Erratum to: Microstructure and Magnetic Properties of SrFe12O19 Nano-sized Powders Prepared by Sol-Gel Auto-combustion Method with CTAB Surfactant
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In this research, nano-sized powders of strontium hexaferrite were synthesized by sol-gel auto-combustion route using stoichiometric ratio of Fe/Sr. The effect cetyltrimethylammonium bromide (CTAB) addition on microstructure and magnetic properties of hexaferrite have been studied. The samples were characterized using X-ray diffraction (XRD), dynamic light scattering (DLS), vibration sample magnetometer (VSM), field emission scanning electron microscope (FESEM), and transmission electron microscope (TEM) techniques. The results revealed that CTAB addition causes a noticeable reduction in the amount of residual α-Fe2O3 phase, since presence of CTAB in the sol facilitates the entrance of Sr2+ ions into the reactions of hexaferrite formation. Also, the morphology of the particles was affected by CTAB addition. Irregular-shaped nanoparticles were synthesized without CTAB additions, while platelet-shaped nanoparticles were obtained by CTAB addition. The mechanism of strontium hexaferrite nanopowder formation has been explained. Magnetic measurements in the sample calcined at 800 °C for 1 h represented that CTAB addition increased the coercivity force (i H c) from 4.9 to 5.2 kOe and maximum magnetization (M max) from 48.4 to 60.4 emu/g, respectively.
KeywordsStrontium hexaferrite Sol-gel auto-combustion Surfactant Magnetic properties
Erratum to: J Supercond Nov Magn (2014)
The original version of this article unfortunately contained mistakes. References were incorrectly cited in the text. The correct version of the article is shown below.
M-type strontium hexaferrite (SrFe12O19) has wide applications due to its appropriate magnetic. properties, corrosion resistance, chemical stability, and high performance-to-cost ratio. It has been recognized that they can be used as permanent magnets and high-density magnetic reordering media; they can also be used in telecommunication and as components in microwaves, higher frequencies, and magneto-optical devices [1, 2, 3, 4]. The purity, size, and morphology of the powders affect its magnetic properties . Therefore, various synthesis methods such as chemical coprecipitation [6, 7], hydrothermal [8, 9], self-propagating high temperature route , mechanical alloying [11, 12], sol-gel [13, 14, 15], and sol-gel auto-combustion  have been developed to synthesize M-type hexaferrite nano-sized powders.
It was observed that some additional Sr was needed to synthesize strontium hexaferrite without residual α-Fe2O3 phase in the sol-gel auto-combustion process , and the same result was also reported for some other wet chemistry routes, since the solubility of Sr(OH)2 is low in an aqueous solution. This low solubility poses problems in maintaining the stoichiometry of the strontium ferrite; therefore, some excess strontium is needed to be introduced into the starting composition .
There are some investigations on the effect of different surfactant additions on synthesis of strontium hexaferrite by sol-gel auto-combustion method using Fe/Sr ratio of 10, which represented that the mean crystallite size and calcination temperature of synthesized single-phase strontium hexaferrite had been reduced by surfactant addition [18, 19].
Up to authors’ knowledge, the effect of CTAB addition on microstructure and magnetic properties of strontium hexaferrite synthesized with Fe/Sr ratio of 12 (stoichiometric Fe/Sr ratio) with sol-gel auto-combustion route was not studied yet. Therefore, in this research, strontium hexaferrite nano-sized powders have been synthesized with the stoichiometric ratio of Fe/Sr by a sol-gel auto-combustion route and the effect of CTAB addition and calcination temperature on phase formation, microstructure, and magnetic properties of strontium hexaferrite nanopowders have been studied.
2 Materials and Methods
In order to synthesize strontium hexaferrite nano sized powders, proper amounts of metal nitrates: 19.16 g Fe(NO3)3.9H2O (99 % Merck) and 0.82 g Sr(NO3)2 (99 % Merck) were dissolved into 50 ml of distilled water to make an aqueous solution. Then, citric acid (C6H8O7 99 % Merck) was added to the above mixture as a chelating agent. The molar ratio of Fe/Sr was fixed to 12 in different samples and the nitrate to citrate ratio was fixed to 1:1. The pH of the solution was increased to 7 by addition of ammonia solution. Then cetyltrimethylammonium bromide (CTAB) C19 H 42BrN (99 % Merck) was added to the solution as surfactant. The resulting sol had been heated at constant temperature of 80 °C on magnetic stirrer to complete the reaction for forming the gel precursor. Then the dried precursor undergoes a self-ignition reaction to form a very fine brown foamy powder. Finally, the samples were calcined at 800 and 900 °C for 1 h.
Magnetic properties have been taken out at room temperature at the maximum applied field of 14 kOe by vibrating sample magnetometer (VSM) model MDK. The phase identification of the combustion products and calcined nanopowders has been performed by Philips X’pert Pro X-ray diffractometer (XRD) using Cu K α radiation (λ = 0.1541 nm).
The XRD patterns were submitted to a quantitative analysis by the Rietveld method using Material Analysis Using Diffraction (MAUD) software . The size of calcined nanoparticles were determined using dynamic light scattering technique (DLS) model Nano ZS90. The morphology and microstructure of the nanoparticles were studied by a field emission gun scanning electron microscope (FESEM) model (TESCAN) and transmission electron microscope (TEM) at 200 kV (Philips CM200). Selected area electron diffraction (SAED) patterns were also taken on TEM.
3 Results and Discussions
XRD patterns of the sample prepared with CTAB addition under calcination temperatures of 800 (Fig. 1b) revealed that addition of the CTAB decreases the amount of residual α-Fe2O3 phase considerably at 800 °C.
It was shown that CTAB addition in the sol-gel auto-combustion process increases the exothermicity of the reaction strongly . So the different morphologies in the combustion product may be as a result of higher combustion exothermicity obtained in the presence of CTAB since it plays the role of fuel in the combustion process. The higher combustion temperature and its higher exothermicity may lead to the formation of more crystallized and aggregated precursors. Also, presence of surfactant in the sol may affect the morphology of the precursors and facilitates the growth of precursors in some preferred directions leading to the formation of platelet-like morphologies, since there are some reports that represent that the presence of CTAB and its concentration in the aqueous wet chemistry methods affect the morphology of the synthesized nanoparticles of different materials (e.g., α-Fe2O3, Cu2O and Au) [27, 28, 29].
Magnetic properties of the samples synthesized without and with CTAB addition with Fe/Sr ratio of 12 calcined at 800 and 900 °C for 1 h
Calcination Temperature (°C)
CTAB addition (Yes/No)
It is known that thei H c of ferrites is strongly affected by grain or particle sizes . The hysteresis loop of the sample prepared with surfactant addition under calcination temperature of 800 °C represents that the intrinsic coercivity force (i H c) of the sample increased to 5.22 kOe which may be as a result of presence of small particles that approached to the super paramagnetic state in the sample,;hile CTAB additions caused formation of coarse particles in the single-domain range.
Increasing the calcination temperature to 900 °C, in the sample synthesized with CTAB addition, reduced thei H c from 5.22 to 4.99 kOe which could be as a result of the growth of the particles during calcination,;ince the coarse particles of hexaferrite in the sample synthesized with surfactant calcined at 900 °C may have a transition from single magnetic domain to multi-magnetic domain by increasing the calcination temperature.
On the other hand, increasing the calcination temperature in the sample synthesized without surfactant addition increases the coercivity force to 5.52 kOe. It seems that the growth of particles causes formation of single-domain particles instead of particles starting to approach the super paramagnetic state and this enhances the coercivity force. Also, reduced amount of α-Fe2O3 may affect the coercivity force while there is no known trend for the relationship between the amount of α-Fe2O3 and coercivity force values [25, 39, 40]. The step which was present in the demagnetization curves of the second and fourth quadrants of the sample without CTAB addition calcined at 800 °C is flattened in the VSM plots of the samples with CTAB addition.
The SrFe12O19 nanopowders were synthesized by sol-gel auto-combustion method with and without CTAB addition. The results revealed that CTAB additions can improve the magnetic properties of the sample calcined at 800 °C and increase its maximum magnetization from 48.41 to 60.40 emu/g and itsi H c from 4950.89 to 5221.47 Oe. This magnetic properties improvement is as a result of reduced amount of residual α-Fe2O3 and presence of some particles with the sizes in the single-domain range. Increasing the calcination temperature to 900 °C in the sample without CTAB addition increases the M max andi H c, while in the sample with CTAB addition increasing the calcination temperature diminishes thei H c as a result of particle coarsening.
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