L10-FePt Nanoparticles Research Articles

Critical switching fluence of L10-FePt nanoparticles with practical size to ultrafast all-optical polarization switching

The magnetic storage industry will be pushed into a new era by emerging magnetic materials and ingenious read/write techniques. Recently, L10-FePt nanoparticles with exceptionally high uniaxial anisotropy, combined with the technique of all-optical magnetic switching, have increasingly drawn the attention of researchers. Focusing on L10-FePt nanoparticles, we report the superparamagnetic size limits and three elaborate stages of magnetic orientation switching, which demonstrate that in terms of the opto-magnetic responses of L10-FePt nanoparticles with the size of 6 nm, the fluence of an ultrashort laser pulse should not be larger than the level of 40 mJ/cm2 in order to achieve the helicity-dependent all-optical switching.

Facile synthesis of fully ordered L10-FePt nanoparticles with controlled Pt-shell thicknesses for electrocatalysis

We report a simple one-step approach for the synthesis of ~4 nm uniform and fully L10-ordered face-centered tetragonal (fct) FePt nanoparticles (NPs) embedded in ~60 nm MCM-41 (fct-FePt NPs@MCM-41). We controlled the Pt-shell thickness of the fct-FePt NPs by treating the fct-FePt NPs@MCM-41 with acetic acid (HOAc) or hydrochloric acid (HCl) under sonication, thereby etching the surface Fe atoms of the NPs. The fct-FePt NPs deposited onto the carbon support (fct-FePt NP/C) were prepared by mixing the fct-FePt NPs@MCM-41 with carbon and subsequently removing the MCM-41 using NaOH. We also developed a facile method to synthesize acid-treated fct-FePt NP/C by using a HF solution for simultaneous surface-Fe etching and MCM-41 removal. We studied the effects of both surface-Fe etching and Pt-shell thickness on the electrocatalytic properties of fct-FePt NPs for the methanol oxidation reaction (MOR). Compared with the non-treated fct-FePt NP/C catalyst, the HOAc-treated and HCl-treated catalysts exhibit up to 34% larger electrochemically active surface areas (ECASAs); in addition, the HCl-treated fct-FePt NP (with ~1.0 nm Pt shell)/C catalyst exhibits the highest specific activity. The HF-treated fct-FePt NP/C exhibits an ECASA almost 2 times larger than those of the other acid-treated fct-FePt NP/C catalysts and shows the highest mass activity (1,435 mA·mgPt –1, 2.3 times higher than that of the commercial Pt/C catalyst) and stability among the catalysts tested. Our findings demonstrate that the surface-Fe etching for the generation of the Pt shell on fct-FePt NPs and the Pt-shell thickness can be factors for optimizing the electrocatalysis of the MOR.

Tailoring Structure and Magnetic Characteristics in L10-FePt Nanoparticles via Ag Doping in Different Valence States

Tailoring Structure and Magnetic Characteristics in <i>L</i>1₀-FePt Nanoparticles via Ag Doping in Different Valence States

A general strategy for synthesizing high-coercivity L10-FePt nanoparticles.

It is extremely desirable but challenging to develop a facile solution phase synthesis to directly prepare well-dispersed L10-FePt nanoparticles (NPs) to meet the requirements of advanced magnets in modern industry and information technology. Here, we report a novel strategy to synthesize hard magnetic L10-FePt NPs via controlled co-reduction of Fe(acac)3 and K2PtCl6 in the presence of oleylamine, in which effective control of the magnetic properties and chemical ordering of L10-FePt NPs was achieved by tuning the mole ratio of the precursors, reaction time and temperature. The highest coercivity of 10.5 kOe can be obtained for the NPs synthesized at 350 °C for 8 h, which is much higher than the coercivities reported by the previous studies on solution-synthesized FePt NPs without annealing or the third elemental additive. The reported one-pot synthesis of L10-FePt NPs may provide an ideal class of building blocks for magnetic energy applications.

Large coercivity FePt nanoparticles prepared via a one-step method without post-annealing

L10 FePt nanoparticles were synthesized by a one-step sol-gel autocombustion method, using nontoxic ferric nitrate, hexachloroplatinic acid, and glycine as starting materials. In contrast to common syntheses, high-temperature post-annealing was not required to form the L10 FePt phase. The entire ignition and combustion process lasted no more than one minute. The L10 FePt phase could form in the presence of the high temperature caused by the exothermic combustion reaction. Adjusting the glycine-to-metal ion molar ratio from 0.5 to 6.0 allowed its effects on the phase transformation and magnetic properties of the products to be investigated. X-ray diffraction indicated that pure phase L10 FePt was obtained at a glycine-to-metal ion molar ratio of 1.5. Transmission electron microscopy indicated that the monodisperse L10 FePt nanoparticles had an average particle size of about 20 nm. The reasons why the as-synthesized L10 FePt nanoparticles were not aggregated and sintered could be attributed to the large amount, a gas is being released and the short duration of heat treatment during this combustion. This finding constituted a significant improvement in the synthesis of L10 FePt nanoparticles. Magnetic measurements showed that the L10 FePt nanoparticles had a coercivity of 15.8 kOe at 300 K, and 23.2 kOe at 5 K. Thus, the L10 FePt nanoparticles had a very large coercivity.

Explorations on size limit of L1-FePt nanoparticles for practical magnetic storage

With the advance of HAMR technology, the storage potential of L10-FePt nanoparticles with practical considerations are of great significance. We present an L10-FePt nanoparticle model based on atomistic spin model with Langevin thermodynamics to simulate the magnetic behaviors of L10-FePt nanoparticles at Curie temperature and room temperature to explore their practical design margins. Given specific composites, the Curie temperatures of L10-FePt nanoparticles decrease with their volume sizes decrease starting from 8 nm, meaning no more laser power increment needed for smaller L10-FePt nanoparticles. However, L10-FePt nanoparticles get unstable more easily while their volume sizes decrease at room temperature within 10 years. Above all, a reasonable size of L10-FePt nanoparticles for stable information retaining should not be less than 8 nm at a certain aspect ratio.

L1 0 -FePt Nanoparticles Cluster as a Magnetic Vortex

Cluster assemblies of 10–30 nm ferromagnetic nanoparticles in two dimensions may show magnetic vortex configurations. The average diameters of vortices with in-plane magnetization are about 2 orders of magnitude greater than the diameters of the anisotropic L10-FePt nanoparticles. In the present work, such configurations which also have core regions have been fabricated by a spin coating method. In the core region, the local magnetization is perpendicular to the vortex surface. This region moves by changing an applied magnetic field. The changes of magnetization in terms of the magnetic field are due to both L10-FePt nanoparticles and vortex microstructures formed by a collective behavior of these interactive ferromagnetic single domains.

Magnetocrystalline properties of Iron-Platinum (L10-FePt) nanoparticles through phase transition

High magneto- crystalline anisotropy (ku=7×106j/m3) of L10-FePt nanoparticles are an excellent candidate for ultra high-density magnetic recording. The 4 nm FePt nanocrystals were prepared by superhydride reduction of FeCl2·4H2O and Pt(acac)2 precursors in the phenyl ether by reduction of the 1, 2-hexadecanediol and LiBEt3H superhydride. The crystal and magnetic structures were studied by XRD and VSM analysis. By TEM and EDS analyses the size distribution and molar concentration of Fe/Pt of the nanoparticles were determined. The results showed that the particles are first superparamagnetic before heat treatment and then a phase transition accrue from disorder fcc to order fct structure after annealing. Finally, the phase transition leads the magnetic anisotropy of hard FePt nanoparticles to increase to 7 kOe.

Size Controlling of L10-FePt Nanoparticles During High Temperature Annealing on the Surface of Carbon Nanotubes

Mono-size FePt nanoparticles with particles size about 2.5 nm have been prepared by polyol method on the surface of carbon nanotubes (CNTs). The CNTs functinalization time and the mass ratio of nanoparticles to CNTs affects on the CNTs surface coating. The as-synthesis nanocomposites have a superparamagnetic behavior with chemically disordered fcc structure at room temperature and they can be transformed into chemically ordered fct structure after thermal annealing above 600 °C. Their magnetic behavior changes from the superparamagnetic to the ferromagnetic with a large coercivity up to 0.83 T for the nanocomposites which annealed at 800 °C. The CNTs surfaces as a substrate prevent the agglomeration of nanoparticles during high temperature annealing and the FePt nanoparticles after annealing at 800 °C have finite size with an average about 10 nm. The structure, composition and magnetic properties of nanocomposite were characterized by X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy and vibrating sample magnetometer.

Thermal dewetting with a chemically heterogeneous nano-template for self-assembled L1(0) FePt nanoparticle arrays.

A design for the fabrication of metallic nanoparticles is presented by thermal dewetting with a chemically heterogeneous nano-template. For the template, we fabricate a nanostructured polystyrene-b-polydimethylsiloxane (PS-b-PDMS) film on a Si|SiO2 substrate, followed by a thermal annealing and reactive ion etching (RIE) process. This gives a template composed of an ordered hexagonal array of SiOC hemispheres emerging in the polystyrene matrix. After the deposition of a FePt film on this template, we utilize the rapid thermal annealing (RTA) process, which provides in-plane stress, to achieve thermal dewetting and structural ordering of FePt simultaneously. Since the template is composed of different composition surfaces with periodically varied morphologies, it offers more tuning knobs to manipulate the nanostructures. We show that both the decrease in the area of the PS matrix and the increase in the strain energy relaxation transfer the dewetted pattern from the randomly distributed nanoparticles into a hexagonal periodic array of L10 FePt nanoparticles. Transmission electron microscopy with the in situ heating stage reveals the evolution of the dewetting process, and confirms that the positions of nanoparticles are aligned with those of the SiOC hemispheres. The nanoparticles formed by this template-dewetting show an average diameter and center-to-center distance of 19.30 ± 2.09 nm and 39.85 ± 4.80 nm, respectively. The hexagonal array of FePt nanoparticles reveals a large coercivity of 1.5 T, much larger than the nanoparticles fabricated by top-down approaches. This approach offers an efficient pathway toward self-assembled nanostructures in a wide range of material systems.

Direct chemical synthesis of well dispersed L10-FePt nanoparticles with tunable size and coercivity

We report a simple, green, one-step solid-phase approach to synthesize well-dispersed single-crystal L10-FePt nanoparticles with coercivity of more than 20 kOe.

Tuning hard and soft magnetic FePt nanocomposites

Nanocomposites formed by hard and soft magnetic phases are very promising for magnetic energy storage and biomedical applications. Highly challenging is the development of simple synthesis methods able to tune the phase composition and a thorough structural, morphologic and magnetic characterization in order to understand and optimize the interactions between hard and soft magnetic phases. Mainly depending on Fe:Pt atomic ratio, multi-phase or single phase FePt nanocomposites have been prepared by thermal treatment of core–shell FePt(Ag)@Fe3O4 nanoparticles at 750 °C for 1 h under flow of a Ar + 5% H2 gas mixture (H2 is necessary to reduce Fe2+ and Fe3+ ions of Fe3O4 to Fe0 atoms and the thermal treatment to form the crystalline soft and hard magnetic FePt phases). Performing Rietveld refinement of the XRD data as well as HR-TEM and electron diffraction analyses, the different phases have been singled out. Besides single phase hard L10 FePt and soft magnetic L12 Fe3Pt nanoparticles, two phase soft α-FePt and γ-FePt and hard and soft magnetic L10 FePt and L12 FePt3 nanocomposites have been formed and the structure and morphology correlated to their magnetic behavior.Moreover, for possible applications, it is important to form stable nanoparticle layers; as-prepared FePt(Ag)@Fe3O4 nanoparticles have been chemically attached on a Si substrate, thermally annealed and the morphology, structure and magnetic properties of the layered nanoparticle sample investigated.

Cluster Monte Carlo methods for the FePt Hamiltonian

Cluster Monte Carlo methods for the classical spin Hamiltonian of FePt with long range exchange interactions are presented. We use a combination of the Swendsen–Wang (or Wolff) and Metropolis algorithms that satisfies the detailed balance condition and ergodicity. The algorithms are tested by calculating the temperature dependence of the magnetization, susceptibility and heat capacity of L10-FePt nanoparticles in a range including the critical region. The cluster models yield numerical results in good agreement within statistical error with the standard single-spin flipping Monte Carlo method. The variation of the spin autocorrelation time with grain size is used to deduce the dynamic exponent of the algorithms. Our cluster models do not provide a more accurate estimate of the magnetic properties at equilibrium.

Growth mechanism and magnetic properties of monodisperse L1(0)-Co(Fe)Pt@C core-shell nanoparticles by one-step solid-phase synthesis.

In this report, we present a novel one-step solid-phase reaction method for the synthesis of L10-CoPt@C core-shell nanoparticles (NPs) using organic metal precursors without surfactants. The obtained CoPt@C NPs have a good face-centered tetragonal single crystal structure and regular shape. The mean size of CoPt is 14 nm with a uniform carbon shell. The evolution of the core-shell structure during the synthesizing process is investigated in detail. Firstly organic metal precursors are decomposed, followed by the formation of grains/clusters in a metal-carbon intermediate state. Then the metal-carbon small grains/clusters agglomerate and recrystallize into single crystal metal alloy NPs covered with a carbon layer. The carbon shell is effective in preventing the coalescence of L10-CoPt NPs during high temperature sintering. The prepared L10-FePt nanoparticles have a high coercivity of up to 12.2 kOe at room temperature. This one-step solid-state synthesizing method could also be employed for the preparation of other types of nanostructures with high crystallinity, monodispersity and chemically ordered phase.

Optical properties of L10 FePt nanoparticles dispersed in a C matrix

Optical properties of L10 FePt nanoparticles dispersed in a C matrix have been studied by using an approach combining spectroscopic ellipsometry and intensity transmission. Dielectric functions of FePt nanoparticles over a wavelength range of 380–1700 nm have been determined based on the Bruggeman effective medium approximation theory. Via analysis using the Drude–Lorentz model, the extracted optical properties provide electronic information of FePt nanoparticles. As compared with the thin film composed of larger grains, decrease of free electron absorption is observed in FePt nanoparticles, and this absorption decrease causes dielectric suppression in the near-IR photon energy range. And it can be due to the increased surface scattering and reduced unscreened plasma frequency in nanoparticles. On the other hand, two interband electronic transitions located at 0.7 and 1.0 eV are derived, which are in good agreement with the reported density of states of L10 FePt. In the present work, they show only a small dependence on FePt particle size. Such a study of optical properties of FePt nanoparticles is important for technological applications and fundamental physics.

Size control synthesis and high coercivity L10-FePt nanoparticles produced by iron (III) acetylacetonate salt

Monodisperse 4.11nm FePt nanoparticles were synthesized via chemical co-reduction of iron (III) acetylacetonate and platinum (II) acetylacetonate by 1,2-hexadecanediol as a reducing agent. Then through the seed mediated growth process smaller sized FePt nanoparticles are used as seeds for the growth of larger sized particles. There is no specific limitation to achieve upper size range by this method. In this work, we have successfully synthesized FePt nanoparticles up to 8.32nm. After annealing these particles become ferromagnetic and their coercivirty increases with increasing particle sizes and reaches a maximum value of 17760Oe for size of 8.32nm.

Room temperature coercivity and interaction effects in L10 FePt nanoparticles

FePt alloy systems in the L10 phase are characterized by high anisotropy values that lead to exceptionally high coercive fields, making them a primary candidate for high density data storage. In this paper, we report the synthesis of L10 FePt nanoparticles by thermal annealing of chemically disordered FePt nanoparticles. The high degree of ordering and high coercivity achieved in these systems is further studied using Monte Carlo simulations by incorporating the effects of anisotropy, interactions, and disorder, with a view to understand the role of these interactions. A comparison of the simulation results with the experiment results also helps us to estimate an anisotropy value of ∼2.25–2.5 MJ m−3.

Effects of Cu addition on the structure and magnetic properties of L10-FePt nanoparticles prepared by sol–gel method

(FePt)100Cu0, (FePt)95Cu5 and (FePt)90Cu10 nanoparticles (NPs) were successfully synthesized by the sol–gel method. The relationship between Cu doping and structure and magnetic properties of L10-FePt NPs was studied. The results indicated that all three samples originated a L10-FePt structure and Cu doping did not destroy the ordered structure of L10-FePt. By increasing the Cu content, c/a ratio of the FePtCu NPs linearly decreased. Pawley refinement showed symmetry of (FePt)95Cu5 NPs was still tetragonal. When the Cu concentration increased from 0 to 10 %, coercivity increased from 7,050 to 11,250 Oe. This result confirms that the prepared alloys can be promising candidates for magnetic storage applications.

Effects of Au content on the structure and magnetic properties of L10-FePt nanoparticles synthesized by the sol–gel method

(FePt)100−xAux (x=0, 5, 10, and 20) nanoparticles were synthesized by the sol–gel method, and effects of Au content on the structural and magnetic properties of samples were investigated. Au doping reduced the phase transition temperature from face-centered cubic (FCC) to face-centered tetragonal (FCT) structure. In addition, additive Au promotes the chemical ordering of L10 FePt NPs and increases the grain size of L10 FePt NPs. When Au content increased from 0 to 10at%, the coercivity (Hc) increased due to the increase in degree of ordering S and grain size of L10 FePt NPs. By increasing the Au content to 20at%, Hc decreased.

Influence of ball milling and annealing conditions on the properties of L1 FePt nanoparticles fabricated by a new green chemical synthesis method

In this work, a new green chemical strategy for the synthesis of L10 FePt alloy nanoparticles is reported. The precursor is a polycrystalline molecular complex (Fe(H2O)6PtCl6), in which Fe and Pt atoms are arranged on alternating planes and milled with NaCl to form nanocrystals. Then the mixture was annealed under reducing atmosphere (5% H2 and 95% Ar) at temperatures varying from 350 °C to 500 °C for 2 h with a heating rate of 5 °C/min. After the reduction, the mixture was washed with water to remove the NaCl and L10 FePt nanoparticles were obtained. The X-Ray Diffraction pattern showed the presence of the characteristic peaks of the fct phase of FePt nanoparticles. Influence of precursor/NaCl ratio and ball milling time on particle size was investigated. Transmission electron microscopy images revealed that smaller precursor/NaCl ratio (10 mg/20 g) and longer milling time (15 h) lead to smaller particle size and narrower size distribution. Milling time does not influence the coercivity much but the decrease of the amount of precursor leads to a decrease of coercivity from 10.8 kOe to 4.8 kOe.