Synthesis and Characteristics of FePt Nanoparticle Films Under In Situ-Applied Magnetic Field
© The Author(s). 2016
Received: 6 April 2016
Accepted: 5 July 2016
Published: 11 July 2016
In situ external magnetic field was applied during the synthesis of FePt nanoparticles via a chemical solution method. FePt nanoparticle films were prepared on Si by a drop-coating method with and without a magnetic field. Annealing at 700 °C in reductive atmosphere was explored to obtain ferromagnetic FePt L10 phase. The effect of in situ-applied magnetic field on the structure, morphology, and magnetic properties of FePt nanoparticle films was characterized. It is found that the applied magnetic field during the chemical synthesis of FePt nanoparticles plays a key role in the crystallinity and magnetic property of FePt nanoparticle films. As-synthesized FePt nanoparticles under the magnetic field are monodispersed and can be self-assembled over a larger area by a dropping method. The applied magnetic field during the synthesis of FePt nanoparticles not only significantly improves the nanoparticles’ c-axis preferred orientation but also benefits the phase transition of FePt nanoparticles from face-centered cubic to face-centered tetragonal structure during the annealing process. The FePt nanoparticle films derived under magnetic field also show some 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, regarded as the most promising candidate for ultra-high-density magnetic recording media [1–3]. Meanwhile, FePt nanoparticles and their multifunctional surfaces have also shown great potentials in biomedical applications such as multimodality imaging probes and target-specific drug/gene delivery [1, 4].
Chemical solution method has become an attractive route to obtain FePt nanoparticles with the controllable size, well-defined shape, and ordered monolayer assemblies since Sun et al. made great success in preparing monodisperse FePt nanoparticles . Based on this work, a lot of studies have been conducted to explore and optimize the synthesis of FePt nanoparticles, such as modifying fabrication methods [6–10], optimizing assembly methods [7, 11–14], and fabricating FePt one-dimensional nanorods/nanowires [15–18].
For example, elements such as Ag , Au , and Sb  with low surface energy were doped into FePt nanoparticles to decrease the phase transition temperature of FePt from face-centered cubic (fcc) to fct structures. However, the morphology of FePt nanoparticles became uncontrolled, and self-assembled arrays over a large area were destroyed after doping. A series of inorganic core-shell structures such as ZnO [22, 23], MnO , NiO , and SiO2  covering on FePt nanoparticles have been synthesized to obtain multifunctional magnetic nanoparticles. In addition, polymer templating [8, 27], micellar approach with SiO2 thin film capping , direct synthesis [29, 30], and salt-matrix technique  have also been attempted. Recently, our group explored the combination of self-assembled FePt nanoparticles and atomic layer-deposited Al2O3 capping for fabrication of patterned magnetic nanocomposites with improved the coercivity and stability of FePt nanoparticles under high-temperature annealing . In addition, sol-gel-derived oxide matrixes also prevented FePt nanoparticles from sintering and aggregation [33, 34].
In this work, we reported that in situ magnetic field was applied during the chemical solution synthesis process of FePt nanoparticles. And the drop-coating process was explored to form FePt nanoparticle films with and without a magnetic field. Under a magnetic field, as-synthesized FePt nanoparticles are monodispersed and can be self-assembled over a larger area by a dropping method. As-prepared FePt nanoparticle films were then annealed at 700 °C for 60 min in forming gas (7 % H2 + 93 % Ar) to obtain the L10 phase of FePt. It is revealed that an applied magnetic field during chemical synthesis not only significantly improves the c-axis preferred orientation but also benefits the phase transition of FePt nanoparticles from fcc to fct structures. The FePt nanoparticle films chemical-synthesized under the magnetic field also show some magnetic anisotropy.
Synthesis of FePt Nanoparticles
In a typical procedure, 49.4 mg of Pt(acac)2 was mixed with 20 mL of phenyl ether under nitrogen flow. The mixture was heated to 50 °C and stirred until the platinum source dissolved in the solvent completely. Then the mixed solution was heated to 150 °C, and 80 μL of Fe(CO)5, 40 μL of OA, and 42.5 μL of OAm were added step by step under in situ-applied magnetic field with continuous stream of nitrogen. Finally, the solution was heated up to 220 °C at the rate of 5 °C/min and refluxed for 30 min under the nitrogen protection. After the black solution cooling down to room temperature naturally, 50 μL of OA, 50 μL of OAm, and absolute ethanol were added into the mixture to a total volume of 80 mL in a clean beaker. The black products were then precipitated by centrifugation at 8000 rpm for 10 min, and the supernatant solution 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 nanoparticles were obtained by repeating the separation process for 2~3 times. The FePt nanoparticles were dispersed in 6 mL of octane and stored in a brown glass bottle under the nitrogen conditions. For comparison study, we also prepared the control samples of FePt nanoparticles using the same processing without an applied magnetic field.
Preparation of FePt Nanoparticle Films
Assembled FePt nanoparticles on the HF-treated n-Si (100) substrates (1.0 × 1.0 cm2) were prepared by dropping a drop of FePt solution (FePt nanoparticles dispersed in octane with concentration of 2 mg/mL). The FePt nanoparticle films were first dried at room temperature and then heated to 120 °C for 2 h in a baking oven to remove organic solvent completely. In situ external magnetic field was applied during the drop-coating process to form FePt nanoparticle films on Si, as illustrated in Fig. 1b. Two SmCo permanent magnets were placed horizontally to produce a magnetic field by inserting 1-cm-high plastic spacer.
The applied magnetic field conditions of different samples during the chemical synthesis process for FePt nanoparticles and the drop-coating process for FePt nanoparticle films
Applied magnetic field
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, respectively. 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 the samples were analyzed by an energy dispersive X-ray spectroscope (EDS) attached to a field emission scanning electron microscope (FESEM, Zeiss). Magnetic properties of the fct-FePt were measured by a superconducting quantum interference device (SQUID, MPMS XL-7, Quantum Design) with a maximum field of 35 kOe.
Results and Discussion
The unannealed I(200)/I(111), annealed I(001)/I(111), D(001), and S of c-axis orientation degree and chemical ordering of 1#, 2#, and 3# samples under different magnetic field conditions before and after annealing
0.46 ± 0.03
0.19 ± 0.02
0.63 ± 0.07
0.72 ± 0.02
0.89 ± 0.03
1.04 ± 0.03
3.47 ± 0.10
0.76 ± 0.03
0.76 ± 0.11
0.65 ± 0.02
2.17 ± 0.07
0.77 ± 0.02
After 700 °C annealing in forming gas, the Bragg peaks of (001), (110), (002), and (201) appear in the XRD patterns of Fig. 2b, suggesting the phase transition from disordered fcc to ordered ferromagnetic fct. It is easily observed that the peak intensity of (001) and (002) from the 2# and 3# samples, respectively, is much stronger than that from the 1# sample. Especially for the 2# sample, the (001) peak intensity has exceeded the usual strongest (111) peak one (PDF #01-43-1359), and the intensity of the (002) peak split from the (200) peak is also higher than the (200) peak one. It means that after annealing, the fct-FePt nanoparticle films from the applied magnetic field synthesis process exhibit a c-axis preferred orientation, i.e., fct-FePt nanoparticles aligning along the c-axis—the easy axis of magnetization perpendicular to the surface of films . In situ-applied magnetic field during the chemical synthesis process has evidently improved the c-axis preferred orientation of annealed FePt nanoparticle films.
where the I(001)/I(111) standard value of 0.3 is obtained from diffraction patterns of fct-FePt powders with random orientation, while the I(001)/I(111) measured values can be calculated from the XRD patterns of the 1#, 2#, and 3# annealed samples.
where c and a are the lattice constants for the fct-FePt, evaluated from the (001) and (110) peaks of the XRD patterns, respectively. The c/a measured values can be calculated for the partially ordered phase. For the fully ordered-phase FePt, the c/a standard value is 0.9639 based on the XRD card of PDF #01-43-1359.
The related data of c-axis orientation degree and chemical ordering of the 1#, 2#, and 3# samples under different magnetic conditions before and after annealing are summarized in Table 2, including unannealed I(200)/I(111), annealed I(001)/I(111), D(001), and S.
It is easily seen from Table 2 that the 2# and 3# samples with applied magnetic field during chemical synthesis show some a-axis preferred orientation than the 1# sample without applied magnetic field before anneal. After annealing, the 2# and 3# samples with an applied magnetic field during chemical synthesis exhibit a significant  preferred orientation with higher D(001) values of 3.47 and 2.17, respectively, whereas the D(001) value of the 1# sample without an applied magnetic field is only 0.19. In addition, an applied magnetic field during the synthesis of FePt nanoparticles also enhances the chemical ordering parameter S of the 2# and 3# samples and benefits the phase transition of FePt nanoparticles from fcc to fct structure during the annealing process.
Why does the applied magnetic field during the chemical synthesis of FePt nanoparticles finally produce c-axis preferred orientation fct-FePt nanoparticle films on Si? It might be related to the nucleation anisotropy induced by an external magnetic field during the chemical synthesis of FePt nanoparticles. The applied magnetic field changes the nucleation barrier of a different orientation, leading to an enhanced a-axis orientation in superparamagnetic FePt particles. During high-temperature annealing, the a-axis orientation fcc nucleation easily induces and yields c-axis orientation fct grains.
In summary, in situ magnetic field was applied during the synthesis of FePt nanoparticles via a chemical solution method. FePt nanoparticle films were formed on Si by a drop-coating method with and without a magnetic field. The influence of in situ-applied magnetic field on the structure, morphology, and magnetic properties of FePt nanoparticle films was characterized deeply. Although in situ magnetic field has no obvious impact on the FePt nanoparticles’ morphology and chemical composition, it is revealed that an applied magnetic field during the synthesis of FePt nanoparticles not only significantly improves the nanoparticles’ c-axis preferred orientation with a larger D(001) of 3.47 but also benefits the phase transition of FePt nanoparticles from fcc to fct structure. The FePt nanoparticle thin films synthesized under a magnetic field also show some magnetic anisotropy. In addition, as-synthesized FePt nanoparticles under a magnetic field are monodispersed and can be self-assembled over a larger area by a dropping method.
This project is supported by the Natural Science Foundation of China (51571111), a grant from the State Key Program for Basic Research of China (2015CB921203). 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 (20120091110049).
XQ, MYG, and XYZ carried out the sample fabrication and device measurements, and XQ drafted the manuscript. XQ and XJL helped to finish the TEM sample preparation and observation. CL and YQC did the data analysis and interpreted the results. ADL and DW participated in the discussion of the results. ADL supervised the whole work and revised the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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