Chuanbing Rong

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.

Investigation on intergrain exchange coupling of nanocrystalline permanent magnets by Henkel plot

In a real magnet, the relation between isothermal remanence Jr(H) and dc demagnetization remanence Jd(H) is expressed as δm(H)=[Jd(H)−Jr(∞)+2Jr(H)]/J(∞). It is believed that nonzero δm is due to the interactions between particles in the magnet. Using Pr2Fe14B as a sample, the relation is examined by the micromagnetic finite element method. The positive value of δm is primarily caused by intergrain exchange coupling. The decrease of intergrain exchange coupling results in the drop of the maximum value of δm. However, the variation of anisotropy in grain boundaries produces no change in the maximum value of δm. A Henkel plot is suggested to be effective for checking intergrain exchange coupling in magnets.

SmCo5∕Fe nanocomposites synthesized from reductive annealing of oxide nanoparticles

Hard magnetic nanocomposites SmCo5∕Fex (x=0–2.9) are synthesized by a simultaneous calcium reduction of Sm–Co–O and Fe3O4 nanoparticles. The composites consist of nanostructured SmCo5 and Fe with their average grain sizes at 29 and 8nm, respectively. The magnetic properties of the composites can be tuned by controlling Fe composition. SmCo5∕Fe1.5 shows an enhanced remanent magnetization at 56emu∕g (45emu∕g for SmCo5). The largest coercivity value of 11.6kOe is achieved with SmCo5∕Fe0.23. The synthesis represents a general process toward SmCo-based exchange-spring nanocomposites for high performance permanent magnet applications.

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.

Formation of Fe3Pt phase in FePt-based nanocomposite magnets

Atomic interdiffusion between FePt and Fe3O4 nanoparticles in annealed FePt-based nanocomposite magnets has been studied by means of structural and magnetic characterizations. The results show that the Fe3Pt phase is formed during the annealing only when the mass ratio x of Fe3O4/FePt is larger than 1/20. When x ≤ 1/20, only FePt single phase is formed. It is interesting to find that the coercivity of the annealed samples increases with a small addition of Fe3O4 before the formation of the Fe3Pt phase. This magnetic hardening behaviour indicates that the composition of the FePt phase can be further adjusted via the post annealing process. The characteristic of recoil loops and Henkel plots also give evidence for the transition from single-phase FePt magnets to nanocomposite magnets with the addition of Fe3O4.

Effects of particle size and composition on coercivity of Sm–Co nanoparticles prepared by surfactant-assisted ball milling

Proceedings of the 11th Joint MMM–Intermag Conference, 18–22 January 2010, Washington, DC, USA. Hard Magnetics Materials.