FePt nanoparticles have great application potential in advanced magnetic materials such as ultrahigh-density recording media and high-performance permanent magnets. [1][2][3] The key for applications is the very high uniaxial magnetocrystalline anisotropy of the L1 0 -FePt phase, which is based on crystalline ordering of the face-centered tetragonal (fct) structure, described by the chemical-ordering parameter S.[4] Higher chemical ordering results in higher magnetocrystalline anisotropy. Unfortunately, as-synthesized FePt nanoparticles take a disordered face-centered cubic (fcc) structure that has low magnetocrystalline anisotropy. Heat-treatment is necessary to convert the fcc structure to the ordered fct structure. Several previous theoretical and experimental investigations have been reported on the size-dependent chemical ordering of FePt nanoparticles. [5][6][7][8][9] It has been observed that the degree of ordering decreases with decreasing particle size of the sputtered FePt nanoparticles. [5,6] Theoretical simulation predicted that the ordering would not take place when the particle size is below a critical value. [8,9] However, there have not been systematic experimental studies on quantitative size dependence of chemical ordering of FePt nanoparticles due to the lack of monodisperse L1 0 -FePt nanoparticles with controllable sizes.There are also few studies reported to date on the quantitative particle size dependence of magnetic properties, including the Curie temperature, coercivity, and magnetization of the L1 0 -FePt phase, although it has been well accepted that there is a size effect on the ferromagnetism of any low-dimensional magnets. [10,11] Additionally, the magnetic properties of FePt ferromagnets, as observed in thin-film samples, [4,12] are affected by the degree of chemical ordering, which is in turn size dependent. It is therefore highly desirable to understand the size and chemical-ordering effects, and their influence on the magnetic properties of the nanoparticles. A major hurdle in obtaining the particle size dependence of structural and magnetic properties of the L1 0 phase is particle sintering during heat-treatments that convert the fcc phase to the fct phase. [13,14] This long-pending problem has been solved recently by adopting the salt-matrix annealing technique. [15,16] With this technique, particle aggregation during the phase transformation has been avoided so that the true size-dependent properties of the fct phase can be measured. In this paper, we report results on quantitative particle size dependence of the chemical-ordering parameter S and selected magnetic properties, including the Curie temperature, T c , magnetization, M s , and coercivity, H c , with the particle size varying from 2 to 15 nm. Figure 1 shows the transmission electron microscopy (TEM) images of the FePt nanoparticles with different sizes before and after annealing in a salt matrix at 973 K for 4 h. The images, from left to right, show nanoparticles with nominal diameters of 2, 4, 6, 8, and 15 nm, res...

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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.

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.

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.

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.

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...

We demonstrate that a SmCo/FeCo based hard/soft nanocomposite material can be fabricated by distributing the soft magnetic ␣-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.