Directed self assembly of patterned magnetic nanostructures
Srivastava, Akhilesh Kumar
Date of Issue2009
School of Materials Science and Engineering
There is tremendous interest in magnetic nanostructured arrays of various shapes, driven by the urgent need for novel high density data storage media. This thesis presents the directed self assembly of curved and high aspect ratio chemically synthesized periodic magnetic nanostructures and their characterization and property evaluation. Chemical synthesis of magnetic structures was used since the shapes studied in this work, e.g., bowl, wire and chain of spheres are difficult to create using conventional physical methods. Chemical methods also offer enhanced homogeneity from the “bottom up” processing of the material, high throughput, low cost, facile surface modification and control of size, shape and size distribution of the nanoparticles. Morphological, structural, surface and magnetic characterization of these nanostructures was carried out using SEM, TEM, XRD, XPS and VSM. Chemical synthesis of cobalt nanobowl arrays with minimal volume shrinkage was carried out by the template directed assembly technique. Borohydride reduction within the interstitials of colloidal templates of polystyrene (PS) spheres followed by annealing resulted in long range periodic metallic Co nanobowl arrays. Surface and structural analysis was carried out to investigate the boron removal and the two phase (h. c. p. and f. c. c.) microstructure of the cobalt nanobowls. The coercivity values of cobalt nanobowl arrays increase with decreasing bowl diameter and are larger than their nanoparticle and thin film counterparts due to shape effects. Cobalt ferrite nanobowl arrays were also synthesized using the colloidal crystal templating methodology, a single phase cubic structure was observed with coercivity values which were enhanced compared to the nanoparticle and thin film counterparts. Cobalt nanoparticles with an average size of 4.3 nm and unusually high saturation magnetization were synthesized by borohydride reduction technique and subsequently directed by an external magnetic field assembly to produce nanowires, the coercivity of such nanowires was found to be greater than that of nanoparticles due to shape anisotropy. The difference between the experimental values of coercivity and the predictions of Jacobs and Bean model was ascribed to the thermal fluctuations that occur for small particle size. Chemical synthesis of cobalt within the pores of an AAO template was also carried out to create periodic arrays of Co nanowires, textured growth of Co nanowires within the pores was observed and the magnetic properties exhibited interesting magnetization cross over behavior. Two phase cobalt chains of spheres and chains of ellipsoids were synthesized by thermal decomposition of cobalt oxalate nanorods. This morphological transition was driven by Rayleigh instability effects. The fragmentation process and decomposition kinetics were found to be in reasonable agreement with the theoretical predictions of Nichols and Mullins. The magnetization reversal of these chains could be explained by the fanning mechanism as predicted by Jacobs and Bean. These results show that directed self assembly can be used in conjunction with chemical synthesis to form patterned magnetic nanostructures with interesting magnetic properties.