Synthesis of magnetic nanofibers using femtosecond laser material processing in air
© Alubaidy et al; licensee Springer. 2011
Received: 6 December 2010
Accepted: 6 May 2011
Published: 6 May 2011
In this study, we report formation of weblike fibrous nanostructure and nanoparticles of magnetic neodymium-iron-boron (NdFeB) via femtosecond laser radiation at MHz pulse repetition frequency in air at atmospheric pressure. Scanning electron microscopy (SEM) analysis revealed that the nanostructure is formed due to aggregation of polycrystalline nanoparticles of the respective constituent materials. The nanofibers diameter varies between 30 and 70 nm and they are mixed with nanoparticles. The effect of pulse to pulse separation rate on the size of the magnetic fibrous structure and the magnetic strength was reported. X-ray diffraction (XRD) analysis revealed metallic and oxide phases in the nanostructure. The growth of magnetic nanostructure is highly recommended for the applications of magnetic devices like biosensors and the results suggest that the pulsed-laser method is a promising technique for growing nanocrystalline magnetic nanofibers and nanoparticles for biomedical applications.
Nanomaterials field is of current interest because it studies materials with morphological features on the nanoscale. Nanosized materials show distinctive properties compared with bulk materials [1–3]. In particular, magnetic nanostructures have recently attracted much attention because of their intriguing properties that are not displayed by their bulk or particle counterparts. These nanostructures are potentially useful as active components for ultrahigh-density data storage, as well as in the fabrication of sensors and spintronic devices .
The growth of nanofibers using ultrafast laser offers advantages of high resolution, high throughput, uniformity, localized heating, simplicity, and reproducibility [5–8]. The time scale of materials heating and cooling of traditional thermal processes is significantly higher than that with femtosecond laser irradiation . The rapid absorption of energy leads to efficient material removal before significant heat diffusion to the substrate occurs. Femtosecond laser radiation has already been used to fabricate nano-sized spikes of semiconductor , metallic [11, 12], and dielectric surfaces  in vacuum.
Magnetic neodymium-iron-boron (NdFeB) nanofibers and nanoparticles have become one of the hotspots in the research field of magnetic materials to meet the demand for miniaturization of electronic components in recent years, and have been successfully prepared by various routes like the sol-gel auto-combustion method , co-precipitation , hydrothermal method , reverse micelles , microemulsion method , alternate sputtering , pulsed-laser deposition , and so on. However, until now there have been no reports on the synthesis and magnetic properties of NdFeB ferrite nanofibers in literatures.
In the present study a magnetic weblike fibrous nanostructure is formed due to the agglomeration of the bulk quantity of nanoparticles created during laser ablation at mega hertz pulse frequency. A distinct characteristic of the fibrous nanostructures is that particles are fused and the agglomeration shows certain degree of organization, unlike the random stacking of particles observed at femtosecond laser ablation at pulse frequency in kilohertz and hertz regime. The effect of pulse repetition rate on the nanofibers size and hence the magnetization was also investigated. The nanostructures were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX), X-ray diffraction (XRD), and magnetic force microscopy (MFM). The mechanism of formation is explained by the well-established theory of vapor condensation induced by ultrafast laser ablation. Also, the fibrous nanostructures have relatively uniform diameters (30-90 nm) and did not observe a wide range of variation in size distribution. This agrees with the characteristics of nanoparticle formation through homogenous nucleation, which tends to generate monosized nanoparticles.
The laser source is a diode-pumped Yb-doped fiber oscillator/amplifier system (Clark MXR Inc.) capable of producing an average power of 15.5 W with pulse repetition frequency between 200 kHz and 25 MHz. A neodymium-iron-boron magnetic specimen of 1" ± 0.008" length by 1" ± 0.008"; width by 0.1" ± 0.005" thickness was cut into four pieces of same size. The tetragonal Nd2Fe14B crystal structure has exceptionally high uniaxial magnetocrystalline anisotropy. This gives the compound the potential to have high coercivity. To generate magnetic nanofibers, the first piece of the magnetic specimen was irradiated with laser using 1040 nm wavelength with 15 W power and a pulse repetition rate of 4 MHz. The experiment was repeated to generate nanofibers on the specimen using the same power and wavelength with frequencies of 8, 13, and 26 MHz. The irradiated sample was characterized using SEM, TEM, EDX, and XRD analysis.
Results and discussion
The laser pulse repetition rate plays a critical role in the formation of magnetic nanofibrous structure . In order for nanoparticles to aggregate and form fibrous structure, a continuous supply of vapor is required to maintain the nucleus density of the expanding plume. Nanoparticles generated from the successive laser pulse are fused to the particles created from the previous laser pulse that are still above the melting temperature and grow as nanofibrous like structure as shown in Figure 1. As the pulse repetition rate of the femtosecond laser increases, the time between successive pulses decreases which gives less time for clusters to agglomerate and generate nanofibers with smaller diameter. It is evident from the SEM images shown in Figure 2a-d that smaller size nanofibers was generated with the increase of the laser pulse repetition rate.
where r is the nanofiber size, λ is the X-ray wavelength, B is the full width at half maximum of the peak (FWHM), and θ is the diffraction angle. From the diffraction peaks in Figure 7, the average nanofiber size was estimated using the above equation and plotted in Figure 8. Those calculations are close to our experimental results as shown in the figure.
The metastable Nd-rich phase is a grain-boundary phase which has an FCC structure. This grain boundary phase exhibits a characteristic contrast which is similar to a metastable high-pressure phase observed previously as FCC γNd . The structure of the phase is, however, closely related to that of NdO and it was frequently reported that oxygen content is fundamental in the formation of this phase . However, oxygen-containing FCC phases as shown in Figure 4 were observed only at high temperatures. Therefore, oxygen presence is not critical for the formation of the FCC phase, although at higher temperature this phase may absorb oxygen more easily than other phases because of the high Nd content. Moreover, oxygen can probably stabilize this metastable phase and at higher temperature it can transform into the stable NdO oxide. It was noticed, however, more than three phases can coexist at a given temperature (e.g., at melting point) only if the fourth element was introduced into the ternary system, i.e., oxygen in Nd-Fe-B system . The FCC phase is presumably a metastable phase with a structure close to the short-range order in the Nd-rich amorphous phase . It probably forms from the undercooled substrate with lower melting point than Nd2Fe14B or from the amorphous phase produced at grain boundaries during the laser ablation process.
We introduced synthesis of NdFeB magnetic fibrous nanostructure and nanoparticle on bulk substrate using femtosecond laser radiation under ambient conditions. The phase structures and microstructures have been investigated using XRD, SEM and EDX analysis. The magnetic nanofibers were grown in the order of few nanometers and organized themselves in weblike structures. Magnetic nanoparticles with diameter in the order of few nanometers were attached to the nanofibrous structure. Increasing the repetition rate of the femtosecond laser results in increasing the number of pulses and hence decreases size of the generated magnetic nanofibers. Increasing repetition rate of the femtosecond laser results in generating smaller size magnetic nanofibers. The magnetic strength of the generated nanofibers can be controlled by changing the repetition rate of the femtosecond laser. These magnetic nanofibers may be utilized in many applications, such as magnetic devices, carriers, tissue engineering materials, and drug delivery.
magnetic force microscopy
scanning electron microscopy
transmission electron microscopy
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