L10-FePt Nanoparticles Research Articles

Magnetic and structural properties of L10 FePt/MnFe2O4 nanocomposites

Monodispersed 4.1nm FePt nanoparticles were successfully synthesized by the chemical polyol process with co-reduction of iron acetylacetonate, Fe(acac)3 and platinum acetylacetonate, Pt(acac)2. High temperature (usually higher than 550°C) annealing is necessary to obtain L10 ordered structure. The agglomeration of particles takes place during post-annealing process. In the present work, we prevent sintering of FePt nanoparticles during the annealing process at temperatures of 750 and 850°C by using nanocomposite structure. In this case, MnFe2O4 nanoparticles were used successfully to preserve particles from sintering. The results show a decrease in coercivity by adding MnFe2O4 in nanocomposite. This might be due to the effective exchange coupling happens between FePt and MnFe2O4 parts. Furthermore, the well isolated L10 FePt nanoparticles with coercivity of 4300Oe are prepared and magnetic properties of L10–FePt–MnFe2O4 nanocomposite are studied.

Decoration of carbon nanotube with size-controlled L10-FePt nanoparticles for storage media

In this work, first multi-wall carbon nanotubes (MWCNTs) with outer diameter about 20–30 nm are synthesized by a CVD method; they have been purified and functionalized with a two-step process. The approach consists of thermal oxidation and subsequent chemical oxidation. Then, monosize FePt nanoparticles along carbon nanotubes surface are synthesized by a Polyol process. The synthesized FePt nanoparticles are about 2.5 nm in size and they have superparamagnetic behavior with fcc structure. The CNTs surfaces as a substrate prevent the coalescence of particles during thermal annealing. Annealing at the temperature higher than 600 ∘C for 2 h under a reducing atmosphere (90 % Ar + 10 % H2) leads to phase transition from fcc to fct-L10 structure. So, the magnetic behavior changes from the superparamagnetic to the ferromagnetic. Furthermore, after the phase transition, the FePt nanoparticles have finite size with an average of about 3.5 nm and the coercivity of particles reaches 5.1 kOe.

Composition-Mediated Order-Disorder Transformation in FePt Nanoparticles

Heat-treated alloy FePt nanoparticles transform into L10 FePt and mixed L10/L12 FePt3 intermetallic phases. Enrichment in Pt in some nanoparticles, rather than intrinsic thermodynamic effects, drives phase segregation. FePt nanoparticles of uniform, equimolar composition are expected to transform into phase-pure, highly ordered L10 FePt nanoparticles.

Self-suspended permanent magnetic FePt ferrofluids

We present the synthesis and characterization of a new class of self-suspended ferrofluids that exhibit remanent magnetization at room temperature. Our system relies on the chemisorption of a thiol-terminated ionic liquid with very low melting point on the surface of L10 FePt nanoparticles. In contrast, all types of ferrofluids previously reported employ either volatile solvents as the suspending media or superparamagnetic iron oxide nanoparticles (that lacks permanent magnetization) as the inorganic component. The ferrofluids do not show any sign of flocculation or phase separation, despite the strong interactions between the magnetic nanoparticles due to the strong chemisorption of the ionic liquid as evidenced by Raman spectroscopy and thermal analysis. Composites with high FePt loading (40 and 70wt%) exhibit a pseudo solid-like rheological behavior and high remanent magnetization values (10.1 and 12.8emu/g respectively). At lower FePt loading (12wt%) a liquid like behavior is observed and the remanent and saturation magnetization values are 3.5 and 6.2emu/g, respectively. The magnetic and flow properties of the materials can be easily fine tuned by controlling the type and amount of FePt nanoparticles used.

Microstructure Control of L10-Ordered FePt Granular Film for Heat-Assisted Magnetic Recording (HAMR) Media

L10-ordered FePt nanogranular film is one of the promising candidates for the next-generation high areal density media for heat-assisted magnetic recording (HAMR). To realize a suitable microstructure for the HAMR media, the granular structure composed of uniformly dispersed L10-FePt nanoparticles with various segregants has been optimized using various combinations of segregants and seed layer materials. Although the microstructure of FePt-C came so close to the ideal one for HAMR, it would not be the final answer as the growth of the columnar grain structure is not possible due to too strong phase separation between FePt and C. In this article, we review recent investigations aiming at achieving the ideal microstructure for HAMR media.

L10 FePt Nanoparticles Processing with Applied Magnetic Field

Due to the strong magnetic anisotropy energy, the L10 FePt nanocrystals are considered as one of promising candidates for magnetic recording media with ultrahigh densities. The surfaces of Si and SiO2 wafers have been covered by FePt nanoparticles (NPs) and heated at 600 °C for 1 h to form the L10 phase. Bonding of NPs with the SiO2 surface in the presence of magnetic field during annealing process controls the L10 FePt NPs size and their uniform surface distribution. In the presence of perpendicular magnetic field to the surface, the size of NPs obtains up to 30 nm with coercivity of 5.8 kOe and in parallel magnetic field, the NPs size reaches to 45 nm with coercivity of 2.9 kOe. These results and the easy axis ordering of samples have been determined by energy-dispersive X-ray spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, X-ray diffraction, and vibrating sample magnetometer analyses.

Monolayers of spin-coated L10 FePt nanoparticles

Monolayers of FePt nanoparticles were synthesized on SiO2/Si substrates by spin-coating method. The effects of spin-coating conditions on surface morphology of FePt system was studied with FE-SEM. A high temperature annealing on the FePt monolayer films resulted in phase transition from fcc into fct (L10 phase), while preventing nanoparticles from sintering. Furthermore, L10 FePt nanoparticles with an average size of 15 nm are coated on SiO2/Si surface. Uniform nanoparticle monolayer was obtained by optimizing the experiment parameters such as spin time and controlling hexane evaporation rate.

Effects of annealing temperature on the structure and magnetic properties of the L10-FePt nanoparticles synthesized by the modified sol–gel method

Abstract The ordered face-centered tetragonal (FCT) FePt nanoparticles (NPs) were successfully synthesized by the modified sol–gel method. The relationship between the annealing temperature (400, 500, 600 and 700 °C) and the structure and magnetic properties of the FePt NPs was discussed. And the possible growth mechanism was given in terms of atomic diffusion. Differential thermal analysis (DTA) showed the dehydration of various water molecules and combustion/decomposition of organic groups at the temperature below 600 °C. The X-ray diffraction (XRD) patterns revealed that as the annealing temperature increased to 500 °C, the L10 FePt NPs were formed as was indicated by the appearance of superlattice peaks (001) and (110). Moreover, based on the XRD results, we found that the chemical order parameter S increased with the increase of the annealing temperature. The transmission electron microscope (TEM) images showed that the NPs produced at 700 °C had a mean diameter of 14 nm. The selected area electron diffraction (SAED) patterns of the samples annealed at 700 °C further demonstrated the synthesis of the L10 FePt NPs. The energy dispersive X-ray spectroscopy (EDS) analysis showed that no impurity phases existed in FePt NPs. The maximum coercivity (10,148 Oe) was obtained for the sample annealed at 700 °C.

Reversible Phase Transfer of Ferromagnetic L10-FePt Nanoparticles

Abstract A convenient and efficient method of reversible phase transfer of ferromagnetic L10-FePt nanoparticles between aqueous and organic phases was developed. From the SiO2-coated L10-FePt nanoparticles prepared by the “SiO2-nanoreactor” method, stable aqueous dispersions of L10-FePt nanoparticles were prepared first by using tetramethylammonium hydroxide (TMAOH). Replacement of TMAOH by mixtures of oleic acid and oleylamine enables transfer of the L10-FePt nanoparticles from aqueous to organic phase. The L10-FePt nanoparticles could be subsequently transferred to aqueous phase by replacing the hydrophobic ligands with TMAOH. Magnetic properties were unchanged during the phase transfer, suggesting that these L10-FePt nanoparticles will be of great importance not only for applications in biomedical fields but also for fabrication of high-performance magnetic devices via bottom-up processes.

Effects of SiO2 content on the structure and magnetic properties of L10-FePt nanoparticles synthesized by the sol–gel method

SiO2-coated L10-FePt nanoparticles were successfully synthesized by the sol–gel method, and the effects of SiO2 content on the structure and magnetic properties of the samples were investigated. Thanks to a silica coating, agglomeration is restrained even if the annealing temperature increases up to 700°C, and the silica shells decrease magnetic dipole coupling between FePt nanoparticles. The results of energy dispersive spectroscopy (EDS) analysis further testify that the L10-FePt NPs are successfully coated with SiO2. The SiO2-coated L10-FePt nanoparticles show a high coercivity of 8096Oe at room temperature in spite of their core size of only 7nm in diameter.

Structural and magnetic properties of L10/A1, FePt nanocomposites

In this work structural and magnetic properties of (L10-FePt/A1-FePt) exchange coupled nanocomposites are presented. Semi spherical “dome-like” nanocomposites with L10 FePt isolated nanoparticles and A1 FePt (fcc) cap layers were obtained by depositing A1-FePt on type L10 FePt nanoparticles in order to understand the influence of the soft magnetic layer thickness on the magnetic properties of the system. Epitaxial growth is confirmed by X-ray diffraction and TEM, while the coercivity decreases dramatically for the L10/A1-FePt system when the thickness of the A1-FePt cap layers is increased. This result can be used to realize ultrahigh magnetic recording media with tunable coercivity, suitable for conventional write heads.

A novel method for synthesis of size-controlled L10 FePt nanoparticles

Monodisperse FePt nanoparticles with an average particle size of 2.7nm are synthesized by a polyol process and an etched silicon structure is also formed by an electrochemical process in HF acid. Then, the etched silicon surface as a substrate is coated with FePt nanoparticles. Chemical bonding between nanoparticles and the etched silicon surface prevents the coalescence of the particles during thermal annealing. Annealing at 600°C for 1h under a reducing atmosphere (90Ar%+10H2%) leads to realize phase transformation from fcc to fct L10 structure and the magnetic behavior changes from the superparamagnetic into the ferromagnetic one. After annealing, the coercivity reaches a maximum value of 3.3kOe for particles coated on the etched wafer. Furthermore, the monosize nanoparticles with an average size of 15nm are distributed uniformly on the etched silicon surface. So, after phase transition to L10, the FePt nanoparticle size limits to the average value (15nm).

Nanostructure and magnetic properties of c-axis oriented L10-FePt nanoparticles and nanocrystalline films on polycrystalline TiN underlayers

We performed a systematic study of the nanostructure and magnetic properties of FePt on templates of either (200)-oriented polycrystalline TiN underlayers with in-plane grain sizes from 5.8 to 10 nm (poly-TiN) or highly (200)-textured TiN underlayers epitaxially grown on single-crystalline MgO (100) substrates (epi-TiN). For small nominal FePt thicknesses (0.7–8.0 nm), FePt forms particulate films with the magnetic easy axis perpendicular to the film plane on every template TiN underlayer. In addition, the coercivity of nominally 1.4-nm-thick FePt at 300 K in the out-of-plane direction increases from 5.3 kOe for 5.8-nm-sized poly-TiN to 12.9 kOe for 10-nm-sized poly-TiN and reaches 16.3 kOe for epi-TiN, which shows that the coercivity strongly depends on the degree of the c-axis orientation. For larger FePt nominal thicknesses (16–64 nm), FePt particles percolate and form continuous films, and the direction of the easy magnetic easy axis becomes random. The coercivity of nominally 64-nm-thick FePt at 300 K in the out-of-plane direction is still as large as 8.8 kOe for 10-nm-sized poly-TiN, but it drastically decreases to 0.5 kOe for epi-TiN. The absence of in-plane texture in the FePt layer on the poly-TiN suppresses the decrease in coercivity, which prevents domain-wall displacement.

(Invited) Development on Self-Assembly Technique for Arrangement of Chemically Synthesized FePt Nanoparticles

This paper reviews our study of bit patterned media (BPM) using L10 (face centered tetragonal)-FePt nanoparticles. For the practical use of L10-FePt nanoparticles to BPM, some breakthroughs are required: a highly ordered two-dimensional array of nanoparticles on a substrate, a controlled orientation of the axis of easy magnetization in vertical direction on a substrate. Previously, we reported about the achievement for uniformity FePt nanoparticles by controlling chemical synthesis conditions. Moreover, we confirmed the selective chemical interaction Pt in the surface of FePt nanoparticles and SH group with (3-mercaptopropyl)tri-methoxysilane (MPTMS). This study presented a new hybrid technique combining the selective chemical bonding and a substrate with a geometrical grid in order to control the array of FePt nanoparticles with uniform direction of their magnetization. We succeed in preparing uniform array of nanoparticles on geometrical grid with size of 200 nm square prepared using ultraviolet nanoimprint lithography (UV-NIL).

Effect of thermal fluctuations on magnetization reversal of L10 FePt nanoparticles

The temperature-dependent switching field, magnetic viscosity and activation volume of L10 FePt nanoparticles with size ranging from 3 to 15 nm have been studied systematically. It is found that the anisotropy constant increases with particle size which is attributed to size-dependent chemical ordering. The temperature dependence of magnetic viscosity and activation volume can be well explained for the 3–8 nm particles by the Stoner–Wohlfarth model incorporating thermal activation, but the model is not suitable for the 15 nm particles. A quantitative analysis of the relation between the switching field and activation volume suggests that the magnetization in the single-crystal 3–8 nm particles reverses via coherent rotation. However, the magnetization reversal in polycrystalline 15 nm particles cannot be well described by coherent rotation although the particle size is much smaller than the single-domain size derived from bulk L10 FePt magnetic material, which may be due to the multiple crystals within each nanoparticle.

Direct synthesis of L10 FePt nanoparticles within carbon nanotubes by wet chemical procedure

This paper reports on the low temperature synthesis of L10 iron–platinum (FePt) particles within multiwall carbon nanotubes using a novel wet chemical method that allows the filling of the nanotube cavity keeping clean its external wall. In the proposed procedure, nanotubes are filled with a precursor salt of hexaaquairon(II) hexachloroplatinate, ([Fe(H2O)6][PtCl6]) and nanoparticles of the magnetically hard phase are directly obtained by heating at 400 °C in a reductive atmosphere. The advantage of such a precursor, allowing one to obtain at low temperature the L10 phase without passing through the soft fcc phase, is due to its structure, where the Fe and Pt atoms are arranged in alternating planes, as in the fct FePt structure. Morphological, structural and magnetic properties of the filled nanotubes have been investigated by transmission electron microscopy, x-ray diffraction and magnetization measurements. The results show the coexistence of nanoparticles in the superparamagnetic and blocked state, depending on the temperature, due to the particle size distribution.

Magnetic properties of perpendicularly orientated L10 FePt nanoparticles

L10 FePt films were deposited on MgO (001) substrates heated to 700°C by magnetron sputtering. Assisted by the misfit of lattice between film and substrate, strong (001) texture was formed. The film at nominal thickness tN = 5 nm was composed of nanoparticles with a size of ∼70 nm, and showed a high coercivity of ∼105 kOe at 4.2 K. At tN =∼50 nm, as the film changed from discontinuous to continuous, the coercivity dropped about one order of magnitude. Micromagnetic simulation implies that the magnetization reversal is a vortex-like nuclear type. The ideal coercivity of a separated single-domain L10 FePt nanoparticle with a size of 70 nm× 70 nm× 5 nm is ∼121 kOe. This tells us that the experimental coercivity has nearly reached the limit of ideal single crystalline nanoparticles.

Phase Transformation and Magnetic Hardening in Isolated FePt Nanoparticles

Isolated monodisperse L10 FePt nanoparticles coated by carbon were obtained by adding enough surfactants that decomposed into carbon after the chemical synthesis and postannealing of the A1 FePt nanoparticles. The effect of isolation between FePt nanoparticles on the phase transition temperature and magnetic properties has been studied systematically by thermal, magnetic, and structural characterizations and analyses. It was found that the A1 to L10 phase transition temperature is dependent sensitively on the amount of isolation medium. The transition temperature shift reaches 150-200degC from nonisolated particle assemblies to completely isolated particles, which may be attributed to the high activation energy of the phase transformation for the isolated particles.

Surface Modification of Chemically Synthesized FePt Nanoparticles

L10-FePt nanoparticle having high magnetic anisotropy is a promising candidate for the future ultra high magnetic recording material. We have succeeded in the fabrication of cubical FePt nanoparticles. It is necessary for high areal recording density to develop a homogeneous assembled technique of the cubical nanoparticle with its magnetic easy axis in the vertical direction on a substrate. It has been known the selective chemical reaction between metal species and functional group of organic molecules. We focused on the selective chemical reaction to control the direction of the magnetic easy axis in the cubical nanoparticle. In this study, the modification of the organic molecule with various functional groups to the surface of the cubical nanoparticle was examined. Consequently, the amino groups were replaced by thiol groups and carboxyl groups on the surface ligands of the nanoparticle were residual, and then, we confirmed bonding thiol group end to Pt atom selectively.

General Strategy for Direct Synthesis of L10 Nanoparticle Alloys from Layered Precursor: The Case of FePt

A general strategy for the synthesis of high chemically ordered L10 alloy nanoparticles is presented. L10 FePt nanoparticles are obtained directly from the atomically layered precursor FePtCl6·6H2O. The method is easy and low cost, and the FePt nanoparticles, obtained at 400 °C, present a very high magnetic anisotropy.