Narayan Poudyal

Advances in nanostructured permanent magnets research

This paper reviews recent developments in research in nanostructured permanent magnets (hard magnetic materials) with emphasis on bottom-up approaches to fabrication of hard/soft nanocomposite bulk magnets. Theoretical and experimental findings on the effects of soft phase and interface conditions on interphase exchange interactions are given. Synthesis techniques for hard magnetic nanoparticles, including chemical solution methods, surfactant-assisted ball milling and other physical deposition methods are reviewed. Processing and magnetic properties of warm compacted and plastically deformed bulk magnets with nanocrystalline morphology are discussed. Prospects of producing bulk anisotropic hard/soft nanocomposite magnets are presented.

Monodisperse face-centred tetragonal FePt nanoparticles with giant coercivity

Monodisperse face-centred tetragonal (fct) FePt nanoparticles with high magnetic anisotropy and, therefore, high coercivity have been prepared via a new heat treatment route. The as-synthesized face-centred cubic FePt nanoparticles were mixed with salt powders and annealed at 700uC. The salts were then removed from the particles by washing the samples in water. Monodisperse fct FePt particles were recovered with the particle size and shape being retained. Coercivity of the isolated particles up to 30 kOe at room temperature has been obtained. (Some figures in this article are in colour only in the electronic version)

Fabrication of bulk nanocomposite magnets via severe plastic deformation and warm compaction

We demonstrate that a SmCo/FeCo based hard/soft nanocomposite material can be fabricated by distributing the soft magnetic α-Fe phase particles homogeneously in a hard magnetic SmCo phase through severe plastic deformation. The soft-phase particle size can be reduced from micrometers to smaller than 15 nm upon deformation. Up to 30% of the soft phase can be incorporated into the composites without coarsening. A warm compaction process of the plastically deformed powder particles then produces bulk nanocomposite magnets of fully dense nanocomposites with energy product up to 19.2 MGOe owing to effective interphase exchange coupling, which makes this type of nanocomposite magnets suitable for high energy-density applications at high temperatures.

High Energy Product Developed from Cobalt Nanowires

Cobalt nanowires with high aspect ratio have been synthesized via a solvothermal chemical process. Based on the shape anisotropy and orientation of the nanowire assemblies, a record high room-temperature coercivity of 10.6 kOe has been measured in Co nanowires with a diameter of about 15 nm and a mean length of 200 nm. As a result, energy product of the wires reaches 44 MGOe. It is discovered that the morphology uniformity of the nanowires is the key to achieving the high coercivity and high energy density. Nanowires of this type are ideal building blocks for future bonded, consolidated and thin film magnets with high energy density and high thermal stability.

Hard magnetic FePt nanoparticles by salt-matrix annealing

Proceedings of the 50th Annual Conference on Magentism and Magnetic Materials. L10 and Other Hard Magnetic Materials L10 and Other Hard Magnetic Materials L10 and Other Hard Magnetic Materials. L10 and Other Hard Magnetic Materials.

Bulk FePt-based nanocomposite magnets with enhanced exchange coupling

Department of Physics, University of Texas at Arlington. NanoTech Institute, University of Texas at Dallas. School of Materials Science and Engineering, Georgia Institute of Technology.

Preparation of Nd–Fe–B nanoparticles by surfactant-assisted ball milling technique

Nd–Fe–B nanoparticles have been obtained by using surfactant-assisted ball milling and subsequent size-selection technique. Structural analyses show that nanoparticles with two particle sizes around 10 and 100nm were obtained. The partially amorphous Nd–Fe–B nanoparticles give their room-temperature coercivities around 0.1 and 1.5kOe for the small and large nanoparticles, respectively. As the temperature decreases to 200K, the coercive force of the large nanoparticles increases by 50% due to the enhancement of the magnetocrystalline anisotropy of the Nd2Fe14B phase in the particles.

Anisotropic bonded magnets fabricated via surfactant-assisted ball milling and magnetic-field processing

Anisotropic bonded magnets are fabricated by surfactant-assisted ball milling in a magnetic field and magnetic field alignment of the milled chip-like nanoparticles of the Sm–Co and Nd–Fe–B materials. It is found that the application of magnetic fields during the ball milling strengthens the anisotropy of the chips and therefore improves the alignment. For SmCo5 phase-based chips, for instance, energy products up to 26.0 MG Oe and 19.1 MG Oe are obtained for the chips and the bonded magnets, respectively. This combined technique opens a new approach to the fabrication of anisotropic bonded magnets for various applications.

Morphological and magnetic characterization of Fe, Co, and FeCo nanoplates and nanoparticles prepared by surfactants-assisted ball milling

We report here the preparation of Fe, Co, and FeCo nanoplates and nanoparticles by ball milling in the presence of surfactants in organic solvents. By controlling the milling and centrifugation conditions, the Fe, Co, and FeCo nanoplates and nanoparticles with different sizes were successfully obtained, from the slurries and from the top part of the solutions, respectively. The thickness of the nanoplates is in the range of 20–200 nm and their diameter is from 5 to 30 μm. The Fe, Co, and FeCo nanoparticles of about 6 nm show superparamagnetic behavior at room temperature and are ferromagnetic at low temperatures with blocking temperatures of 33, 103, and 54 K, respectively. It is found that the surfactants play multifold roles in the process.

High thermal stability of carbon-coated L10-FePt nanoparticles prepared by salt-matrix annealing

Department of Physics, University of Texas at Arlington. Ames Laboratory and Department of Materials Science and Engineering, Iowa State University. NanoTech Institute, University of Texas at Dallas.