Structural and magnetic properties of MnCoGe ferromagnetic thin films produced by reactive diffusion

International audienceThe structure, the chemistry, and the magnetic properties of MnCoGe thin films elaborated by reactive diffusion were investigated. In situ X-ray diffraction (XRD) was used to study phase formation during thin film reaction. MnCo, MnGe, and CoGe binary systems were studied before investigating phase formation during Mn-Ge-Co ternary system reaction. Three pure layers of Mn, Ge, and Co were successively deposited by magnetron sputtering on SiO2 to form a 200 nm-thick Co/Ge/Mn stack, and annealed. Six phases were observed during reaction, first following the sequential phase formation observed for the binary systems at the two Mn/Ge and Ge/Co interfaces, and ending with the formation of a single ternary compound MnCoGe at 673 K. The structure and the composition of the MnCoGe films were characterized using XRD, atomic force microscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy. The magnetic properties of the films were studied using superconducting quantum interference device (SQUID) and ferromagnetic resonance (FMR) measurements. The obtained MnCoGe thin films are polycrystalline with the stoichiometric composition Mn:Co:Ge(1/3:1/3:1/3), and show high porosity. They are made of grains exhibiting both the Ni2In-type hexagonal structure and the TiNiSi-type orthorhombic structure

Electron microscopy and Auger spectroscopy study of the wetting of the grain boundaries in the systems Mo-Pb, Mo-Sn, Mo-Ni and Ni-Pb

Understanding the mechanism of the intergranular penetration of a liquid phase into a metallic solid is an important problem. The structural and chemical characterization of nanometric films at grain boundaries is now possible by using high resolution electron microscopy associated with X-ray micro-analysis, electron energy loss spectroscopy and Auger spectroscopy. In order to study this problem, two different classes of model materials were selected according to their crystallographic structure: a bcc metal (molybdenum) and an fcc one (nickel). The wetting element was either lead or tin or nickel. In a first approach, the metallic matrix was polycrystalline. The conditions in which the liquid phase penetrates into the grain boundaries were studied by using special preparation and observation techniques. In particular, the use of a Focused Ion Beam microscope (FIB) allowed the preparation of thin foils located very precisely inside the matrix as well as multi-scale observations. These specimens were further observed in electron microscopy with a very high resolution

Ferromagnetic MnCoGe thin films produced via magnetron sputtering and non-diffusive reaction

MnCoGe thin films were produced using simultaneous magnetron sputtering of Mn, Co, and Ge on SiO2, followed by non-diffusive reaction. The MnCoGe compound begins to form at ∼588 K, and structural characterizations show that the obtained MnCoGe film is polycrystalline with the hexagonal Ni2In-type structure. This structure is found to be stable from 873 K down to room temperature, the expected hexagonal/orthorhombic structural transition being prevented. The film exhibits a lower average Mn composition than the standard MnCoGe stoichiometry. Furthermore, small clusters (<3 nm) forming planar distributions parallel to the sample surface are observed. They are regularly located every∼11 nm in the specimen depth. They mainly contain Mn and O atoms. Magnetic characterizations show very good magnetic properties, allowing the perpendicular and parallel magnetocrystalline anisotropy constants to be measured down to 100 K, using the Chappert model to fit ferromagnetic resonance measurements. The film magnetic properties match the properties of bulk stoichiometric MnCoGe in the hexagonal structure, with a Curie temperature of ∼269 K and a negligible coercive field at room temperature. The only difference between the magnetic properties of bulk and thin film specimens appears to be the film shape anisotropy, forcing the internal magnetic field to be contained in the film plane.INITIATIVE D'EXCELLENCE AIX MARSEILLE UNIVERSIT

Structural, dielectric, ferroelectric and tuning properties of Pb-free ferroelectric Ba0.9Sr0.1Ti1-xSnxO3

International audienceA series of Pb-free ferroelectric materials Ba0.9Sr0.1Ti1-xSnxO3 (BSTS-x) with 0 <= x <= 0.15 was successfully prepared via solid-state reaction method. The effect of Sn substitution on the crystal structure, microstructure, dielectric behavior, ferroelectric and tunable features of BSTS-x ceramics were systematically investigated. Room temperature (RT) x-ray diffraction (XRD) analysis using the Rietveld refinement method reveals that all the synthesized BSTS-x ceramics were well crystallized into single perovskite structure. The results show a tetragonal phase for 0.00 <= x <= 0.02, which evolves to orthorhombic and tetragonal coexisting phases for 0.05 <= x <= 0.07. The composition x = 0.10 showed a mixture of tetragonal, orthorhombic and rhombohedral phases at RT, while a single cubic phase is observed for x = 0.15. The crystal phases determined by XRD were confirmed by Raman spectroscopy. Enhanced dielectric permittivity with a maximum value of epsilon'similar to 35000 is observed for x = 0.10 at RT. The ferroelectric behavior of BSTS-x ceramics was investigated through polarization hysteresis loops and tunability measurements. High tunability of 63% at RT and under the low DC-applied electric field of 1.40 kV/cm is achieved for x = 0.10