Monodisperse FePt nanoparticles with particle size of about 2 nm have been prepared by 1,2-hexadecanediol reduction of iron acetylacetonate and platinum acetylacetonate in dioctyl ether. The as-synthesized particles have a chemically disordered fcc structure and can be transformed into chemically ordered fct structure after thermal processing at temperatures above 500°C. The ordered fct FePt phase has high magnetic anisotropy and thus large coercivity up to 1.8 T.

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 700˚C. 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.

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.

Morphological control of FePt nanoparticles has been systematically studied. By varying synthetic parameters including precursors, solvents, amount of surfactants, and heating rate of the solution, the particle size from 2 to 9 nm can be tuned with 1 nm accuracy. While most particles are spherical in shape, cubic particles can be obtained when particles are greater than 7 nm. Rod-shape nanoparticles have also been obtained. The as-synthesized nanoparticles are found to be superparamagnetic at room temperature and their blocking temperature is size dependent that increases with particle size. After annealing in a reducing atmosphere, the nanoparticles form hard magnetic films with ordered fct structure and high coercivity up to 2.7 T.

An efficient heat activating mediator with an enhanced specific absorption rate (SAR) value is attained via control of the iron oxide (Fe3O4) nanoparticle size from 3 to 32 nm. Monodispersed Fe3O4 nanoparticles are synthesized via a seed-less thermolysis technique using oleylamine and oleic acid as the multifunctionalizing agents (surfactant, solvent and reducing agent). The inductive heating properties as a function of particle size reveal a strong increase in the SAR values with increasing particle size up to 28 nm. In particular, the SAR values of ferromagnetic nanoparticles (>16 nm) are strongly enhanced with the increase of ac magnetic field amplitude than that for the superparamagnetic (3-16 nm) nanoparticles. The enhanced SAR values in the ferromagnetic regime are attributed to the synergistic contribution from the hysteresis and susceptibility loss. Specifically, the 28 nm Fe3O4 nanoparticles exhibit an enhanced SAR value of 801 W g-1 which is nearly an order higher than that of the commercially available nanoparticles.

To transfer face-centered-cubic ͑fcc͒ FePt nanoparticles to the face-centered-tetragonal ͑fct͒ phase with high magnetic anisotropy, heat treatments are necessary. The heat treatments lead to agglomeration and sintering of the nanoparticles. To prevent the particles from sintering, salts as the separating media ͑matrix͒ have been used for annealing the nanoparticles in our experiments. The fcc nanoparticles produced by chemical synthesis were mixed with NaCl powders. The mixture was then annealed in forming gas ͑93% H 2 +7%Ar͒ in different conditions to complete the fcc to fct phase transition. After the annealing, the salt was washed out by water and monodisperse fct FePt nanoparticles were obtained. Detailed studies on the effect of the NaCl-to-FePt weight ratios ͑from 1:1 to 400:1͒ have been performed. It was found that a suitable NaCl-to-FePt ratio is the key to obtain monodisperse fct FePt nanoparticles. A higher NaCl-to-FePt ratio is needed for larger particles when the annealing conditions are the same. Increased annealing temperature and time should be accompanied by a higher NaCl-to-FePt ratio. Magnetic measurements show very high coercivity ͑up to 30 kOe͒ of the monodispersed fct nanoparticles by the salt-matrix annealing. 1 The chemically synthesized FePt nanoparticles, however, are of face-centered-cubic ͑fcc͒ phase without magnetic anisotropy. To transfer FePt nanoparticles from fcc phase to face-centered-tetragonal ͑fct͒ phase, heat treatments above 600°C are necessary, which undesirably lead to sintering of these nanoparticles.Since 2000, great efforts have been made to produce monodisperse fct FePt nanoparticles 2-8 driven by potential applications of the magnetically anisotropic nanoparticles in high-density recording media and high-performance nanocomposite magnets. Recently, we obtained monodisperse fct FePt nanoparticles with retained size and shape by using salts as the annealing separating media. 9 The salts can be completely removed after the annealing just by washing the samples in water. High coercivity up to 30 kOe of the fct particles has been obtained. In this paper we report detailed results in controlling the particle morphology and properties by adjusting the salt-to-FePt particle ratio. EXPERIMENTThe fcc FePt nanoparticles with size of 4, 8, and 15 nm were synthesized by chemical solution methods.1,10-13 Sodium chloride ͑NaCl͒ was selected as a separating media in this investigation due to its chemical stability and high solubility in water. NaCl was first ball milled for 24 h to reduce the particle size. The ball-milled NaCl powder was then dispersed in hexane and mixed with hexane dispersion of assynthesized fcc FePt nanoparticles. The mixture was stirred until all the solvent evaporates. Then the mixture was annealed in forming gas ͑93% H 2 +7%Ar͒ in different conditions to complete the fcc to fct transition. The annealed powders were washed in de-ionized water and centrifuged for several times to remove all the NaCl.9 Different NaCl-toFePt weight ratios from 1:1 to 400:1 were t...

Controlled synthesis of mesoporous iron oxide nanowire nanohybrids with enhanced visible light-driven photocatalytic activity.

Articles you may be interested inSynthesis and magnetic properties of size-tunable Mn x Fe3− x O4 ferrite nanoclusters Size effects on structural and magnetic properties of CoFe2O4 nanoparticles prepared by co-precipitation method AIP Conf.Single crystalline CoFe 2 O 4 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 CoFe 2 O 4 nanoparticles. V C 2015 AIP Publishing LLC. [http://dx.