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Thermal treatment of self-assembled FePt nanoparticles reduces the interparticle distances, resulting in dramatic changes in the type and strength of interparticle interactions. Consequently, magnetic properties such as the hysteresis and magnetization reversal mechanisms are strongly affected. It is suggested that controlled annealing of self-assembled nanoparticles may offer a novel approach for producing hard magnetic nanocomposites. Index Terms--Exchange coupling, FePt nanoparticles, magnetization reversal, self-assembly.

Permanent magnets are a class of critical materials for information storage, energy storage, and other magneto‐electronic applications. Compared with conventional bulk magnets, magnetic nanoparticles (MNPs) show unique size‐dependent magnetic properties, which make it possible to control and optimize their magnetic performance for specific applications. The synthesis of MNPs has been intensively explored in recent years. Among different methods developed thus far, chemical synthesis based on solution‐phase reactions has attracted much attention owing to its potential to achieve the desired size, morphology, structure, and magnetic controls. This Minireview focuses on the recent chemical syntheses of strongly ferromagnetic MNPs (Hc>10 kOe) of rare‐earth metals and FePt intermetallic alloys. It further discusses the potential of enhancing the magnetic performance of MNP composites by assembly of hard and soft MNPs into exchange‐coupled nanocomposites. High‐performance nanocomposites are key to fabricating super‐strong permanent magnets for magnetic, electronic, and energy applications. [ABSTRACT FROM AUTHOR]

We report a flame‐reaction method to synthesize high‐performance SmxCoy (x=1, y=5; x=2, y=17) particles on a multigram scale. This flame reaction allows the controlled decomposition of Sm(NO3)3 and Co(NO3)2 to 320 nm SmCo‐O (SmCoO3 + Co3O4) particles. A 5.8 g sample of SmCo3.8‐O particles was coated with CaO and then reduced at 900 °C by Ca to give 4.2 g of 260 nm SmCo5 particles. The SmCo5 particles are strongly ferromagnetic and the aligned particles in epoxy resin exhibit a large room‐temperature coercivity (Hc) of 41.8 kOe and giant (BH)max (maximum magnetic energy product) of 19.6 MGOe, the highest value ever reported for SmCo5 made by chemical methods. This synthesis can be extended to synthesize Sm2Co17 particles, providing a general approach to scaling up the synthesis of high‐performance SmxCoy nanomagnets for permanent magnet applications. [ABSTRACT FROM AUTHOR]

We report a new strategy to prepare a composite catalyst for highly efficient electrochemical CO2 reduction reaction (CO2RR). The composite catalyst is made by anchoring Au nanoparticles on Cu nanowires via 4,4′‐bipyridine (bipy). The Au‐bipy‐Cu composite catalyzes the CO2RR in 0.1 m KHCO3 with a total Faradaic efficiency (FE) reaching 90.6 % at −0.9 V to provide C‐products, among which CH3CHO (25 % FE) dominates the liquid product (HCOO−, CH3CHO, and CH3COO−) distribution (75 %). The enhanced CO2RR catalysis demonstrated by Au‐bipy‐Cu originates from its synergistic Au (CO2 to CO) and Cu (CO to C‐products) catalysis which is further promoted by bipy. The Au‐bipy‐Cu composite represents a new catalyst system for effective CO2RR conversion to C‐products. [ABSTRACT FROM AUTHOR]

We report a new strategy to prepare a composite catalyst for highly efficient electrochemical CO2 reduction reaction (CO2RR). The composite catalyst is made by anchoring Au nanoparticles on Cu nanowires via 4,4′‐bipyridine (bipy). The Au‐bipy‐Cu composite catalyzes the CO2RR in 0.1 m KHCO3 with a total Faradaic efficiency (FE) reaching 90.6 % at −0.9 V to provide C‐products, among which CH3CHO (25 % FE) dominates the liquid product (HCOO−, CH3CHO, and CH3COO−) distribution (75 %). The enhanced CO2RR catalysis demonstrated by Au‐bipy‐Cu originates from its synergistic Au (CO2 to CO) and Cu (CO to C‐products) catalysis which is further promoted by bipy. The Au‐bipy‐Cu composite represents a new catalyst system for effective CO2RR conversion to C‐products. [ABSTRACT FROM AUTHOR]

We report the magnetotransport properties of self-assembled Co@CoO nanoparticle arrays at temperatures below 100 K. Resistance shows thermally activated behavior that can be fitted by the general expression of R∝ exp{(T0/T)ν}. Efros–Shklovskii variable range hopping (ν=1/2) and simple activation (hard gap, ν=1) dominate the high and low temperature region, respectively, with a strongly temperature-dependent transition regime in between. A giant positive magnetoresistance (MR) of >1400% is observed at 10 K, which decreases with increasing temperature. The positive MR and most of its features can be explained by the Zeeman splitting of the localized states that suppresses the spin dependent hopping paths in the presence of on-site Coulomb repulsion. [ABSTRACT FROM AUTHOR]

We present magnetization data for polymer-mediated, self-assembled films of 4 nm FePt nanoparticles with thicknesses between one and four nanoparticle layers. As synthesized and deposited, the assemblies contain weakly magnetic material whose volume is equivalent to an outer 0.5 nm shell of the nanoparticles. During a 580 °C anneal, this material is incorporated into the magnetic domains of the nanoparticles. The fraction of nanoparticles transformed by annealing into ferromagnetic L1[sub 0] FePt, the structure essential for magnetic storage applications, is found to vary with sample thickness; samples thinner than four nanoparticle layers show no significant fraction of ferromagnetic (at 300 K) L1[sub 0] structured nanoparticles under these annealing conditions, while in films comprised of four nanoparticle layers, less than half of the particles are ferromagnetic at 300 K. Possible causes of these observed results are discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have produced assemblies of monodisperse 4 nm FePt nanoparticles using polymer-mediated layer-by-layer deposition at room temperature. The process leads to good control of particle assembly thickness and offers great potential for future fabrication of ultra-high density magnetic storage media. Vibrating sample magnetometry with fields up to 9 T was applied to study the magnetic properties of the particle assemblies as a function of annealing condition while near edge x-ray absorption fine structure (NEXAFS) spectroscopy and x-ray diffraction (XRD) were used to investigate the chemical nature and structural properties within particles. It was found that the coercivity can be as high as 22.7 kOe for samples annealed at 800 °C, the moment density (normalized to the particle volume) for the sample annealed at 650 °C is estimated close to the value for bulk FePt, at 1140 emu/cm[SUP3]. NEXAFS spectroscopy shows that the Fe in the as-deposited assemblies is partly oxidized, and the oxidation is greatly reduced by annealing. XRD studies on the assemblies annealed at high temperature revealed the increased atomic order and the formation of the high-anisotropy L1[SUB0] phase within the particles. However, the high-temperature annealing also resulted in nanoparticle agglomeration. [ABSTRACT FROM AUTHOR]

Monodisperse 4 nm FePt magnetic nanoparticles are synthesized by superhydride reduction of FeCl2 and Pt(acac)(sub 2) at high temperature. Chemical analysis of the assemblies indicate that more iron oxide is present in the thinner assemblies annealed at lower temperature or for shorter time.

We report a general chemical approach to synthesize strongly ferromagnetic rare‐earth metal (REM) based SmCo and SmFeN nanoparticles (NPs) with ultra‐large coercivity. The synthesis started with the preparation of hexagonal CoO+Sm2O3 (denoted as SmCo‐O) multipods via decomposition of Sm(acac)3 and Co(acac)3 in oleylamine. These multipods were further reduced with Ca at 850 °C to form SmCo5 NPs with sizes tunable from 50 to 200 nm. The 200 nm SmCo5 NPs were dispersed in ethanol, and magnetically aligned in polyethylene glycol (PEG) matrix, yielding a PEG‐SmCo5 NP composite with the room temperature coercivity (Hc) of 49.2 kOe, the largest Hc among all ferromagnetic NPs ever reported, and saturated magnetic moment (Ms) of 88.7 emu g−1, the highest value reported for SmCo5 NPs. The method was extended to synthesize other ferromagnetic NPs of Sm2Co17, and, for the first time, of Sm2Fe17N3 NPs with Hc over 15 kOe and Ms reaching 127.9 emu g−1. These REM based NPs are important magnetic building blocks for fabrication of high‐performance permanent magnets, flexible magnets, and printable magnetic inks for energy and sensing applications. A new chemical process is developed to prepare strongly ferromagnetic SmCo5 nanoparticles (NPs) via controlled reduction of SmCo‐O multipods. The 200 nm SmCo5 NPs embedded in polyethylene glycol are aligned magnetically and show the room temperature coercivity of 49.2 kOe. The synthetic method has also been extended to prepare ferromagnetic Sm2Co17 and Sm2Fe17N3 NPs. [ABSTRACT FROM AUTHOR]

Using nanoparticles (NPs) to catalyze multiple chemical reactions in one-pot and to achieve high-yield syntheses of functional molecules/materials is an important direction in NP chemistry, catalysis and applications. In this article, we report a nanocomposite of AgPd NPs anchored on WO2.72 nanorods (NRs) (denoted as AgPd/WO2.72) as a general catalyst for formic acid dehydrogenation and transfer hydrogenation from Ar-NO2 to Ar-NH2 that further reacts with aldehydes to form benzene-fused heterocyclic compounds. The AgPd/WO2.72 catalysis is Ag/Pd dependent and Ag48Pd52 is the most active composition for the multiple chemical reactions. The high activity of AgPd/WO2.72 arises from strong interfacial interaction between AgPd and WO2.72, resulting in AgPd lattice expansion and electron polarization from AgPd to WO2.72. The syntheses proceed in one-pot reactions among formic acid, 2-nitrophenol (or 2-nitroaniline, or 2-nitrothiophenol) and aldehydes in dioxane/water (2/1 v/v) at 80–90 °C, leading to one-pot syntheses of benzoxazoles, benzimidazoles and benzothiazoles that are key ring structures present in functional compounds for pharmaceutical, optical and polymer applications. [ABSTRACT FROM AUTHOR]

Superparamegnetic cobalt nanoparticles (NP's) with different crystalline structures and sizes raging from 4 to 9 nm in diameter is investigated using ferromagnetic resonance (FMR) techniques. The results from both single crystalline and polycrystalline cobalt NP's reveal that a particle's effective anisotropy and thus its magnetic properties, are extremely sensitive to internal structure and overall shape.

Superlattices of self-assembled, monodisperse FePt nanomagnet arrays have been studied using magneto-optical (MOKE) spectroscopy in the photon energy range 0.8-5.3 eV. The nano-magnets are chemically synthesized and subsequently deposited on Si[O.sub.2] substrates for structural, magnetic and optical characterization. Large room temperature coercivities up to 9000 Oe are obtained after annealing to temperatures up to 580 [degrees] C. They are attributed to the transformation from the chemically disordered fcc phase to the chemically ordered [L1.sub.0] fct phase of FePt. The chemical ordering process is accompanied by changes in the electronic structure of the materials, which leads to characteristic MOKE spectral changes. In particular, the occurrence of a strong MOKE peak at 2 eV photon energy is observed. Polar and transverse (out-of-plane and in-plane) Kerr hysteresis studies indicate 3D random distribution of the magnetic easy axes in these superlattices. These nanomagnet assemblies with the control on magnet spacing and spatial order are prospective candidates for future ultrahigh density magnetic recording media with potential areal densities beyond Tbit/[in.sub.2]. Index Terms--Chemical synthesis, magnetic recording, nanomagnets, self-assembly, Tbit/[in.sup.2] areal density.

High temperature solution phase decomposition of Fe[(CO).sub.5] and reduction of Pt[(acac).sub.2] in the presence of stabilizers oleic acid and oleyl amine are employed to produce 4 nm diameter FePt nanoparticles. The Fe and Pt composition of the nanoparticle materials can be tuned by adjusting the molar ratio of Fe[(CO).sub.5] to Pt[(acac).sub.2], and the compositions ranging from [Fe.sub.30][Pt.sub.70] [Fe.sub.80][Pt.sub.20] are obtained. The nanoparticle materials are easily dispersed into alkane solvent, facilitating their self-organization into nanoparticle superlattices. As synthesized FePt nanoparticles possess disordered fcc structure and show superparamagnetic behavior. Thermal annealing induces the change of internal particle structure and thus the magnetic properties of the particles. Composition dependent structure analysis shows that an annealed FePt nanoparticle assembly with a composition around [Fe.sub.55][Pt.sub.45] will lead to the highly ordered fct phase. This [Fe.sub.55][Pt.sub.45] nanoparticle assembly yields high coercivity, and will be a candidate for future ultra-high density magnetic recording media applications. Index Terms--FePt nanoparticle, high Ku FePt, magnetic recording, nanoparticle synthesis, self-assembly, thermal annealing.