained the desired value of 100% polarization at E F and thus Fe 3 O 4 is not a true half-metallicity ferromagnet [6]. Magnetite crystallizes in the inverse spinel cubic structure (space group Fd  3m. In an unit cell with a lattice constant of a = 8.396 Å, 32 oxygen anions O 2 form a close-packed face-cen- tered-cubic fcc lattice, 8 Fe 3+ cations locate in the tetrahedrally coordinated A-sites, while 16 octahedrally coordinated B-sites are occupied randomly by 8 Fe 3+ and 8 Fe 2+ cations. The bulk crys- tallographic structure of magnetite parallely to the (0 0 1) plane can be represented as a stacking sequence of two alternative layers; the A-layer contains only tetrahedral Fe ions while the B-layer con- sists of oxygen and octahedral Fe ions. The smallest interlayer spacing of the A–B layer is of 0.105 nm, while that of the A–A layer and B–B layer is of 0.21 nm. In the B-layers, only half of the octahe- dral sites are occupied and Fe ions form rows along the [1 1 0] directions which are rotated by 90° with respect to one another in successive octahedral planes. A well-known feature of magnetite is the so-called Verwey transition (T V ) around 125 K reflected by distinguished anomalies in many physical properties. Despite of extensive studies, the nature of the Verwey transition remains still puzzling [7]. For over 60 years it has been described as a metal– insulator transition or a charge order–disorder transition related to the electrons resonating between Fe 3+ –Fe 2+ adjacent octahedral sites [8]. Recent studies demonstrating an absence of charge order- ing at low temperatures suggest that the Verwey transition is caused by strong electron–phonon interaction [9]. LDA band struc- ture calculations have proved that the combination of the on-site Coulomb interaction between 3d electrons and the electron–pho- non coupling implied an opening of a gap (0.2 eV) at the Fermi energy is the key feature responsible for the Verwey transition [10]. Recently an increasing interest is focused on the growth of iron oxide thin films on various oxide and metal substrates due to their importance in the model catalyst studies, magnetic thin films, sur- face geochemistry and corrosion. The growth of magnetite thin films with a high crystalline quality is especially important for spin dependent transport devices, as the mobile electrons may be 100% polarized. The very small lattice mismatch (0.31%) between the Fe 3 O 4 film and the MgO substrate in the (0 0 1)-plane provides 0039-6028/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2008.04.036 * Corresponding author. Tel.: +48 12 6626314; fax: +48 12 6372243. E-mail address: tarnawsk@ap.krakow.pl (N.-T.H. Kim-Ngan). Surface Science 602 (2008) 2358–2362 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc