Self-assembled FePt Nanoparticles Research Articles

Fabrication and magnetic properties of FePt/Al2O3 composite film by atomic-layer-deposition

Self-assembled face-centered cubic FePt nanoparticles were capped by a amorphous Al2O3 capping layer with the thickness of 10nm using the atomic layer deposition (ALD) technology, and transmission electron microscopy indicates that the FePt nanocrystals are well-separated. After annealing the composite film at 700°C, well-monodispersed face-centered tetragonal FePt particles can be obtained, yielding a coercivity value Hc=5.9kOe with the magnetocrystalline anisotropy of 3.86MJ/m3. The thermal factor for the composite film is 68.5, meeting the industry requirement (KuV/kBT ≥ 50). The protection of amorphous Al2O3 matrix can effectively inhibit grain growth and particle aggregation, and preserve the ordered domains of FePt nanoparticles during the L10 ordering transition. The combination of ALD-capping layer and self-assembled FePt nanoparticles provides a new potential approach to fabricate patterned magnetic recording media with ultrahigh areal density.

Reduction of Ordering Temperature of Self-Assembled FePt Nanoparticles by Addition of Au and Ag

The [FePt]94Au6 and [FePt]90Ag10 nanoparticle arrays were synthesized on Si substrates by a reverse micellar method, combined with plasma treatment and in-situ deposition of a SiO2 overlayer, and the post annealing step was performed to drive the face-centered cubic to tetragonal phase transition. These FePt nanoparticles exhibit a quasi-hexagonal order with tailored inter-particle spacing and particle size. The effects of the Ag and Au on the structural and magnetic properties of FePt were investigated. The results indicate that both Au and Ag additives can remarkably enhance the coercivity and reduce the ordering temperature, however, the optimum composition is different for them. The optimum composition is determined to be [FePt]94Au6 and [FePt]90Ag10, respectively, for which the ordering temperature of FePt nanoparticles is reduced by -100 degrees C. After 600 degrees C annealing, the [FePt]94Au6 and [FePt]90Ag10 nanoparticles are totally ferromagnetic with apparent larger coercivities of -7.0 kOe, which is about 3.8 kOe larger than that of the pure FePt nanoparticles. The mechanism of the chemical ordering acceleration may be attributed to the defects and strains caused by the Au/Ag additives.

Magnetic properties of FePt nanoparticle assemblies embedded in atomic-layer-deposited Al2O3

Self-assembled face-centered cubic FePt nanoparticles on Si substrates were embedded into amorphous Al2O3 capping layers with various thicknesses in the range 5–20 nm using atomic layer deposition (ALD) technology. The effect of the Al2O3 layer thickness on the structure, mono-dispersibility, and magnetic properties of the FePt/Al2O3-matrix composite films was investigated. After annealing at 700 °C in a reducing atmosphere for 1 h, well-dispersed face-centered tetragonal (fct) FePt particles could be obtained for the samples with 10 nm-thick and greater Al2O3 layers. Experimental results suggest that the protection of the amorphous 10 nm-thick Al2O3 matrix can effectively inhibit grain growth and particle aggregation, and preserve the ordered domains of FePt nanoparticles during the L10 ordering transition through annealing. The 5 nm fct FePt-nanoparticles/10 nm-thick Al2O3-matrix sample shows higher coercivity of 5.9 kOe. The combination of ALD-capping layer and self-assembled FePt nanoparticles provides a potential new approach to fabricate patterned magnetic recording media with ultrahigh areal density.

Formation mechanism and composition distribution of FePt nanoparticles

Self-assembled FePt nanoparticle arrays are candidate structures for ultrahigh density magnetic storage media. One of the factors limiting their application to this technology is particle-to-particle compositional variation. This variation will affect the A1 to L10 transformation as well as the magnetic properties of the nanoparticles. In the present study, an analysis is provided for the formation mechanism of these nanoparticles when synthesized by the superhydride reduction method. Additionally, a comparison is provided of the composition distributions of nanoparticles synthesized by the thermal decomposition of Fe(CO)5 and the reduction of FeCl2 by superhydride. The latter process produced a much narrower composition distribution. A thermodynamic analysis of the mechanism is described in terms of free energy perturbation Monte Carlo simulations.

Enhance the structural stability of the FePt nanoparticle monolayer by adding gold overlayer

The stability during the annealing of monolayer structures and the coalescence of self-assembled FePt nanoparticles on PEI functional substrate with/without the Au overlayer were studied. During the annealing, the iron X-ray absorption K-edge was found shifted to the lower energy, which indicates a reduction of Fe atoms on the particle surface as the annealing temperature increases. Using the X-ray diffraction and anomalous grazing incidence small angle X-ray scattering technique, the nanoparticle was found to be intact under the annealing process after a 10 nm of Au overlayer was deposited on the top of FePt monolayer nanoparticles.

Enhanced Magnetic Properties of Self-Assembled FePt Nanoparticles with MnO Shell

Self-assembled FePt/MnO nanoparticles with different morphology and size were synthesized with a polyol process. With the MnO coating, FePt nanoparticles exhibit a high blocking temperature and magnetic moment. The low-temperature hysteresis loop of FePt nanoparticles can be shifted through the AFM pinning of the MnO shell. The aggregation of FePt nanoparticles during the L10 phase transformation can be significantly decreased by coating with the MnO shell.

Agglomeration and sintering in annealed FePt nanoparticle assemblies studied by small angle neutron scattering and x-ray diffraction

In this work we give a detailed account of complementary small angle neutron scattering and x-ray diffraction studies of polymer mediated, self-assembled FePt nanoparticle arrays as a function of annealing temperature. The combination of these two techniques provides significantly greater physical insight than is available using either individually. Since both methods integrate over a large number of particles statistically meaningful data can be obtained in contrast to imaging techniques where typically only small areas are analyzed. The data show that the median particle size increases with annealing at temperatures of $580\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and above. The data also demonstrate that the distribution of particle diameters is significant and increases with annealing temperature. These results allow a comprehensive structural model of the annealed assemblies to be developed in terms of particle sintering and agglomeration. This enhanced understanding will allow new strategies to be pursued in realizing the potential of nanoparticle assemblies as a monodispersed data storage medium.

Structural and magnetic model of self-assembled FePt nanoparticle arrays

Chemically ordered, self-assembled FePt nanoparticle arrays with high magnetic anisotropy are considered as a candidate medium for data storage beyond 1 Tbit/in2. We report comprehensive structural and magnetic studies on thin (three-layer) assemblies of polyethylenimine (PEI) and 4 nm Fe58Pt42 nanoparticles using x-ray diffraction, small angle neutron scattering, and magnetometry. We show that prior to annealing FePt nanoparticles in the PEI-FePt assembly consist of a metallic magnetic core surrounded by a weakly magnetic or nonmagnetic shell. High temperature annealing creates the desired L10 chemical ordering and results in high coercivity FePt nanoparticles. However, we find that the high temperatures necessary to establish full chemical ordering leads to particle sintering and agglomeration. Understanding the magnetic and physical properties of these assemblies allows future research directions to be clarified for nanoparticle arrays as data storage media.

Silicide formation and particle size growth in high-temperature-annealed, self-assembled FePt nanoparticles

L1 0 FePt nanoparticle assemblies consisting of a few layers of 4-nm-diameter particles, are a potential data storage medium beyond 1 Tbit/in2. However, annealing at temperatures >500 °C is required to form the high anisotropy L10 phase. Recent studies have shown a substantial drop in magnetization for Tanneal>650 °C. We show that this reduction in magnetization is due to silicide formation as a result of a chemical reaction with the native oxide or Si substrate. We also show that full L10 ordering is established only after annealing at 725 °C for 60 min and note that particle agglomeration occurs under these conditions.

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.

X-ray diffraction and absorption spectroscopy studies of the structure of self-assembled FePt nanoparticles during annealing

The superlattice structures and coalescence of self-assembled FePt nanoparticles were studied by X-ray diffraction and absorption spectroscopy techniques. The as-synthesized superlattice structure was found to be an FCC-like one with 21±0.5 well-packed layers. During the annealing, the iron absorption K edge was found shifted to a higher energy as the annealing temperature increase. The shift of this iron K edge might be attributed to the quantum size effect, surface effect or oxidation effect.

Disk substrate deposition techniques for monodisperse chemically synthesized FePt nanoparticle media

Self-assembled FePt nanoparticles hold promise for future ultrahigh-density magnetic recording media because of their high anisotropy and capability to be formed into small and uniform grains. By using a special spin coater, we were able to form a dense array of FePt particles across the entire surface of a 2.5 in. disk substrate. Our method can control the number of layers of FePt nanoparticles. The media deposited with FePt nanoparticles by the spin coater was annealed in a vacuum. We measured read–write properties of the FePt nanoparticle media on a spin stand, and succeeded in detecting a signal of 290 kfci.

Patterning self-assembled FePt nanoparticles

We describe a potential way to extend the ordered domain of self-assembled FePt nanoparticles. The FePt particles, with an average diameter of 3 nm, were prepared by simultaneous thermal decomposition of Fe(CO) 5 and chemical reduction of Pt(acac) 2 and then were dispersed in a mixture of hexane and octane. When self-assembling on a plain silicon wafer, FePt nanoparticles formed ordered hexagonal arrays in a range of tens to a few hundred nanometers. A silicon wafer with patterned holes of a photoresist film, made using UV-lithographing technique, was used as a template to direct the stacking direction of the FePt nanoparticles. The FePt dispersion was dropped on the patterned holes of the photoresist film. After being heat-treated at 100°C for 30 min under vacuum condition, the photoresist was stripped out by dipping the sample in acetone. The patterned disks, with an average diameter of 2.0 μm and a height of 250 nm, of self-assembled FePt nanoparticles were examined using SEM and Auger mapping. Their magnetic properties were measured using AGM. The Auger electrons of neither Fe LMM nor Pt MNN could be detected from the sample, which indicated the adsorption of oleic acid and oleylamine on the surface of FePt nanoparticles. The coercivity of patterned FePt significantly increased with the annealing temperature above 600°C.

Thermal stability of self-assembled FePt nanoparticles

We have produced self-assembled FePt nanoparticles by chemical synthesis and subsequent thermal annealing. The self-assembled samples were annealed in nitrogen or forming gas (95%Ar+5%H2) atmosphere for varying annealing time and temperatures. Thermal annealing above 500 °C resulted in phase transformation of the as-synthesized FePt from the chemically disordered fcc to chemically ordered fct structure which has a high uniaxial magneto-crystalline anisotropy. With increasing annealing temperature, the coercivity and the thermal stability factor KV/kT were found to increase due to improved chemical ordering, irrespective of the annealing atmosphere. For samples annealed at 580 °C for 30 min in forming gas, the maximum coercivity obtained was 6.95 kOe. The samples annealed in forming gas showed higher coercivity and KV/kT values than those annealed in nitrogen for the same annealing time and temperature. We have also measured the Curie temperatures of the FePt nanoparticle assemblies. The Curie temperatures were found to increase with increased annealing time for the samples annealed in nitrogen at 580 °C.

Coercivity and remanence in self-assembled FePt nanoparticle arrays

FePt-based nanostructured materials are excellent candidates for high density recording beyond 1 Tbit/in2. We calculate remanence, coercivity, and loop shape of annealed monodisperse FePt nanocrystals, using a modified Stoner–Wohlfarth model. To justify the simplifications of a Stoner–Wohlfarth model detailed finite element micromagnetic simulations were performed. Magnetic measurements on arrays of chemically synthesized FePt nanoparticles show remanence ratios of about 0.6 which is greater than that predicted for a series of noninteracting Stoner–Wohlfarth particles. A small fraction of the particles (5%) is assumed to remain in the disordered fcc phase with low magnetocrystalline anisotropy. Both remanence and coercivity are highly sensitive to the strength of the exchange interactions within a multiple twined nanocrystal. The calculated values are in the range from Jr/Js=0.52, Hc=0.77 MA/m to Jr/Js=0.61, Hc=1.2 MA/m. The results of the modified Stoner–Wohlfarth model are confirmed by finite element micromagnetic simulations taking into account magnetostatic interactions and allowing nonuniform magnetic structures within a particle.

Reduction of the fcc to L10 Ordering Temperature for Self-Assembled FePt Nanoparticles Containing Ag

[Fe49Pt51]88Ag12 nanoparticles were prepared by the simultaneous polyol reduction of platiunum acetylacetonate and silver acetate and the thermal decomposition of iron pentacarbonyl, giving 3.5 nm diameter FePt particles with Ag atoms substituted in the lattice. The addition of Ag promoted the fcc to tetragonal phase transition, thereby reducing the temperature required for this transition by some 100 to 150 °C compared with pure FePt nanoparticles. After heat treatment at 400 °C for 30 min, the coercivity of the films containing [Fe49Pt51]88Ag12 nanoparticles was more than 3400 Oe, while the films containing FePt nanoparticles were superparamagnetic. This decrease in phase transformation temperature allowed us to decrease the particle coalescence and loss in particle positional order seen when FePt nanoparticles were transformed at temperatures above 550 °C.

Interparticle interactions in annealed FePt nanoparticle assemblies

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.

Thermal effects in self-assembled FePt nanoparticles with partial chemical ordering

We have studied the thermal stability of self-assembled FePt nanoparticles with partial chemical ordering. The magnetic viscosity was measured at zero field after partial dc demagnetization as a function of the initial remanent magnetization. The viscosity curves are in good agreement with a calculation using a noninteracting Stoner–Wohlfarth, Arrhenius–Néel model with a broad distribution of anisotropy fields and randomly oriented easy axes. The shape of the zero field viscosity curve differs from curves measured for thin film media with in-plane and perpendicular anisotropy, in which interactions are known to be important.

Self-Assembled Structures of Gas-Phase Prepared FePt Nanoparticles

ABSTRACTWe report on a non-lithographic method for the preparation of self-assembled FePt nanoparticles via inert-gas condensation. Prior to deposition the particles can be sintered in flight at temperatures as high as TS = 1273 K. Whereas un-sintered particles have irregular shapes, particles sintered at elevated temperatures TS ≤ 793 K show a regular faceting. (High resolution) transmission electron microscopy ((HR)TEM) shows that these regularly faceted particles are of icosahedral structure. When being deposited onto amorphous carbon films, the gas-phase sintered particles are found to have a high mobility. In particular, for the high-temperature sintered FePt nanoparticles, we observe that this mobility leads to the formation of particle arrays with hexagonal close-packed arrangements. Within these ordered patches, the particles are separated from one another. Analytical investigations using energy filtered TEM (EFTEM) show that a carbon layer is formed between the particles. Magnetization analyses give results showing that the gas-phase sintered particles are superparamagnetic at room temperature with a blocking temperature of TB = 49K.

Self-Assembled Structures of Gas-Phase Prepared FePt Nanoparticles

AbstractWe report on a non-lithographic method for the preparation of self-assembled FePt nanoparticles via inert-gas condensation. Prior to deposition the particles can be sintered in flight at temperatures as high as TS = 1273 K. Whereas un-sintered particles have irregular shapes, particles sintered at elevated temperatures TS ≥ 793 K show a regular faceting. (High resolution) transmission electron microscopy ((HR)TEM) shows that these regularly faceted particles are of icosahedral structure. When being deposited onto amorphous carbon films, the gas-phase sintered particles are found to have a high mobility. In particular, for the high-temperature sintered FePt nanoparticles, we observe that this mobility leads to the formation of particle arrays with hexagonal close-packed arrangements. Within these ordered patches, the particles are separated from one another. Analytical investigations using energy filtered TEM (EFTEM) show that a carbon layer is formed between the particles. Magnetization analyses give results showing that the gas-phase sintered particles are superparamagnetic at room temperature with a blocking temperature of TB = 49K.