synthesis of L1 0 -FePt NPs has been recently reported, such as adding a third metal element (Au, Ag), which can reduce the ordering temperature to 400°C, but the fraction of the ordered phase is usually small. [9] Compared with the method of solution phase synthesis of fcc-FePt nanoparticles followed by high temperature annealing to get L1 0 -FePt nanoparticles, one step method attracts more attention. He and Zhao have developed a simple one step synthesis of L1 0 -FePt NPs by simultaneous decomposition of Fe(NO 3 ) 3 ·9H 2 O, H 2 PtCl 6 ·6H 2 O and direct catalytic graphitization of a carbon precursor (citric acid) at high temperature in solid phase, [10] but the size of the particles was ultrasmall (mostly 2~4 nm) even sintered at temperature of 850°C for 2 hours, therefore the coercivity was just 4.56kOe. More recently, Hu and co-workers reported a solvent-free chemical synthesis of L1 0 -FePt nanoparticles using the synthesized intermediate [Fe(H 2 O) 6 ]PtCl 6 as metal precursors, which was then mixed with different ratio of NaCl before sintering. [11] For the obtained FePt nanoparticles, which can obtain chemical ordered phase at annealing temperatures as low as 400 o C due to the intermediate precursor itself has a certain chemical order, the coercivity can be as large as 10.9 kOe. In a certain degree, this method has much improvement, but it still needs at least two steps.Here, we report a simple approach for large scale and direct synthesis of L1 0 -FePt NPs with tunable coercivity and controlling size. Unlike the previously reported method, no organic solvents, surfactant, chelating agent/catalyst or intermediate precursors are used, and the composition of Fe and Pt is easy to control, which is precursors (Fe(acac) 3 and Pt(acac) 2 ) in an alumina crucible with a heating rate of 5 °C/min under Ar flow. X-ray diffraction (XRD) characterization was carried out on a Bruker AXS D8-Advanced diffractometer with Cu Kα radiation (λ = 1.5418 Å).High-resolution TEM (HRTEM) and the high angle annular dark-field scanning TEM (HAADF-STEM) images were obtained on FEI Tecnai F20 200 kV TEM. The composition of the particles was semi-quantitatively determined by energy dispersive X-ray spectroscopy (EDS). Selection area electron diffraction (SAD) was used for structure characterization. Magnetic properties were measured using a Magnetic Property Measurement System (SQUID MPMS).The procedure of the synthesis L1 0 -FePt nanoparticles is shown in Figure 1. The design of our experiment is to let Fe and Pt atoms nucleate and grow into FePt nanoparticles on a substrate and then the substrate is removed and nanoparticles are colleted. Here, Fe(acac) 3 and Pt(acac) 2 (acac = acetylacetonate) were chosen as precursors of Fe and Pt. NaCl was selected as the substrate, which has a high melting point of 801 °C. NaCl is a perfect option since it is easy to be ground down to small size and also easy be removed with deionized water after synthesizing. For synthesizing L1 0 -FePt, NaCl particles can also work as insulation media pr...

By using Fe(CO)x-OAm (oleylamine, x < 5) as the Fe precursor to slow down the formation rate of FePt nanoparticles (NPs), a time dependence of the NPs' nucleation and growth process was observed by transmission electron microscopy (TEM). The complexing temperature of OAm and Fe(CO)5 at which Fe(CO)x-OAm was formed has a strong influence on the nucleation rate and growth process of the NPs. TEM analyses indicated that the NPs with isotropic shape were single crystalline throughout the synthesis and were formed by a diffusion-controlled Ostwald-ripening (OR) growth mechanism. The nanorod particles were first formed via joining of arbitrarily oriented single crystals and the two crystals formed a uniform particle afterwards, as described by the oriented-attachment (OA) mechanism. The ratio of OAm to Fe(CO)5 used in the preparation of Fe(CO)x-OAm has a significant influence on the growth process, and subsequently the shape, size and size distribution of the FePt NPs. By adjusting the ratio and its complexing temperature, single-crystal FePt NPs with controllable size and isotropic shape were obtained. The insight into the exploration of the specific roles of the reaction conditions and the formation mechanisms provided important information for controlling the morphology of the nanoparticles.

Nanoparticles covered with surfactants are often used to study particle motion patterns and self-assembly processes in solutions. Surfactants have influence on the interparticle interactions and therefore on the particle motion tracks and final patterns. In this study, CoPt nanoparticles are synthesized in aqueous solution without any surfactant. In situ transmission electron microscopy observation is performed to monitor the self-assemble process. Two types of magnetic nanoparticle superlattice arrays are formed: hexagonal equal distance superlattice arrays when particle size is 3 nm, and tight unequal distance superlattice arrays when particle size is 4.5 nm. It is interesting to observe that two small arrays merge into a large one through rotational and translational movements. A Monte Carlo simulation is carried out which successfully restores the whole process. It is identified that the underlying forces are van der Waals and magnetic dipolar interactions. The latter is responsible for orientation of each particle during the whole process. This investigation leads to a better understanding of the formation mechanism of magnetic nanoparticle superlattice arrays.

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