Electron microscopy characterization and structure optimization of iron-platinum self-assembled nanoparticles for ultrahigh-density magnetic data storage

High magnetocrystalline anisotropy materials are in the center of current research interest in magnetic recording. Among the pursued research directions on these materials are nanogranular thin films, self-assembled nanoparticles and patterned media. Ultrahigh-density recording requires a bit dimension of only a few nanometers with a precise control of microstructure, grain size, size dispersion and grain isolation. The data storage industry standards require that these materials comply with a 10-year storage period requirement which, in terms of material properties translates into Ku V/kBT > 50, where K u is the magnetocrystalline anisotropy constant, V is the volume of the bit and kBT is the thermal energy at the storage temperature. L10 materials have magnetocrystalline anisotropy energy constants one order of magnitude larger than currently employed Co-based hexagonal alloys and among these, FePt has the highest value of the magnetocrystalline anisotropy constant which allows a bit size of only 3 nm. Self-assembled FePt nanoparticles offer a low-cost, high storage density alternative and are regarded as the possible next generation of advanced materials in magnetic data storage.; Chapter 1 presents the motivation of this research and the experimental challenges encountered in optimization of the self-assembled FePt nanoparticle media for magnetic data storage. The previous literature on ordering of nanostructured FePt films and the crystallography of the fcc and L1 0 structures are reviewed. Chapter 2 presents the electron microscopy techniques employed for the structural characterization of nanoparticles, the principle of preparation of monodispersed nanoparticles with application to metallic nanoparticles and the procedure developed by us in order to achieve the phase transformation fcc → L1 0 in FePt nanoparticle monolayers capped with oleic acid/oleylamine without sintering. Chapter 3 presents the notion of self-assembly in the context of nanoparticle texture and symmetry of the self-assembly, showing some of their mutual influences and how they can be observed in electron microscopy. The procedure of determining the distribution angle of axial texture in thin films is presented at the end of this chapter. Chapter 4 presents the classification of first-order phase transformation which includes the transformation of interest to us fcc → L10. An overview of the main experimental findings from the existing literature on the phase transformation in bulk FePt, nanogranular thin films and nanoparticles is given in Sections 4.2.1, 4.2.2 and 4.2.3 respectively. Chapter 5 introduces the notion of order parameter in order-disorder transformations, its dependence on texture in the case of nanoparticles and details the electron microscopy method employed by us to measure the order parameter in nanoparticles. Chapter 6 presents quantitative results on the microstructure, texture and order parameter and their evolution with annealing, when the thermal annealing sintering-prevention procedure developed by us is employed. In Chapter 7 the main conclusions of our research are shown together with their implications for nanoparticle data storage. Future directions of research of the phase transformation in nanoparticles are suggested in Chapter 8.