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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SmCo5 magnets are synthesized by the facile Ca reduction of core/shell‐structured Co/Sm2O3 nanoparticles, as schematically illustrated in the figure. The magnets exhibit coercivities reaching 24 kOe at 100 K and 8 kOe at room temperature. The synthesis represents an important first step towards the fabrication of SmCo‐based exchange‐spring nanocomposites for high‐performance permanent magnet applications.

This paper reports an improved synthesis of hard magnetic fct-FePt nanoparticles (NPs) from reductive annealing of fcc-FePt/Fe3O4/MgO NPs followed by MgO removal. The fcc-FePt/Fe3O4 NPs are made by a one-pot reaction of Pt(acac)2 with Fe(CO)5 in the presence of oleic acid and oleylamine and are coated with a layer of MgO via the thermal decomposition of Mg(acac)2. The MgO coating prevents FePt from sintering under high temperature reductive annealing conditions. The fct-FePt NPs obtained from the 650 °C annealing of the fcc-FePt/Fe3O4/MgO NPs show a coercivity value of 2 T at 300 K, suitable for various nanomagnetic applications.

Hard magnetic nanoparticles based on the Sm(2)Co(17) and SmCo(5) systems have been successfully produced using a surfactant-assisted ball milling technique. A size-selection process has been developed to obtain nanoparticles of different sizes with narrow size distribution. Significant room-temperature coercivity up to 3.1 kOe has been achieved with the Sm(2)Co(17)-based nanoparticles of an average size of 23 nm. It has been found that surfactants play multifold roles in the processing.

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Hard magnetic nanocomposites SmCo 5 /Fe x ͑x =0-2.9͒ are synthesized by a simultaneous calcium reduction of Sm-CoO and Fe 3 O 4 nanoparticles. The composites consist of nanostructured SmCo 5 and Fe with their average grain sizes at 29 and 8 nm, respectively. The magnetic properties of the composites can be tuned by controlling Fe composition. SmCo 5 /Fe 1.5 shows an enhanced remanent magnetization at 56 emu/ g ͑45 emu/ g for SmCo 5 ͒. The largest coercivity value of 11.6 kOe is achieved with SmCo 5 /Fe 0.23. The synthesis represents a general process toward SmCo-based exchange-spring nanocomposites for high performance permanent magnet applications.