Hexagonal ferrites MFe12O19 (M = Ba, Sr, and Pb) are well known as permanent magnetic materials. They have magnetoplumbite structure and exhibit large spontaneous magnetization with strong anisotropy along the c-axis . Moreover, hexagonal ferrites are technologically very useful materials due to their application in permanent magnets, high-density magnetic recording media, and microwave devices [2–4]. The conventional solid mixture for the preparation of hexagonal ferrite powders involves a high temperature, resulting in the loss of the fine particle nature. Several solution techniques including co-precipitation , sol–gel , hydrothermal , and ultrasonic spray pyrolysis  are used to prepare magnetic nanoparticles.
The research in magnetic oxides driven by the prospective applications of these materials and on the novel properties that they possess as nanocrystalline particles, powders and thin films has received a lot of interests [9–11]. In particular, Pb–M hexaferrite (PbFe12O19) crystallizes at temperatures lower than which happen for Ba and Sr–M hexaferrites. Thus makes it interesting for magnetic recording media [12–15]. The preparation of lead hexaferrite, as bulk material, presents some problems due to lead evaporation during thermal treatment and sintering process [15,16].
In this work we synthesis nanoparticles by microwave method. Microwave systems provide the opportunity to complete reactions in minutes, and have manifold applications in academic and industrial environments alike. There are some important parameters in microwave synthesis such as choosing solvent, kind of reactions, of heating time and microwave power which can affect on synthesized products.
Because of uniform heat distribution, ultra heating, choice of heating or selectivity, the reaction rate increases and subsequently reduce the time and energy required for synthesis of different nanomaterials. Thus many products with various structures have been synthesized by this method [17-20].
MATERIALS AND METHODS
Materials and Instruments
Pb(NO3)2 , Fe(NO3)3 9H2O, NaOH, NH3 32%, ethylene glycol and acetone were purchased from Merck and all the chemicals were used as received without further purifications. Room temperature magnetic properties were investigated using a vibrating sample magnetometer (VSM) device, made by Meghnatis Daghigh Kavir Company (Iran) in an applied magnetic field sweeping between ±10000 Oe. XRD patterns were recorded by a Philips, X-ray diffractometer using Ni-filtered CuKα radiation. SEM images were obtained using a LEO instrument model 1455VP. Prior to taking images, the samples were coated by a very thin layer of Pt (using a BAL-TEC SCD 005 sputter coater) to make the sample surface conductor and prevent charge accumulation, and obtaining a better contrast.
Synthesis of PbFe12O19 nanoparticles
0.012 mole of Fe(NO3)3 9H2O and 0.001 mole of Pb(NO3)2 and surfactant (SDS,C-TAB and citric acid) were dissolved in 100 ml of ethylene glycol and put under microwave with 510 W. 16 ml of NaOH solution (1M) was then slowly added to the solution until reaching pH to around 10. A brown precipitate was then centrifuged and rinsed with distilled water. Finally the obtained precipitate was calcinated at 850 oC and its colour goes from brown to black. Fig. 1 shows the schematic diagram for experimental setup for nanoparticles preparation used in the microwave procedure.
Fig 1. Schematic of ferrite preparation.
RESULTS AND DISCUSSION
The XRD pattern of PbFe12O19 nanoparticles is shown in Fig. 2. The pattern consists with the typical diffraction pattern of pure hexagonal phase (JCPDS No.: 17-0660) with P63-mmc space group.
The crystallite size measurements were also carried out using the XRD data and using Scherrer equation:
where β is the width of the observed diffraction peak at its half maximum intensity (FWHM), K is the shape factor, which takes a value of about 0.9, and λ is the X-ray wavelength (CuKα radiation, equals to 0.154 nm). The average crystallite size was found to be about 18 nm.
Fig. 2. XRD pattern of the PbFe12O19 nanoparticles
The XRD pattern of PbFe12O19 nanoparticles synthesized by SDS surfactant is illustrated in Fig. 3. The pattern reveals the typical diffraction patterns of pure hexagonal phase (JCPDS No.: 17-0660) with P63-mmc space group and consists with pure lead hexa ferrite.
Fig 3. XRD pattern of the PbFe12O19 by SDS.
SEM images of PbFe12O19 nanoparticles prepared using SDS surfactant are illustrated in Fig. 4. In this condition, mono-disperse product with mediocre size of around 80 nm was synthesized.
Fig. 4. SEM images the lead ferrite nanoparticles synthesized by SDS.
Fig. 5 shows TEM image of lead ferrite nanoparticles synthesized by SDS.
Fig. 5. TEM image the lead ferrite nanoparticles synthesized by SDS.
Fig. 6 illustrates SEM images of PbFe12O19 nanoparticles synthesized by CTAB surfactant. It is observed that nanoparticles are homogenous and rod-like nanostructures were formed.
Fig 6. SEM images of PbFe12O19 nanoparticles obtained by CTAB.
Fig. 7 shows SEM images of PbFe12O19 nanoparticles synthesized using citric acid. It seems homogenous hexagonal plate-like nanoparticles were synthesized. The average particle size is found to be around 60 nm, showing that, particle size, morphology and magnetic property of samples can easily be controlled by using different surfactants.
Fig. 7. SEM images of PbFe12O19 nanoparticles with citric acid.
Hysteresis loop for PbFe magnetite nanoparticles prepared with microwave method is shown in Fig.8. Interestingly from Fig. 9, it was observed that surfactant free sample has coercivity of 800 Oe, which is much less than which for product prepared by SDS (about 3000 Oe). The outcomes indicate the direct effect of morphology and particle size on the magnetic property of the prepared ferrite [21-24]. Saturation magnetization of this sample is also lower than ferrite which prepared with SDS and it is around 26.7 emu/g. Fig.9 shows that Saturation magnetization for lead ferrite nanoparticles synthesised by SDS is around 43 emu/g.
Fig. 8. AGFM loop of PbFe12O19 calcinated at 850 ◦C.
Fig. 9. AGFM loop of PbFe12O19 by addition of SDS.
Fourier transform infrared (FT-IR) spectrum of synthesized nanoparticles was recorded in the range of 400–4000 cm-1 and result is shown in Fig. 10. Absorption peaks around 400 to 600 cm-1 are related to metal-oxygen Fe-O and Pb-O bonds. The spectrum exhibits broad absorption peaks between 3500-3600 cm−1, corresponding to the stretching mode of O-H group of hydroxyl group that are adsorbed on the surface of nanoparticles.
Fig. 10. FT-IR spectrum of PbFe12O19 nanoparticle
Synthesis and characterization of PbFe12O19 nanoparticles were reported. Effect of precursor and surfactant on the morphology and particle size of the products was investigated. SEM images show that it is possible to change formation and nanoparticle size by adding surfactants. AGFM confirmed that nanoparticles exhibit ferromagnetic behaviour. However, by adding of surfactant, coercivity of magnetic nanoparticles was increased. The results also show that microwave method is a suitable approach for preparation of lead ferrite as promising candidate for industrial applications.
This work has been supported financially by Arak University Research Council (AURC) under the Grant Number of 94-2113.
CONFLICT OF INTERESTS
The authors declare that there is no conflict of interests regarding the publication of this paper.