Magnetic Properties Of FePt Nanoparticles Research Articles

Evolutions of microstructure and magnetic property of wet-chemical synthesized FePt nanoparticles assisted by high magnetic field

A high magnetic field up to 6 T was introduced into the wet-chemical process to synthesize FePt nanoparticles. The effects of high magnetic field on the composition, crystal structure, shape, size and magnetic property of FePt nanoparticles were studied. Under the high magnetic fields, the grain size of FePt nanoparticles refined, and the shape transformed from cube to truncated-cube and concave-cube to cube. However, the high magnetic fields show no significance effects on the composition and crystal structure of FePt nanoparticles. As a result, the magnetic properties of FePt nanoparticles were tuned by high magnetic fields. The coercivity at low-temperature (10 K) and the blocking temperature both decreased at 6 T. The results highlight the feasibility of applying an in-situ high magnetic fields to tune magnetic properties of the wet-chemical synthesized nanoparticles.

High magnetic coercivity of FePt–Ag/MgO granular nanolayers

L10-FePt ferromagnetic nanoparticles have a hight coercivity of Tesla order. Thus, these nanoparticles, with size of 10 to 15 nm and uniform surface distribution, are suitable in magnetic data storage technology with density of more than 1GB. In order to improve structural and magnetic properties of FePt nanoparticles, some elements and combinations have been added to compound. In this research, we show that due to the presence of the Ag, the phase transition temperature of FePt from fcc to L10-fct phase decreases. The presence of Ag as an additive in FePt–Ag nanocomposite, increases the magnetic coercivity. This nanocomposite, with 10% Ag, was deposited by magnetron sputtering on the MgO heat layer. VSM results of 10 nm nanoparticles show that coercivity has increased up to 1.4 T. XRD and FESEM results confirm that the size of the L10-FePt nanoparticles are 10 nm and their surface distribution are uniform. Ag gradually form nano scale clusters with separate lattice and FePt–Ag nanocomposite appears. The result of this process is emptiness of Ag position in FePt-fcc lattice. So, the mobility of Fe and Pt atoms in this lattice increases and it can be possible for them to move in lower temperature. This mechanism explain the effect of Ag on decreasing the transition temperature to fct-L10 phase, and hight coercivity of FePt nanoparticles.

High pressure synthesis of FePt nanoparticles with controlled morphology and Fe content

Magnetic nanoparticles (MNPs) are intensively researched due to their high potential in biomedicine, catalysis, and high density information storage. FePt NPs could be an alternative for commonly used magnetite NPs and the synthesis of FePt NPs is an active area of research. The challenge is to increase the Fe content and saturation magnetisation of FePt NPs so that they can be used in many practical applications. Fine tuning of synthetic methods is required in order to achieve the enhanced magnetic properties of FePt nanoparticles and novel methods are being sought. Herein, use of an autoclave is shown to increase the Fe content, crystallinity and the subsequent magnetic properties of FePt pseudo cube nanoparticles compared to those synthesised under atmospheric pressure. Decreasing the amount of oleic acid is also shown to increase the iron content and can lead to elongated FePt nanoparticles under normal pressure. Further application of nanoparticles synthesised in organic media often requires functionalisation or exchange of stabiliser chemicals. Greater demand for control over such functionalisation requires more information about nanoparticle–stabiliser chemical interactions. Infra-red studies indicate mono and bidentate coordination with oleic acid, however shifts of spectra show that the strength of the bidentate interactions weaken with increasing oleic acid amount.

Influence of surface segregation on magnetic properties of FePt nanoparticles

Surface segregation leads to chemical disordering in magnetic alloy nanostructures and thus could have profound impact upon the magnetic properties of these nanostructures. In this study, we used the first-principles density functional theory calculation method to determine how Pt surface segregation (exchanging interior Pt with surface Fe atoms) would affect the magnetic properties of L10 ordered FePt nanoparticles. For both cuboid and cuboctahedral FePt nanoparticles, we predicted that the Pt surface segregation process could cause a decrease in total magnetic moments, a change in (easy and/or hard) magnetization axes, and a reduction in magnetic anisotropy.

One-Step Synthesis of High-Coercivity L10-FePtAg Nanoparticles: Effects of Ag on the Morphology and Chemical Ordering of FePt Nanoparticles

High coercivity L10-ordered (FePt)1–xAgx nanoparticles (NPs) were directly synthesized using a one-step solution method without post-thermal annealing. The effect of silver additive on the morphology, chemical ordering, and magnetic properties of FePt NPs was studied. Magnetic measurements showed that the FePt NPs without Ag additive were soft magnetic with a coercivity of 300 Oe, whereas a record coercivity of 7600 Oe was obtained when the molar ratio of Ag reached 29% in FePtAg. High-resolution transmission electron microscopy and X-ray diffraction analyses proved that Ag atoms in the L10-phase FePtAg NPs were segregated on the surface of the particles, which revealed that the diffusion of Ag atoms out of the FePt grains is the major driving force behind the L10 structure formation.

Magnetic Properties of FePt Nanoparticles Prepared by Sonoelectrodeposition

Sonoelectrodeposition is a useful technique to make metallic nanoparticles, using ultrasound during electrodeposition to remove nanoparticles as they grow on the cathode surface. This paper reports some structural and magnetic properties of FePt nanoparticles prepared by this method. The as-prepared Fe45Pt55nanoparticles were ferromagnetic at room temperature. Upon annealing at 700°C for 1 h under H2atmosphere, the saturation magnetization and the coercivity of the nanoparticles were improved significantly. The annealed nanoparticles showed a high coercivity of 13.5 kOe at 2 K and of 9 kOe at room temperature. Sonoelectrodeposition is a promising technique to make large quantity of FePt nanoparticles.

Monolayer-directed Assembly and Magnetic Properties of FePt Nanoparticles on Patterned Aluminum Oxide

FePt nanoparticles (NPs) were assembled on aluminum oxide substrates, and their ferromagnetic properties were studied before and after thermal annealing. For the first time, phosph(on)ates were used as an adsorbate to form self-assembled monolayers (SAMs) on alumina to direct the assembly of NPs onto the surface. The Al2O3 substrates were functionalized with aminobutylphosphonic acid (ABP) or phosphonoundecanoic acid (PNDA) SAMs or with poly(ethyleneimine) (PEI) as a reference. FePt NPs assembled on all of these monolayers, but much less on unmodified Al2O3, which shows that ligand exchange at the NPs is the most likely mechanism of attachment. Proper modification of the Al2O3 surface and controlling the immersion time of the modified Al2O3 substrates into the FePt NP solution resulted in FePt NPs assembly with controlled NP density. Alumina substrates were patterned by microcontact printing using aminobutylphosphonic acid as the ink, allowing local NP assembly. Thermal annealing under reducing conditions (96%N2/4%H2) led to a phase change of the FePt NPs from the disordered FCC phase to the ordered FCT phase. This resulted in ferromagnetic behavior at room temperature. Such a process can potentially be applied in the fabrication of spintronic devices.

Observation of two kinds of ESR spectra in FePt nanoparticles

The size effects on magnetic properties of FePt nanoparticles were investigated through ESR experiment in X-band at room temperature. For particle size (D) of ≥ 2.6 nm, the absorption spectra were the Gaussian type, whereas for D = 2.0 nm, it was reproduced with the summation of the Gaussian and Lorentzian types. It has been well known that a face-centered-tetragonal (fct) structure becomes unstable for D of below 2-4 nm. Two kinds of magnetic components for FePt nanoparticles with D = 2.0 nm, observed in the ESR spectrum at room temperature, are considered to be attributed to the coexistence of the particles with fct structure and those with face-centered cubic structures.

Magnetic Properties of FePt Nanoparticles Prepared by a Micellar Method

FePt nanoparticles with average size of 9 nm were synthesized using a diblock polymer micellar method combined with plasma treatment. To prevent from oxidation under ambient conditions, immediately after plasma treatment, the FePt nanoparticle arrays were in situ transferred into the film-growth chamber where they were covered by an SiO2 overlayer. A nearly complete transformation of L10 FePt was achieved for samples annealed at temperatures above 700 °C. The well control on the FePt stoichiometry and avoidance from surface oxidation largely enhanced the coercivity, and a value as high as 10 kOe was obtained in this study. An evaluation of magnetic interactions was made using the so-called isothermal remanence (IRM) and dc-demagnetization (DCD) remanence curves and Kelly–Henkel plots (ΔM measurement). The ΔM measurement reveals that the resultant FePt nanoparticles exhibit a rather weak interparticle dipolar coupling, and the absence of interparticle exchange interaction suggests no significant particle agglomeration occurred during the post-annealing. Additionally, a slight parallel magnetic anisotropy was also observed. The results indicate the micellar method has a high potential in preparing FePt nanoparticle arrays used for ultrahigh density recording media.

Dependence of the Properties of Magnetic Nanoparticles on the Interparticle Separation

Dependence of the magnetic properties of FePt nanoparticles with an average diameter, D, of 5 nm on the interparticle separation, d, has been investigated by employing different spacer molecules. The observed decrease in the blocking temperature and Curie−Weiss temperature with increase in d as well as the increase in saturation magnetization with increase in d have been explained on the basis of a simple model for the case d ≪ D where the magnetic nanoparticles are treated as finite dipoles coupled through anisotropic multipolar interactions.

Composition‐dependent magnetic properties of FePt nanoparticles synthesized by the SiO2‐nanoreactor method

AbstractWe investigated effects of compostion on magnetic properties of FePt nanoparticles synthesized by the “SiO2‐nanoreactor” method in the composition range from Fe41Pt59 to Fe60Pt40. The magnetization studies implied that, in all samples, FePt nanoparticles can form the L10 structure although only a minor amount of unconverted precursor fcc‐FePt nanoparticles which behave superparamagnetic at room temperature still remained. It was revealed that composition of Fe54Pt46 has the smallest amount of the unconverted fcc‐FePt nanoparticles. Together with the results of composition‐dependence of coercivity, it can be concluded that slightly iron‐rich compositions is optimum to produce high quality L10‐FePt nanoparticles. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesis and Magnetic Properties of FePt Nanoparticles with Hard Nonmagnetic Shells

Chemically synthesized FePt nanoparticles were coated with nonmagnetic SiO2 and MnO shells by sol-gel and polyol processes. TEM images show that the FePt/SiO2 nanoparticles exhibit a thick spherical shell. The size and morphology of the MnO shell can be controlled by changing the reaction temperature, the molar ratio of surfactants/Mn(acac)2, and/or the concentration of precursor. The morphology of the MnO shell can be either spherical-like or cubic-like, depending on whether the molar ratio of surfactants/Mn(acac)2 is less than or larger than 2. From XRD measurements, the spherical core/shell nanoparticles exhibit 3D random crystallographic orientation, while the cubic core/shell nanoparticles prefer (200) texture. The magnetic moment of FePt particles can be enhanced by coating with SiO2 and MnO shells. Furthermore, the agglomeration of FePt particles upon the thermal annealing can be significantly inhibited with SiO2 and MnO shells.

Improvement of magnetic properties of FePt nanoparticles by adding Mn

FePtMn nanoparticles were synthesized by the simultaneous reduction of iron acetylacetonate, platinum acetylacetonate, and manganese acetylacetonate with 1,2-hexadecanediol as the reducing reagent in the polyol process. The coercivity of annealed nanoparticles strongly depends on the amount of Mn addition. A maximum coercivity of 19kOe at room temperature was obtained for (Fe32Pt68)87Mn13 nanoparticles annealed at 700°C for 1h. The presence of Mn in the FePt L10 lattice may induce local strain so the ordering is accelerated. In addition, the high coercivity can presumably be partially related to the local bonding of Pt–Mn (or Fe–Mn) in the L10 structure.

Direct synthesis of Pt based L1 structured nanoparticles (invited)

Equiatomic FePt and CoPt nanoparticles with the ordered L10 structure are attractive as ultrahigh density magnetic recording media. In a recent work, chemically synthesized fcc-FePt nanoparticles with narrow size distribution and their self-assembled array with close-packed microstructure has been achieved successfully. However, the particles coalesced during the subsequent annealing step necessary to obtain L10 FePt nanoparticles. In the present study, we have successfully demonstrated the direct synthesis of L10 FePt nanoparticles at low temperature of 553K using the “modified polyol method” without subsequent annealing, whose diameter is 5–10nm and intrinsic magnetocrystalline anisotropy field (Hk) is 31kOe. This indicates that precisely controlling the reaction kinetics, especially low reduction rate through optimizing the polyol/Pt mole ratio and type of polyol are very important for directly synthesizing the L10 FePt nanoparticles. Furthermore, we investigated the size, morphology and composition dependence of the magnetic properties of FePt nanoparticles in order to clarify the L10 ordering mechanism. As a result, clear evidence of the existence of the critical diameter for the thermodynamical L10 ordering is not observed in the size range above 2nm. Furthermore, the recrystallization and sintering process can be a driving force for promoting the L10 ordering, and hence, Fe and Pt atom diffusion at the grain boundary plays an important role to the L10 ordering of FePt nanoparticles.

Effects of surface step and substrate temperature on nanostructure of L1–FePt nanoparticles

The variation of the particle size, particle density, and its relation to the hard magnetic properties of FePt nanoparticles with respect to substrate temperatures and substrate surface morphologies has been investigated. On geometrically flat surfaces, densely dispersed FePt nanoparticles with a particle density of 1012 cm−2 were obtained at a substrate temperature below 573 K, while substrate temperatures between 623 and 823 K are necessary for obtaining well-oriented and well-isolated L10–FePt nanoparticles with large coercivity. Isolated particles fabricated above 573 K did not coalesce largely upon annealing at 873 K, which can be attributed to the “anchoring effect” of Pt “seed” particles. The coercivity of FePt nanoparticles was measured at 300 K damped with decreasing particle sizes below 10 nm. High-density areal packing of Fe/Pt nanoparticles could be fabricated at 673 K on a slightly inclined NaCl(001) substrate with surface steps. These particles coalesced easily upon annealing along the 〈100〉 step edges.