Kevin Elkins

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)

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.

Magnetic hardening in ultrafine FePt nanoparticle assembled films

Ultrafine FePt nanoparticles have been synthesized via a novel chemical solution synthesis route. Without using a reducing agent, the stoichiometric FePt nanoparticles were produced by the decomposition of iron acetylacetonate and platinum acetylacetonate in octyl ether in the presence of oleic acid and oleyl amine as the surfactants. The particle size was found to be around 2 nm with a narrow size distribution. The particles were then deposited on substrates an dh eat treated afterwards. Upon annealing, the nanoparticles sintered together to form continuous thin films and at the same time to transform from disordered face centred cubic (fcc) structure to the highly anisotropic ordered face centred tetragonal (fct) structure and therefore magnetic hardening was realized in the thin films. Coercivity up to 27 000 Oe at room temperature has been obtained in the annealed samples. The very high coercivity indicates a highly completed fcc–fct phase transition which may be related to the very fine original particle size. (Some figures in this article are in colour only in the electronic version)

Phase Transformation of FePt Nanoparticles

To transfer face-centered cubic (fcc) FePt nanoparticles to face-centered tetragonal (fct) structure with high magnetic anisotropy, heat treatments are necessary. The heat treatments, however, often lead to agglomeration and sintering of the nanoparticles. To prevent the particles from sintering, a new method, the salt-matrix annealing, has been adopted in our experiments recently. The fcc nanoparticles produced by chemical synthesis were mixed with NaCl or other salt powders. The mixture was then annealed at temperatures up to 750degC to complete the fcc to fct phase transition. After the annealing, the salt was washed out by water and monodisperse fct FePt nanoparticles free of non-magnetic impurities were obtained. Magnetic measurements show very high coercivity (up to 30 kOe) of the monodispersed fct nanoparticles. Size dependence of the phase transition and alignment of the ferromagnetic nanoparticles have also been investigated

Synthesis and characterization of FeCo nanowires with high coercivity

Ferromagnetic FeCo nanocrystals with high coercivity have been synthesized using a reductive decomposition method. The sizes and shapes of the nanocrystals were found to be dependent on reaction parameters such as the surfactant ratio, the precursor concentration and the heating rate. Synthesized nanocrystals have a body-centered cubic crystal structure for both particles and nanowires and the (110) crystalline direction is along the long axis of the nanowires. The coercivity and magnetization of the FeCo nanocrystals are found to be dependent on morphology. Nanowires of Fe60Co40 with saturation magnetization of 92 emu g(-1) and coercive force of 1.2 kOe have been obtained in this study.

Synthesis and characterization of CoFe2O4 nanoparticles with high coercivity

Single crystalline CoFe2O4 nanoparticles with high coercivity were prepared via a one-step hydrothermal method. The shape and size of the nanocrystals (in the range of 20–100 nm) can be controlled by varying synthesis parameters such as the concentration of NaOH and CTAB. X-ray diffraction and Raman spectra analysis confirmed that all the as-synthesized nanoparticles have a face centered cubic spinel crystal structure. HRTEM observation of particles shows interlayer spacing 0.48 nm of (111) lattice planes. A coercive force up to 5.0 kOe and saturation magnetization of 73 emu/g was achieved at room temperature for the 40 nm CoFe2O4 nanoparticles.

Processing of MnBi bulk magnets with enhanced energy product

We report magnetic properties and microstructure of high energy-product MnBi bulk magnets fabricated by low-temperature ball-milling and warm compaction technique. A maximum energy product (BH)max of 8.4 MGOe and a coercivity of 6.2 kOe were obtained in the bulk MnBi magnet at room temperature. Magnetic characterization at elevated temperatures showed an increase in coercivity to 16.2 kOe while (BH)max value decreased to 6.8 MGOe at 400 K. Microstructure characterization revealed that the bulk magnets consist of oriented uniform nanoscale grains with average size about 50 nm.

Mesoporous iron oxide nanowires: synthesis, magnetic and photocatalytic properties

A surfactant- and template-free approach is described for the synthesis of mesoporous α-Fe2O3, Fe3O4 and α-Fe nanowires (NWs). In this approach, α-FeOOH NWs (length 550 nm and diameter 30 nm) were first prepared by hydrolysis of FeCl3. On subsequent thermal treatment in a fluidized bed reactor in the presence of a forming gas (Ar 93% + H2 7%), α-FeOOH transformed to mesoporous NWs of ɑ-Fe2O3, Fe3O4 and ɑ-Fe by controlling the process parameters such as reaction time and temperature. The obtained NWs of α-Fe2O3, Fe3O4 and α-Fe were ferromagnetic at room temperature with a coercive field (Hc) of 412, 583 and 628 Oe respectively. The aligned NWs showed 1.6 to 2 times-enhanced remanence in the parallel direction relative to the perpendicular direction due to magnetic anisotropy. These mesoporous magnetic NWs with a high specific surface area (82 m2 g−1 for α-Fe2O3 NWs) were used in photocatalysis due to the high adsorptivity of three probe dye molecules. The as-prepared α-Fe2O3 NWs exhibited only modest photocatalytic activity; however, the catalytic activity could be further enhanced by decorating the mesoporous ɑ-Fe2O3 NWs with 10 nm sized ZnO nanoparticles. The developed ɑ-Fe2O3/ZnO nanowire nanohybrids could eliminate 100% of the probe dyes: methylene blue, Rhodamine B and methyl orange within 90 min irradiation with solar light, underlining the high photocatalytic degradation efficiency of the nanohybrid. The nanowire nanohybrids could be easily recovered by applying an external magnetic field and reused for at least 4 times without significant loss of their photocatalytic activity.

A Novel Approach to Synthesis of FePt Magnetic Nanoparticles

Chemical reduction of ferric acetylacetonate (Fe(acac)3) and platinum acetylacetonate (Pt(acac)2) using polyol as a reducing agent as well as an effective surfactant, has successfully yielded monodisperse FePt nanoparticles with a size of approximately 2 nm. When annealed samples were compared to FePt nanoparticles synthesized using oleic acid and oleylamine as the surfactants under identical conditions, nearly 30% increase in coercivity (Hc) was achieved with the new, simple and economic method.