IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009 4357 Epitaxial Growth and Magnetic Properties of Half-Metallic Fe O on Si(100) Using MgO Buffer Layer Sameh S. A. Hassan , Yongbing Xu , Jing Wu , and Sarah M. Thompson Spintronics and Nanodevice Laboratory, Departments of Electronics, University of York, York, YO10 5DD, U.K. Department of Solid State Physics, National Research Centre, Cairo, Egypt Departments of Physics, University of York, York, YO10 5DD, U.K. The growth and magnetic properties of epitaxial magnetite Fe O on Si(100) using MgO buffer layer have been studied by reflection high-energy electron diffraction, X-ray photoelectron spectroscopy, and magneto-optical Kerr effect. The epitaxial Fe O films were prepared by in situ post growth annealing of ultrathin epitaxial Fe films at 220 C in an oxygen partial pressure of mbar. The epitaxial relationship was found to be Fe O (100) //MgO(100) //Si(100) with the MgO film grown cube-on-cube orientation with the Si substrate. While the Fe unit cell rotates 45 to match that of the MgO buffer layer, it rotates 45 back upon oxidation. A low saturation field has been observed indicating low density of antiphase boundaries. Index Terms—Magnetic oxides, magnetite, molecular beam epitaxy, Si, spin injection, spintronics, ultra-thin films. I. INTRODUCTION T HE magnetic/semiconductor hybrid spintronic structures are one of the most promising approaches for the second generation spintronics integrating semiconductors [1]. The sil- icon semiconductor has a great potential in spin-based applica- tions. In addition to its great dominance of the current device industry, it has also been expected to have enhanced spin life- time and diffusion length, which is larger than that in GaAs and many orders of magnitude larger than that in metals [2], [3] due to the low spin orbit scattering and lattice inversion symmetry [4]. Yet little progress has been realized in integrating Si with magnetic materials compared to GaAs [5], [6]. One of the rea- sons is the fact that Si has an indirect band gab which precludes the optical spin injection and detection in Si. The other reason is the interface quality and the formation of silicides which could have a deteriorating effect that potentially hinders the electrical spin injection [7]. It was reported recently that the interface of magnetite thin films and Si substrates consists of crystalline iron silicade and amorphous oxide bilayer, which have a lower mag- netic moment [8]. Therefore, there is an intensive need for the use of buffer layer to grow magnetic thin films on Si as it could prevent the interaction at the interface. Several groups have tried growing magnetite thin films on Si with and without buffer layer and varied results were found from polycrystalline to oriented and epitaxial films [9]–[12]. Very recently, Appelbaum et al. have demonstrated for the first time the transport and coherent manipulation of the electron spin in silicon [4] Using Al O tun- neling barrier, a value of 30% spin polarization has also been achieved from Fe into Si. In the present work, we have successfully grown a fully epi- taxial magnetite (Fe O ) on Si(100) using MgO buffering layer. This hybrid structure is very promising for spin injection not only because Fe O has a high Curie temperature far above room temperature and has been predicted to have 100% spin polarization, but also because the MgO buffering layer plays an Manuscript received March 06, 2009; revised May 06, 2009. Current version published September 18, 2009. Corresponding author: Y. Xu (e-mail: Yx2@york.ac.uk). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2009.2025600 important rule in overcoming some technological challenges, such as the conductivity mismatch problem [13] and reducing the lattice mismatch. Moreover, MgO could also work as a spin filter [14], [15] and as a protector of the Si substrate against in- termixing with Fe O . II. EXPERIMENTAL PROCEDURE The Si substrates have been prepared prior to the growth in order to obtain hydrogen terminated and oxide free Si. The substrates were first chemically cleaned using the recipe in Table I [16]. Every step was followed by rinsing in contin- uously flowing deionized water and then blow dried by gas. The substrates were annealed at 830 K for about 55 min in UHV molecular beam epitaxy (MBE) chamber until sharp clear (2 1) RHEED streaks were observed. A 10 nm layer of MgO was grown at 540 K using ebeam evaporation of MgO crystals. The deposition was carried out first under the UHV conditions ( mbar) for the first 60 s and then in an oxygen environment ( mbar ), which was found to enhance the crystalline quality of the film [17]. The delay time of introducing the oxygen gas has been reported to be useful to prevent the formation of amorphous MgO at the interface [18]. A 5 nm Fe thin film was grown afterwards at room temperature and subsequently annealed for 10 min at 220 C and oxygen atmosphere of mbar in order to get a completely oxidized Fe O . The samples have been further characterized for their chemical composition using XPS. The XPS measurements were done with MgK radiation. The magnetic properties of the films were studied by ex situ MOKE at room temperature using an electromagnet with a maximum field of 1.6 kOe and a diode laser. III. EXPERIMENTAL RESULTS AND DISCUSSION A. Growth and Structural Characterization The overall growth process of films was in situ monitored by RHEED at an electron acceleration voltage of 10 kV along four directions of the Si(100) substrate. Fig. 1(a) illustrates the RHEED patterns of the Si(100) substrate after annealing at 550 C at the UHV MBE chamber. The figure shows clearly that the pre-treated Si substrate surface is very smooth, indicated by the sharp streaky diffraction lines and the Kikuchi lines in the pattern. In addition, no clear diffusive scattering could be 0018-9464/$26.00 © 2009 IEEE