Highly uniform and well-aligned one-dimensional Ni nanochains with controllable diameters, including 33, 78, and 120 nm, have been synthesized by applying an external magnetic field without any surface modifying agent. The formation can be explained by the interactions of magnetic dipoles in the presence of applied magnetic field. Magnetic measurements demonstrate that the shape anisotropy dominates the magnetic anisotropy. The demagnetization factor, ∆N, is in the range of 0.23–0.36.
KeywordsNi nanostructure Magnetic anisotropy X-ray absorption spectroscopy Electron microscopy
In the past few years, magnetic nanostructures have aroused considerable interests due to their exceptional properties and potential applications in catalysis, sensors, high-density magnetic records, bio-imaging, and many other domains [1–6]. Because most physical and chemical properties of these nanoparticles depend on their sizes and shapes, the shape control of nanostructures has become a new and interesting research field [7–9]. In particular, one-dimensional (1D) magnetic nanostructures have become one of the focused points [10–13].
Recently, magnetic field-assisted hydrothermal process has been proven to be an efficient way for directing the growth of 1D magnetic material [14–18]. However, previous investigations indicated that the surfactants such as polyvinyl pyrrolidone, hexadecylamine, and cetyltrimethyl ammonium bromide were still necessary, and the diameters of the fabricated nanochains were usually larger than 100 nm [15, 18–20]. Template-free magnetic field-induced synthesis of well-aligned Ni nanochains with smaller diameters has rarely been reported .
Hence, in this work, we proposed a magnetic field-induced solution-phase synthesis and self-assembly approach of aligned Ni nanochains using environment-friendly reagents. The growth mechanism and the magnetic anisotropy of the Ni nanochains have been discussed in terms of the applied magnetic field.
Setup and Apparatus
Magnetic field-induced Synthesis of Nickel Nanochains
All chemicals used in this experiment were analytical grade and used without further purification. In a typical experiment, 40 ml ethylene glycol (EG) was first put into a three-necked flask and heated up to 120°C by a heating tape. Then a 0.4-ml solution of hydrated hydrazine (50%) was added and maintained at 120°C for 5 min. An external magnetic field (5 kOe) was applied, and 20-ml as-prepared NiCl2·6H2O solution of 6 × 10−4 mol/l (2.9 mg, dissolved in 20 ml EG, room temperature) was added into the flask. The whole solution was stirred by flowing N2 gas of 20 ml/min at all times. After refluxing for 1 h, a black product was obtained, filtrated, and then rinsed with ethanol and deionized water for 5–6 times. The synthesis process is summarized in the flowchart in Fig. 1b.
In this report, three nickel nanochains with average diameters of 33 ± 3 (sample 1), 78 ± 8 (sample 2), and 120 ± 12 nm (sample 3) have been synthesized at temperature of 150, 120, and 90°C, respectively. More than three runs have been made for reproduction at each fixed temperature and magnetic field. The diameter of the nanochains is analyzed statistically from the images of about 100 randomly selected nanochains appeared in a scanning electron microscopy (SEM) micrograph of the sample. The standard deviation is determined as about 10%. The influence of the applied magnetic field on the morphologies of the nanochains has also been investigated.
Magnetic field-induced Reorientation of Nickel Nanochains
To obtain well-aligned Ni nanochains array, freshly synthesized nickel nanochains were each dispersed in ethanol and ultrasonically washed for 10 min. Then, the suspension was dropped onto the silicon slice under a 2-kOe magnetic field and dried in air.
The structural and chemical information of the as-prepared products was studied using X-ray diffraction (XRD, X’Pert Pro MPD system, Cu Kα), scanning electron microscopy (SEM, S-4800, Hitachi Company), transmission electron microscopy (TEM, 2100F, JEOL Company) equipped with an energy dispersive X-ray spectroscopy (EDS) apparatus, and X-ray absorption spectroscopy (XAS) with sample current in total electron yield mode at SINS beamline at Singapore Synchrotron Light Source . For the XRD and XAS measurements, powder samples were used. The SEM micrographs were taken with the nanochains dispersed over the silicon substrates. For the TEM, high-resolution TEM (HRTEM) observations and EDS analysis, the as-grown nanochains were dispersed in ethanol and dropped onto a carbon film supported on a copper grid and dried in air. Magnetic properties of the as-synthesized nanochains were measured using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design), with the nanochains prepared over the silicon substrates.
Results and Discussions
For the axially aligned nanochains dispersed over the Si-substrate as shown in Fig. 2c, an interesting feature is observed. The nanochains appear to bundle up, 5 or 6 in each bundle. This is an indication that there exists intra-chain dipolar interaction which is likely to be of antiferromagnetic ordering, as revealed by the work of Bliznyuk et al. . This is an interesting issue to study. However, it is difficult to show evidence for this property by the magnetization measurements alone in the present work.
A facile and effective field-induced method has been employed to prepare highly uniform, size-controllable, well-aligned Ni nanochains without any surface active agent. The formation can be explained by the interactions of magnetic dipoles in the presence of applied magnetic field. In-plane M(H) curves with the applied field parallel and perpendicular to the direction of the aligned nanochains indicate that the shape anisotropy is a significant factor for the magnetization reversal. The greatly enhanced magnetic anisotropy of the well-aligned magnetic nanochains implies their potential applications in the fields of magnetic records, magnetic switches, etc.
This work was supported by the National Natural Science Foundation of China (Nos. 50671003 and 10874006), the National Basic Research Program of China (Nos. 2009CB939901 and 2010CB934601), and the Program for New Century Excellent Talents in University (NCET-06-0175).
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