(679b) Tunable Magnetic Core-Shell Nanoparticles: An Interplay between Composition, Size, Shape and Architecture

Over the past century, magnetic materials have played a key role in the development of the American economy through their incorporation in power plants, memory storage, and cancer treatments. Of particular interest is the creation of permanent magnet replacements, typically requiring expensive rare earth elements, for the application in energy generation (windmills) and energy conversion (electric vehicles). Typically, these permanent magnet replacements focus on iron based oxides due to their relative abundance in the earth’s crust and natural magnetic moment. Unfortunately, these oxides are relatively weak magnets with dipoles which are highly susceptible to external magnetic fields, regardless of the composition. However, the saturation/remanent magnetism and coercivity can be engineered, producing magnets which are more robust to external fields (harder magnets), by increasing the shape anisotropy and controlling coupling across “hard”-“soft” magnetic interfaces. Specifically, highly anisotropic, core-shell iron oxide nanostructures are investigated to develop design criteria for engineering magnetic properties based on the relationship between material composition, structuring and architecture. The magnetic response of these nanostructures is known to be dependent on the crystallinity of the nanostructure, requiring careful synthesis to engineer the formation of defects within the crystal.

In this work, a modified thermal decomposition method is used in order to control the composition, morphology, architecture of Fe₃O₄ nanostructures with critical dimensions between 20-30 nm. Initially, composition of the nanostructures is determined through the synthesis of an M_xFe_1-x oleate (M= Co²⁺, Ni²⁺, Fe^2+/3+ or Mn^2+/3+) during the initial precursor synthesis, where the Co dopant increases coercivity and Ni/Mn dopants increases the remanent magnetization. These precursors are then used to synthesize spheres, cubes, plates, and pyramids based on the ratio of oleate to oleic acid precursor via a steric hindrance mechanism on the difference surface facets. Finally, core-shell nanostructures, with conformal and non-conformal coatings, are fabricated through a subsequent deposition on a magnetic core, in order to facilitate magnetic coupling across the interface, and non-magnetic structures, to overcome the ~20 nm critical dimension that results in agglomeration of the particles. Furthermore, this two-step process allows for the synthesis of high aspect ratio 1D and 2D magnetic structures which have not been previously reported. In addition to standard characterization techniques, the chemical and structural control will be highlighted using high resolution electron microscopy and specially resolved energy spectroscopy coupled with magnetic characterization to introduce design parameter for engineered materials.