Magnetic and structural properties of thin films and nanoparticles studied by scattering methods

Abstract

The present work concerns the magnetic and structural proprieties of magnetic thin films and magnetic nanoparticles studied by scattering methods. The structural properties of epitaxially grown Fe/Cr/Fe trilayer were studied with anomalous x-ray scattering. Two different x-ray energies have been used; (i) one E$_{1}$=5985eV to match the maximum contrast of the Fe/Cr interface close to the Cr absorption K-edge and (ii) a second one E$_{2}$=6940eV where the Fe/Cr interface displays the lowest contrast. The specular reflectivity and longitudinal diffuse scans together with omega scans for both energies were measured. The simulations within the frame of Distorted Wave Born Approximation (DWBA) allowed us to describe quantitatively the morphology of each interface. The roughness, Hurst parameter and the thickness of every layer as well as an oxidation effect at the surface of the sample are derived. The strength and limitations of the method are discussed. The influence of confinement on the magnetic behavior, was investigated in MnO embedded in a porous glass. We studied the magnetic order and phase transition of MnO nanoparticles by polarized neutron scattering. The nanopores filled with MnO are wormlike structures with a typical diameter of 7nm. From the temperature dependence of the magnetic ($\frac{1}{2}$ $\frac{1}{2}$ $\frac{1}{2}$) Bragg intensity we obtained that for the MnO nanoparticles the phase transition is continuous with a Néel temperature T$_{N}$=122K. This is in contrast to bulk MnO, which exhibits a first order phase transition at 118K. Furthermore, we observed that a part of the MnO nanoparticle material remains disordered even at 10K, which indicates frustration at the interface MnO to glass. The continuous character of the transition and the unusual temperature dependence with a reduced order parameter was described and modelled by surface induced disorder. In order to study the spin-canting effect in magnetic nanoparticles, we explored a synthesis route of Co nanoparticles in which we employed water-in-oil microemulsions that are stabilized by the nonionic surfactants of the ethoxylated alkyl- or arylether series such as C$_{12}$E$_{5}$ and Igepal CO520 and contain the NaBH$_{4}$-solution. Co nanoparticle formation is then induced by injecting a solution of Co(AOT)$_{2}$ in hexane. We found out that the water inside the microemulsion leads already to oxidation of the particles. [...