Flat epitaxial ferromagnetic CoFe 2 O 4 films on buffered Si(001) R. Bachelet a , P. de Coux a,b , B. Warot-Fonrose b , V. Skumryev c , J. Fontcuberta a , F. Sánchez a, ⁎ a Institut de Ciència de Materials de Barcelona-CSIC, Campus de la UAB, 08193 Bellaterra, Barcelona, Spain b CEMES-CNRS, 29 rue Jeanne Marvig, BP 94347, Toulouse Cedex 4, France c Institució Catalana de Recerca i Estudis Avancats (ICREA), Barcelona, Spain, and Dep. de Física, Univ. Autònoma de Barcelona, 08193 Bellaterra, Spain abstract article info Available online 31 December 2010 Keywords: Oxides on silicon Spinels films CoFe 2 O 4 RHEED Ferromagnetic films of spinel CoFe 2 O 4 have been grown epitaxially on Si(001) using CeO 2 /YSZ double buffer layers. The heterostructures were built in a single process by pulsed laser deposition with real-time control by reflection high-energy electron diffraction. YSZ and CeO 2 grow cube-on-cube on Si(001) and CoFe 2 O 4 grows with (111) out-of-plane orientation, presenting four in-plane crystal domains. The interface with the buffer layers is smooth and the CoFe 2 O 4 surface is atomically flat, with roughness below 0.3 nm. The films are ferromagnetic with saturation magnetization around 300 emu/cm 3 . The properties signal that CoFe 2 O 4 is a good candidate for monolithic devices based on ferromagnetic insulating spinels. © 2010 Published by Elsevier B.V. Complex oxides are main candidates to new materials presenting either multifunctionality or enhanced properties for device fabrica- tion. But the elusive epitaxial integration on silicon wafers, with films properties usually severely degraded, has stemmed this great potential. The exceptions are some high-k oxides, including the perovskite SrTiO 3 [1] and the spinel γ-Al 2 O 3 [2], that can be grown epitaxially on silicon with a stable atomically-flat interface. In contrast, the epitaxial growth of other functional complex oxides requires a buffer layer, yttria-stabilized zirconia (YSZ) being the most common choice. For example, ferroelectrics as Pb(Zr,Ti)O 3 [3,4] or Bi 3.25 La 0.75 Ti 3 O 12 [5], as well as ferromagnetic La 2/3 Sr 1/3 MnO 3 and related manganites [6,7] can grow epitaxially on YSZ buffered Si(001). Films of insulating ferromagnets with spinel structure, with attractive magneto-optical and high-frequency properties, have been much less studied, although the epitaxial growth of (Ni,Zn)Fe 2 O 4 on buffered Si(001) has been reported [8]. We have deposited spinel CoFe 2 O 4 (CFO) films on Si(001) using a CeO 2 /YSZ double buffer layer. The films are epitaxial, with (111) out- of-plane orientation and four in-plane crystal domains, and they present very flat surface and interface with the CeO 2 buffer layer. The films present saturation magnetization around 300 emu/cm 3 . CFO/CeO 2 /YSZ heterostructures were deposited by pulsed laser deposition in a single process on Si(001) substrates. The oxygen partial pressure and the substrate temperature during deposition were 3×10 -4 mbar and 800 °C for YSZ and CeO 2 buffer layers, and 0.1 mbar and 550 °C for CFO. A KrF excimer laser (λ = 248 nm) operating at a repetition rate of 5 Hz was focused sequentially on stoichiometric ceramic targets at a fluence ∼ 1.2 J/cm 2 . The target-substrate distance was around 50 mm. The Si(001) substrates were used without removing the native oxide, although they were heated to the deposition temperature of YSZ under vacuum (~7 × 10 -7 mbar at 800 °C). Film deposition was started at the base pressure (oxidation of YSZ by reducing SiO x ) and oxygen was introduced after ~ 8 s [9]. Layers thickness (t) and growth rate were ∼ 160 nm and 0.32 Å/pulse for YSZ, ∼110 nm and 0.55 Å/pulse for CeO 2 , and from∼ 30 to ∼ 75 nm and ∼ 0.08 Å/pulse for CFO. A differentially pumped 30 keV reflection high-energy electron diffraction (RHEED) system was used to monitor the intensity of the specular spot in off- axis conditions. The crystal structure, epitaxial relationships and out- of-plane lattice strain were investigated by X-ray diffraction. High resolution transmission electron microscopy (HRTEM) analysis was conducted in cross-section geometry. Atomic force microscopy (AFM) in dynamic mode was used to investigate the surface morphology of the films. Magnetization loops were measured at 10 K by superconducting quantum interference device (SQUID). The time dependence of the intensity of the RHEED specular spot is plotted in Fig. 1 for the early and the last growth stages of YSZ, CeO 2 and CFO. The intensity drops when the YSZ deposition starts (marked by the horizontal arrow in Fig. 1a), and it decreases more after introduction of oxygen (vertical arrow). Then, with constant oxygen pressure, the intensity increases progressively during the growth of around 10 nm of YSZ, and later the intensity remains basically constant. This suggests enhancement of crystalline ordering up to this thickness, which agrees with the higher crystal disorder reported for very thin YSZ films respect to thicker ones [9]. Although periodic variations cannot be observed, the significant intensity recovery (Fig. 1b) ocurring during a long time of few tens of seconds at the end of the YSZ deposition (t=160 nm) suggests two-dimensional growth. In the sequential deposition of CeO 2 (Fig. 1c) on the YSZ buffer, the two complete periodic intensity oscillations signal layer-by-layer growth. After, a slight monotonous increase of intensity is observed up to 25 nm the oscillations are barely distinguishable; at the Thin Solid Films 519 (2011) 5726–5729 ⁎ Corresponding author. E-mail address: fsanchez@icmab.es (F. Sánchez). 0040-6090/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.tsf.2010.12.200 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf