Synthesis and characteristics of FePt nanoparticle films under in-situ applied magnetic field
FePt nanoparticle with L10-phase has extremely high magnetocrystalline anisotropy, good chemical stability, and resistance to oxidation, and has been considered as the most promising candidate for untra-high-density magnetic recording media.
In this work, in-situ magnetic field was applied during the synthesis of FePt nanoparticles via a chemical solution method. FePt nanoparticle films were prepared by a dropping method. The effect of in-situ applied magnetic field on the structure, morphology and magnetic properties of FePt nanoparticle films was characterized. Under magnetic field as-synthesized FePt nanoparticles are monodispersed and can be self-assembled over larger area by a dropping method. The chemically ordered L10-phase FePt can be obtained after annealing at 700 °C for 60 min in forming gas (7% H2+93% Ar). It is revealed that applied magnetic field during the synthesis of FePt nanoparticles not only significantly improves the nanoparticles’ c-axis preferred orientation with the larger perpendicular c-axis preferred orientation degree D(001) of 3.47, but also benefits the phase transition of FePt nanoparticles from fcc to fct structure during the annealing process. The FePt nanoparticle films synthesized under magnetic field also shows some magnetic anisotropy.
Keywords: L10-phase FePt; Chemical solution synthesis; Applied magnetic field; C-axis oriented; Magnetic anisotropy
With the rapid development of magnetic recording technique, the superparamagnetic effect becomes the bottleneck to further increase magnetic storage density. The ferromagnetic L10 FePt assemblies with face-centered tetragonal (fct) structure has extremely high magnetocrystalline anisotropy, good chemical stability, and resistance to oxidation [1-3], considered as the most promising candidate for ultra-high-density magnetic recording media.
Chemical solution method has become an attractive route to obtain FePt nanoparticles (NPs) with the controllable size, well-defined shape, and ordered monolayer assemblies since Sun et al. made great success in preparing monodisperse FePt NPs . Based on this, a lot of studies have been conducted to explore and optimize the synthesis of FePt NPs, such as modifying fabrication methods [5-13], optimizing assembly methods [7,14-21] and fabricating FePt one-dimensional nanorods /nanowires [22-28] and so on.
As-prepared fcc-FePt NPs need to be transformed to ferromagnetic fct-FePt, high temperature annealing will produce severe grain growth and particle aggregation, leading to the decrease of the particle positional order . Great efforts have been made to suppress the unfavorable phenomenon upon annealing and worked. For example, element such as Ag , Au , and Sb  with low surface energy is doped into FePt NPs to abstain from the influence of annealing by decreasing the phase transition temperature of FePt. However, one defect is that the phase transition temperature is too high to avoid particle aggregation, another is that the morphology of FePt nanoparticle will become uncontrolled and self-assembled array over large area are destroyed after Sb doping. In addition, the core-shell structure of inorganic substance such as ZnO [33,34], MnO , NiO  and SiO2  covering on FePt NPs solves the problem of sintering and aggregation of NPs. However, as the thickness and morphology of core-shell structure is uncontrolled and there exists strong magnetic dipole interaction between FePt magnetic NPs, making it difficult for self-assembled of NPs and orderly array over large area fail to form. Recently it reported that nonmagnetic films like Al2O3 deposited by atomic layer deposition (ALD) upon FePt NPs self-assembly array can improve the stability of FePt NPs under high temperature, preventing NPs from sintering and aggregation . Other work like dispersing FePt NPs into the TiO2 substrate by sol-gel is a good way to protect FePt NPs during annealing , but element Fe of FePt will be lost in acidic TiO2 sol.
In this work, we reported that in-situ magnetic field was applied during the synthesis process of FePt NPs and the dip coating process to form FePt NPs films. The FePt NPs were prepared via chemical reduction of Pt(acac)2 and thermal decomposition of Fe(CO)5 under different magnetic conditions in the presence of oleic acid (OA) and oleylamine (OAm) at 220â„ƒ. The prepared FePt NPs films were than annealed at 700 â„ƒ for 60 min in forming gas (7% H2 + 93% Ar) to form the L10 phase of FePt. It is revealed that applied magnetic field not only significantly improves the c-axis preferred orientation, but also benefits the phase transition of FePt NPs from fcc to fct structure. The FePt NPs thin film synthesized under magnetic field also shows some magnetic anisotropy. Under magnetic field as-synthesized FePt NPs are monodispersed and can be self-assembled over larger area by a dropping method.
2.1 Synthesis of FePt NPs
FePt nanoparticles were synthesized through a standard polyol process with a modified synthetic condition using standard airless procedures under a gentle flow of pure nitrogen (N2) [12,39]. Typically, the FePt nanoparticles were prepared via chemical reduction of Pt(acac)2 and thermal decomposition of Fe(CO)5 under different magnetic conditions in the presence of oleic acid (OA) and oleylamine (OAm) at 220â„ƒ.
In a typical procedure, 0.125 mmol of Pt(acac)2 was mixed with 20 mL of phenyl ether under the gentle nitrogen gas flow. The mixture was heated to 50°C, and stir until the platinum source dissolved completely in the solvent. After that the mixed solution was heated to 150°C and 40 μL of oleic acid (OA),42.5 μL of oleylamine (OAm), and 80 μL of Fe(CO)5 were added step by step under different magnetic conditions with continuous stream of nitrogen. After that, the solution was heated up to 220 °C at the rate of 10 °C per minute., and refluxed for 30 min under the nitrogen protection. After the prepared black solution cooling down to the room temperature naturally, 50 μL of oleic acid (OA), 50 μL of oleylamine (OAm) and absolute ethanol were added into the mixture to a total volume of 80 mL. The black products were then precipitated by centrifugation (8000 r/min for 10 min) and the solution supernatant was discarded. The precipitate was then dissolved in 10 mL of hexane and precipitated again in 40 mL of absolute ethanol by centrifugation. The black FePt NPs were synthesized by repeating the separation process for 2~3 times. The magnetic NPs were dispersed in 6 mL of octane and stored in brown glass bottle under the nitrogen conditions.
2.2 Preparation of FePt NPs films
Assembled FePt NPs on the HF-treated n-Si (100) substrates (1.0×1.0 cm2) were prepared by droping a drop of 2 mg/mL FePt solution (FePt NPs dispersed in octane) including a small amount of OA and OAm. As the organic solvent on the surface of FePt NPs was dried under the protection of N2 at room temperature, the FePt NPs were then heated to 120 °C and maintained for 2h in the baking oven to remove the organic solvent completely. In-situ magnetic field was applied in a patr of the samples during the dip coating process to form FePt NP films and another part were in nonmagnetic field for comparison. Three kinds of samples with different external magnetic field applied during the synthesis process and the dip coating process were listed in Table 1. The prepared FePt NP films were than annealed at 700 â„ƒ for 60 min in forming gas (7% H2 + 93% Ar) with a rising speed of 5 °C/min to form ordered fct-FePt before characterization.
The structure and crystalline phase were characterized by means of X-ray diffraction (XRD, D/max 2000, Rigaku) using Cu Kα radiation (λ = 1.5406 Å) operated at 40 kV and 40 mA. The morphology and microstructure of various samples were characterized using a transmission electron microscopy (TEM, Tecnai G2 F20 S-twin, FEI) operating at 200 kV. The compositions of all samples were analysed by the energy dispersive X-ray spectroscopy (EDS) attached to a field-emission scanning electron microscopy (FESEM, Zeiss). Magnetic properties of the fct-FePt were measured by a superconducting quantum interference device (SQUID, MPMS XL-7, Qauntum Design) with a maximum field of 35 kOe.
3. Results and discussion
Figure 1 (a) and (b) show the XRD patterns of unannealed and annealed FePt NPs films under different magnetic conditions. In Fig. 1 (a), the emergence of two broad peak at 40.3 o and 46.9 o of all samples which represent the Bragg peaks (111) and (200) illustrate the fcc-FePt NPs of average grain size of 4.1 nm calculated by Scherrer equation were obtained. It is obvious that in sample 2# and 3#, the peak (200) are stronger and closer to the highest peak (111) where diffraction is most likely to occur compared with sample 1# without magnetic field applied, indicating that in-situ magnetic field applied during the synthesis process exhibit the trend for FePt NPs to align perpendicular to the (100) crystal plane. While magnetic field applied during dip coating process make no obvious effect before anneal via comparing sample 2# with 3#. High temperature annealing make the phase transform from fcc to fct as indicated by the emergence of the Bragg peaks of (001), (110), (002) and (201) as shown in Fig. 1 (b). The Bragg peak (001) and (002) are much stronger with the magnetic field applied during the synthesis process among which the intensity of peak (001) has been ahead of main peak (111) and peak (002) split from peak (200) are higher than peak (200) apparently. It means that the fct-FePt NPs films with the magnetic field applied during the synthesis process after high temperature annealing exhibit c-axis preferred orientation that is fct-FePt NPs align along the c-axis perpendicular to the surface of films which is the easy axis of magnetization . Magnetic field applied during both during the synthesis process and the dip coating process has slightly improve c-axis preferred orientation, inferior to sample 2#.
We define the degree of c-axis preferred orientation D(001) of fct-FePt in direction  as follows :
where (I(001)/I(111))standard=0.3 is got in diffraction patterns of fct-FePt powder with random orientation, while (I(001)/I(111))measure can be calculated from the XRD patterns of annealed sample 1#, 2# and 3#.
where c and a are the lattice constants for the fct-FePt, evaluated from the (001) and (110) Bragg peaks of the XRD patterns and the axial ratio (c/a)measure for the partially ordered phase can be calculated then. For the fully ordered-phase FePt, (c/a)complete = 0.9657.
Some data of samples under different magnetic conditions are listed in Table 2, including unannealedI(200)/I(111), annealed I(001)/I(111), degree of the chemical ordering parameter S and degree of c-axis preferred orientation D(001).
It is easily seen from Table 2 that samples 2# and 3# with external magnetic field applied have a certain degree of  preferred orientation before anneal, making  preferred orientation more obvious after anneal. Comparing the degree of the chemical ordering parameter S of all samples, we can see that applied magnetic field during the synthesis of FePt nanoparticles not only significantly improves the NPs’ c-axis preferred orientation with the larger perpendicular c-axis preferred orientation degree D(001) of 3.47, but also benefits the phase transition of FePt NPs from fcc to fct structure during the annealing process. The reason for obvious c-axis preferred orientation may attribute to the anisotropy induced by external magnetic field during the nucleation of FePt for that applied magnetic field changed the barrier of nucleation in different orientation ,making the ratio I(200)/I(111) bigger in superparamagnetic particles and a-axis orientation enhanced, which is more likely to be transformed to c-axis orientation during the process of films formation and high temperature annealing.
This project is supported by the Natural Science Foundation of China (Grant No. 51202107), a grant from the State Key Program for Basic Research of China (Grant No. 2011CB922104), and the Fundamental Research Funds for the Central Universities. Ai-Dong Li also thanks the support of Priority Academic Program Development in the Jiangsu Province and the Doctoral Fund of Ministry of Education of China (Grant No. 20120091110049).
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