As-synthesized FePt Nanoparticles Research Articles

Synthesis and Characteristics of FePt Nanoparticle Films Under In Situ-Applied Magnetic Field

In situ external magnetic field was applied during the synthesis of FePt nanoparticles via a chemical solution method. FePt nanoparticle films were prepared on Si by a drop-coating method with and without a magnetic field. Annealing at 700 °C in reductive atmosphere was explored to obtain ferromagnetic FePt L10 phase. The effect of in situ-applied magnetic field on the structure, morphology, and magnetic properties of FePt nanoparticle films was characterized. It is found that the applied magnetic field during the chemical synthesis of FePt nanoparticles plays a key role in the crystallinity and magnetic property of FePt nanoparticle films. As-synthesized FePt nanoparticles under the magnetic field are monodispersed and can be self-assembled over a larger area by a dropping method. The applied magnetic field during the synthesis of FePt nanoparticles not only significantly improves the nanoparticles’ c-axis preferred orientation but also benefits the phase transition of FePt nanoparticles from face-centered cubic to face-centered tetragonal structure during the annealing process. The FePt nanoparticle films derived under magnetic field also show some magnetic anisotropy.

Synchrotron Small Angle X-Ray Scattering Spectra of Iron-Based Magnetic Fluids

Magnetic fluid is a special class of materials which possesses the advantages of a liquid state of the carrier and a magnetic state of the particles. In addition to the conventional uses in mechanical engineering, magnetic fluids containing magnetite (Fe3O4) superparamagnetic nanoparticles are under research and development for drug delivery, hyperthermia and MRI contrast agents. On the other hand, iron-platinum (FePt) is investigated as materials for ultrahigh density recording. Before their assembly into patterned media, the as-synthesized FePt nanoparticles in superparamagnetic state are commonly stored in forms of magnetic fluids. In this work, iron-platinum (FePt) nanoparticles with their surface modified by oleic acid and oleyleamine were synthesized from the polyol process. The starting material was an environmental friendly iron(III) acetylacetonate and the products were dispersed in n-hexane. In small-angle X-ray scattering (SAXS) measurements at the Synchrotron Light Research Institute, Thailand, each magnetic fluid was injected into a sample cell with aluminum foil windows and the X-ray of wavelength 1.55 Å from BL2.2 was used. The measured SAXS intensity profiles as a function of the scattering vector from 0.27 to 2.30 nm-1 were fitted and compared between two different reactions. Nanoparticles synthesized by using a higher amount of Fe(acac)3 were matched with monodisperse spheres of radius 2.4±0.3 nm. The other reaction with a reducing agent gave rise to smaller nanoparticles of two size distributions. From this work, the potential of synchrotron radiation to complement conventional characterization techniques in the investigation of nanoparticles for high density recording and biomedical applications is underlined.

Synthesis and characterization of chemically ordered FePt magnetic nano-particles

Monodispersed FePt alloy magnetic nano-particles are prepared by reduction of platinum acetyl acetonate and iron acetyl acetonate salts together in the presence of oleic acid and oleyl amine stabilizers by polyol process. The particle size of FePt is in the range of 2–3 nm confirmed by transmission electron microscopy (TEM). As-synthesized FePt nano-particles are chemically disordered with face centre cubic (fcc) structure where as after vacuum annealing these particles changed to face centre tetragonal (fct) ordered structure confirmed by the X-ray diffraction technique. Magnetic coercivity of 5.247 KOe was observed for fct structure.

Synthesis and Characterization of Magnetic FePt/Au Core/Shell Nanoparticles

We report a two-step chemical solution process for synthesis of FePt/Au core/shell nanoparticles. First, FePt nanoparticles were synthesized using a typical polyol process. The as-synthesized FePt nanoparticles were then used as seed materials and were coated with gold shells by reduction of the gold precursors. The coated nanoparticles have been characterized using various structural and morphological measurements to confirm the FePt/Au core/shell morphology. The as-synthesized FePt/Au nanoparticles were found to be superparamagnetic and became ferromagnetic after annealing at 400 °C. The surface modification of FePt/Au core/shell nanoparticles from hydrophobic to hydrophilic was achieved through ligand exchange to make them water dispersible for potential biomedical applications.

Direct synthesis and characterizations of fct-structured FePt nanoparticles using poly( N-vinyl-2-pyrrolidone) as a protecting agent

FePt alloy in a bulk state is well known as a magnetic material. FePt nanoparticles, which are protected by poly( N-vinyl-2-pyrrolidone) (PVP) and have a face-centered tetragonal (fct) structure at a size of a few nanometers in diameter, have been directly synthesized by a polyol process in high-boiling point tetraethylene glycol used as a reducing reagent for the reduction of Fe(III) acetylacetonate and Pt(II) acetylacetonate. Their magnetic properties (coercivity and saturation magnetization) were dependent on the size and made progress as their diameters increased. The size in diameter was easily controlled by altering the content of PVP, the time for refluxing, and reaction temperature. FePt nanoparticles showed diameter-dependent coercivities at room temperature and they abruptly increased at over 4 nm in diameter. Ferromagnetic FePt nanoparticles with an fct structure were also synthesized at relatively low reaction temperature without refluxing. Likewise, as-synthesized FePt nanoparticles prepared by refluxing at 251 °C for 3 h displayed the fct structure and clearly indicated the ferromagnetism at room temperature. Reaction kinetics such as long refluxing time and slow temperature elevation rate were found to be important key factors to synthesize the ferromagnetic FePt nanoparticles although the reaction temperature was very critical as well.

Phase transfer catalysis: Synthesis of monodispersed FePt nanoparticles and its electrocatalytic activity

Using dibenzo-24-crown-8-ether (DB24C8) as phase transfer catalyst, the monodispersed iron–platinum (FePt) alloy nanoparticles with size of ∼17 nm were synthesized by reduction of H 2PtCl 6·6H 2O and FeCl 2·4H 2O in the solvothermal system. The structure, magnetic property and electrocatalytic activity of FePt nanoparticles were characterized by transmission electron microscopy (TEM), X-ray diffraction system (XRD), vibration sample magnetometer (VSM) and CHI 820 electrochemical analyser (three electrodes system, the reference electrode is saturated calomel electrode (SCE), the counter electrode is platinum electrode and the glassy carbon electrode is used as working electrode (GCE)), respectively. The results show that the as-synthesized FePt nanoparticles have a chemically disordered fcc structure and can be transformed into chemically ordered fct structure after annealing treatment above 400 °C, simultaneously accompanying with the coercivity changed from 5 to 2400 Oe. CVs of 0.5 M H 2SO 4/0.5 M CH 3OH on GCE modified with FePt nanoparticles monolayer illustrate that the as-synthesized FePt nanoparticles have strong electrocatalytic activity toward the oxidation of CH 3OH in aqueous solution.

Preparation of iron–platinum nanoparticles in water/triton/cyclohexane microemulsions

Abstract Iron–platinum (FePt) nanoparticles at two different molar ratios of Fe2+ to Pt4+ of 1:1 and 3:1 have been prepared by coreduction of H2PtCl6 and FeCl2 with hydrazine in water/triton X-100/cyclohexane microemulsions. The as-synthesized FePt nanoparticles were characterized by XRD, EDX and TEM. The FePt nanoparticles showed disordered fcc structure having 4.7 nm crystallite size—larger than the superparamagnetic limit. Higher content of iron resulted in larger FePt nanoparticles. Magnetic properties of the as-synthesized FePt nanoparticles were investigated using AGM. The hysteresis loops revealed that FePt nanoparticles exhibited soft magnetic behavior with high saturation magnetization. However, the FePt nanoparticles with the molar ratio of 3:1 possessed lower values of the coercive force (Hc) and magnetization.

Size‐Dependent Chemical and Magnetic Ordering in L10‐FePt Nanoparticles

FePt nanoparticles have great application potential in advanced magnetic materials such as ultrahigh-density recording media and high-performance permanent magnets. The key for applications is the very high uniaxial magnetocrystalline anisotropy of the L10-FePt phase, which is based on crystalline ordering of the face-centered tetragonal (fct) structure, described by the chemical-ordering parameter S. 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. It has been observed that the degree of ordering decreases with decreasing particle size of the sputtered FePt nanoparticles. Theoretical simulation predicted that the ordering would not take place when the particle size is below a critical value. However, there have not been systematic experimental studies on quantitative size dependence of chemical ordering of FePt nanoparticles due to the lack of monodisperse L10-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 L10-FePt phase, although it has been well accepted that there is a size effect on the ferromagnetism of any low-dimensional magnets. Additionally, the magnetic properties of FePt ferromagnets, as observed in thin-film samples, 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 L10 phase is particle sintering during heat-treatments that convert the fcc phase to the fct phase. This long-pending problem has been solved recently by adopting the salt-matrix annealing technique. 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, Tc, magnetization, Ms, and coercivity, Hc, 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, respectively. The upper and lower rows are images of as-synthesized and salt-matrixannealed nanoparticles, respectively. As shown in Figure 1, the particle size is retained well upon annealing. Both the assynthesized and annealed nanoparticles are monodisperse with a standard deviation of 5–10 % in diameter. TEM observations also revealed that when the particle size is smaller than or equal to 8 nm, the fct nanoparticles are monocrystalline, whereas the 15 nm fct particles are polycrystalline. It is interesting to see that the L10 nanoparticles, tiny ferromagnets at room temperature, are dispersed very well without agglomeration despite the dipolar interaction between the particles, if a solvent with high viscosity is chosen and if the solution is diluted. Extensive TEM and X-ray diffraction (XRD) analyses have proved that the technique of salt-matrix annealing can be applied to heat-treatments of the FePt nanoparticles without leading to particle agglomeration and sintering, if a suitable salt-to-particle ratio and proper annealing conditions are chosen. Figure 2 shows the XRD patterns of the 4 nm, as-synthesized, fcc-structured nanoparticles and the particles annealed in a salt matrix at 873 K for 2 h, 973 K for 2 h, and 973 K for 4 h (from bottom to top), respectively. As shown in the figure, the positions of the (111) peaks shift in the higher-anC O M M U N IC A TI O N

Langmuir-Blodgett self-assembly and electrochemical catalytic property of FePt magnetic nano-monolayer

Dispersed-well FePt nanoparticles with particle size ~5 nm have been prepared by hydrazine hydrate reduction of H2PtCl6·6H2O and FeCl2·4H2O in ethanol–water system. By employing as-synthesized FePt nanoparticles, the monolayer can be formed by LB Technique. The structural, magnetic properties and electrochemical properties of FePt monolayer were respectively studied by XRD, TEM, VSM and CHI 820 electrochemical workstation. The as-synthesized particle has a chemically disordered fcc structure and can be transformed into chemically ordered fct structure after annealing treatment above 400°C. The coercivity of ordered fct FePt phase can be up to 2515Oe. CVs of 0.5 M H2SO4/0.5M CH3OH on GCE modified with FePt nanoparticles monolayer films illustrate that the as-synthesized FePt is a kind of active electrochemical catalyst.

Preparation of magnetic nanoparticles by pulsed plasma chemical vapor synthesis

FePt nanoparticle is expected as a candidate for the magnetic material of the high density recording media. We attempted to synthesize FePt alloy nanoparticles using 13.56 MHz glow discharge plasma with the pulse operation of a square-wave on/off cycle of plasma discharge to control the size of nanoparticles. Vapors of metal organics, Biscyclopentadienyl iron (ferrocene) for Fe and (Methylcyclopentadienyl) trimethyl platinum for Pt, were introduced into the capacitively coupled flow-through plasma chamber, which consisted of shower head RF electrode and grounded mesh electrode. Synthesis experiments were conducted at room temperature under the conditions of pressure 0.27 Pa, source gas concentration 0.005 Pa, gas residence time 0.5 s and plasma powers 60 watts. Pulse width for plasma duration was chosen from 0.5 to 30 s and plasma off period was 4 s to each pulse operation. Visual observations during the particle growth showed plasma emission in the bulk region was increased with the particle growth. These were theoretically explained by using the model for both transient particle charging in the plasma and single particle behavior in the stationary plasma as well as assuming the similarity between the negative charged particle and negative gas containing plasma. Synthesized nanoparticles were directly collected onto TEM grid, which was placed just below the grounded mesh electrode in the plasma reactor downstream. TEM pictures showed two kinds of particles in size, one of which was nanometer size and isolated with crystal structures and the other appeared agglomerate of nanometer size particles. The size of agglomerated particle was controlled in the 10–120 nm range by varying the plasma-on time from 0.5 to 30 s, although the nanometer size particles did not change. The composition of FePt alloy particles could be altered by adjusting the source gas feed ratio. Also magnetization of FePt nanoparticles was measured by use of SQUID (superconducting quantum interference device) magnetometry measurements. As-synthesized FePt nanoparticles did not exhibit loop-shape characteristic, which indicated superpamagnetic property. Annealed nanoparticles with the composition of Fe58Pt42 at 650°C in atmospheric hydrogen showed clear hysterisis loop with the coercivity as large as 10 KOe.

Preparation and properties of ferromagnetic FePt dispersion

FePt nanoparticles were prepared via the coreduction of platinum and iron acetyl acetonate precursors in tetraethylene glycol at various temperatures. The X-ray diffractogram of the as-synthesized FePt nanoparticles at temperatures above 573 K exhibited (0 0 1) and (1 1 0) superlattice reflections. TEM–EDX studies revealed that the average particle diameter was 5 nm with nearly equiatomic composition. The saturation magnetization and coercivity at room temperature (RT) were found to be 50 emu/g and 2.1 kOe, respectively. The high anisotropy field of about 32 kOe and magnetic ordering at RT revealed the presence of the highly anisotropic L1 0 phase. These particles were partially dispersed in an organic solvent using oleyl amine as surfactant. The dispersed FePt nanoparticles will be useful in future technological applications.

FePt and Fe nanocomposite by annealing self-assembled FePt nanoparticles

We present a simple fabrication of FePt and Fe nanocomposite by annealing self-assembled FePt nanoparticles. 5 nm FePt nanoparticles were prepared by simultaneous reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl in the presence of oleic acid and oleylamine as stabilizers. The as-synthesized FePt nanoparticles can self-organize well on a clean SiO/Si substrate and, then, were annealed in a vacuum using a rapid thermal process at different temperatures for 0.5 h. It is found that the annealing temperature plays a key role in determining the final products. The samples annealed at 550 °C and 700 °C consist of a FePt-face-centered-tetragonal (fct) phase with a coercivity of around 1000 and 8800 Oe, respectively, whereas, the samples annealed at 580 °C are composed of two phases: one is the FePt-fct phase and the other is a Fe-body-centered-cubic phase. The hysteresis loop of the sample annealed at 580 °C is also two-phase like with a kink at low field and its coercivity is around 8000 Oe. That indicates that Fe can be segregated from FePt by proper annealing, forming FePt and Fe nanocomposites.

Chemical ordering of FePt nanoparticle self-assemblies by rapid thermal annealing

Self-assembled 4 nm FePt nanoparticle arrays were treated with rapid thermal annealing (RTA) process. The phase transformation from the chemically disordered face-centered-cubic structure of the as-synthesized FePt nanoparticles to the chemically ordered face-centered-tetragonal structure was realized by RTA, with both the annealing temperature and time being greatly reduced, as compared to conventional annealing. The onset of chemical ordering occurred at the annealing condition of around 400°C for only 5 s. Temperature-dependent coercivity measurements revealed strong thermal effects of the nanoparticle assemblies. Curie temperatures ( T c) of the annealed assemblies were derived from the temperature-dependent hysteresis measurements. T c of the annealed assemblies increases with increasing annealing temperature, but is about 200–300 K lower than that of bulk FePt (750 K). The drastic reduction of T c may be attributed to size effect and partial chemical ordering of FePt nanoparticles.