Influence of Calcination Temperature on Structural, Morphological and Magnetic Properties of M-Type Strontium Hexaferrite Powder Prepared by Sol-Gel Auto Combustion Route

INTRODUCTION 
Nanoscience field comprises innovating as well as making novel nanostructures, but the hard task is to generate a connection between their characteristics to possess multifunctional instruments. And, this lead to introduce a bigger incorporated interdisciplinary science recognized as nanotechnology, which keeps on evolving lastingly [1]. Hexaferrite is still the highly pertinent material for the applied uses and most of stiff ferrite manufacture. They’re very remarkable materials for innumerable uses. And, of specific attention is the strontium hexaferrite, which has appealed the attention of numerous investigators due to their fresh electromagnetic characteristics, and their usage in a broad range of uses. And, that’s due to its description via high Curie temperature, high saturation magnetization, high corrosion resistance and chemical stability [2, 3]. Also, it has been known that it can be utilized as lasting magnets, telecommunication, registering media, and parts in microwave, higher-frequency, and magneto-optical instruments [4-7]. Owing to the qualities that listed before as well as its less cost, strontium hexaferrite is regarded to be a preferred nominee for lasting magnets utilized for the industrial uses that being ecologically friendly, like the rotors of generator employed in the wind energy or electric vehicles [1]. The M-type hexagonal ferrites represent a class of hard magnetic and promising materials, they contribute certainly in distinctive hi-tech and industrialized areas owing to their chemical vitality, inexpensive with easy manufacturing methods, and a high uniaxial magneto-crystalline anisotropy [8]. M-type hexagonal ferrite strontium hexaferrite (SrM) belongs to magnetic oxide known as the “magnetoplumbite” phase of ferrites. The SrM possesses a hexagonal structure with a distance set of P63/mmc [9, 10]. It also possesses the benefit of an elevated Curie temperature of (733 K) and remains the high broadly utilized magnets owing to their less price of manufacture [11]. The M-type hexaferrite’s hard magnetic characteristic is ascribed to the uniaxial magneto-crystalline anisotropy of the Fe3+ ions dispersed throughout (5) symmetry locations: One tetrahedral (↓4f1), three octahedral (↓4f2, ↑12k and ↑2a), and one trigonal bipyramidal (↑2b) [12]. In preceding documents, it was noted that there’s certain unwanted middle non-ferromagnetic phases, which result in deprived magnetic characteristics and uneven form for the particles of derived SrFe12O19 [13]. In the present research, a single-phase SrFe12O19 nanopowder with a comparatively uniform size has been prepared successfully via the sol-gel auto combustion technique. For the synthesis of SrFe12O19 nanopowders, the calcination procedure was performed as a result of the single phase nanoferrite SrM produced and studied extensively. As a consequence, the grain size, shape and particle size were well controlled, and different techniques, like XRD, Rietveld method, FE-SEM, Raman spectroscopy, and vibrating sample magnetometer (VSM) were applied for the present study. 

MATERIALS AND METHODS
SrFe12O19 hexaferrite was synthesized using citrate sol-gel auto combustion method. A stoichiometric amount of strontium nitrate, Sr (NO3)2 (>99%), and ferric nitrate nonahydrate Fe (NO3)3.9H2O (>99%), were dissolved into a least quantity of deionized water at a continuous stirring. The citric acid (C6H8O7) purchased from Thomas Baker was supplemented to the blended solution as a fuel. pH of the last solution was modified to (7) employing ammonia solution and heated to (90°C) to let the whole water evaporate and leave a viscous dense gel. And, the final viscous gel was heated to (275°C), so that the auto-combustion takes place and makes a fluffy powder. Eventually, the resulted powder was calcined at (1000°C) and (1100°C) for (4 hr) for forming the SrFe12O19 hexaferrite phase. In the present work, the crystalline phase identification was achieved utilizing the XRD measurement that has been performed in Day Petronic Company-Iran with a diffractometer (XRD model: Analytical Philips- XPERT-PRO) provided with CuKα1 (λ =1.54178 Å) employing a (40 kV) generator voltage and a (40 mA) current, and the scans were run through a range (5.0-79.97°) of 2θ, utilizing a step size of [2θ, 0.026°]. Raman spectroscopy (Micro-Raman spectroscopy-785 nm laser) was employed for investigating the vibrational spectra of the whole samples, while the surface morphology was studied utilizing FE-SEM (FE-SEM; Model Mira3-XMU, TESCAN, made in Japan). The magnetic measurements were conducted at the room temperature employing Vibrating Sample Magnetometer (VSM) (LBKFB model Meghnatis Daghigh Kavir Company). 

RESULTS AND DISCUSSION 
Structural studies 
Fig. 1 shows the typical XRD spectra of the pure strontium hexaferrite SrFe12O19 powder synthesized using sol-gel auto combustion method. The samples were prepared as-burnt (275), calcined at 1000oC and 1100oC which reveals that the most intense diffraction peaks were at 2θ= 30.34, 31.39, 32.33, 34.19, 35.42, 37.14, 40.40, 55.20, 56.84, 63.15 and 72.02 belong the plains (110), (008), (017), (114), (021), (023), (025), (127), (0211), (220) and (228), respectively. The indicated XRD patterns reveal a single-phase M-type Sr hexaferrite which being in good agreement with the standard data (ICDD 01-079-1411) which has a hexagonal crystal system with space group p 63/mmc (number194). The results confirm the formation of crystalline structure. And, this permits for the impact assessment of the calcined processing upon the SrM materials’ structural characteristics, which exhibits that the calcination can be utilized for improving the crystallinity degree. The diffracted peaks are chiefly indexed to the M-type Sr hexaferrite phase, and all samples were highly oriented along the (017) and (114) reflection planes. Moreover, a slight extra Fe2O3 secondary phase is exist at a position of 2θ=33.170o along the direction (104) plane, according to the reference code (ICDD 98-001-2729), and Fig. 1 depicts this phase as indicated by (red stars) at (as burnt) temperature and 1000oC. The patterns of X-ray diffraction obviously reveal that the Fe2O3 peak intensity disappeared at the calcination temperature of 1100oC. And, this can be clarified via the grains growth as well as the whole crystallization of the M-type Sr hexaferrite. The disappearance of the second impurity phase at 1100oC shows that the creation of M-type Sr hexaferrite is encouraged via elevating the sample synthesis temperature. Thus, the value of temperature (1100OC) is selected as the optimum conditions. It’s obvious that the well-defined, sharp and extreme peaks specify the virtuous crystalline materials.  
 Crystallite size (D), unit cell volume (Vcell), and X-ray density (ρx), of the whole powder samples are assessed from the subsequent formulas [14, 15]:

 Where:
D: Crystallite size 
λ: X-ray wavelength of Cu kα radiation 
β: Full Width at Half Maximum (FWHM) 
θ: The angle of diffraction angle (deg.) 
M: Molecular mass 
NA: Number of Avogadro 
Also, (δ=1/D2) is dislocation density, which represents a linear crystallographic defects within the crystal structure [16]. The stated variables change in terms of calcined temperature is shown in Table 1. The broadening of the peaks indicated that all the particles were in nanometer range. The perceived crystallite size (D) verified that the fabricated SrFe12O19 compound possesses nano-crystalline nature. While the obtained parameters like the lattice parameters a and c, the volume of unit cell and the density of x-ray (ρx ) for the all samples were in a good agreement and has very small shifts compared with the results founds from the standard data (ICDD 01-079-1411). It is also worth mentioning that the density being a significant parameter which is needed to verify the fabrication of fresh materials. The density values computed for the fabricated powder as well as the densities for the samples calcined at (2) various temperatures, which go on increasing with the increasing calcined temperature, are given in Table 1. Also, the raising density may be the outcome of reduced porosity, since the calcined temperature increases the voids no. or the free volume decrease resulting a higher density[17].
XRD patterns of all samples, which are displayed in Fig. 2(a, b, c), were analyzed using Rietveld refinement with the aid of the Full-Prof Suite software. Through the refinement, the wavelength correction, lattice parameters, zero correction, atomic position, and scale factor were refined at the same time. And, the peaks’ shape was characterized via pseudo-Voigt function, as well as the background was stated via a linear interpolation between the chosen background points of a group.
The fitting was arbitrated via the fit goodness, lengthways with the χ2 factor which is the refined structure factor that derived via the Rietveld method (less than 10 is sensible), as comprised in Table 2. And, it can be seen from the peaks in Fig. 2, and that the profiles for the noted and computed ones are flawlessly corresponded with each other as well as the whole investigational peaks. The small values of χ2 exhibit that the optimized value of extraction is too near to the sample’s actual value, and the fabricated sample is too virtuous. Also, Table 2 includes the values for the fit qualities of the fit which is the Bragg R-factor. A virtuous covenant between the noted and calculated reflection, as revealed via determining a minor value for one of the RBragg indexes, gives a valued sign that the model is making a virtuous job of remaking the crystallographic remarks [17]. And, the R-factors values are < 10% of the existed pattern as well as a refined pattern verified the refinement goodness, as documented via different investigators [18-21]. Additionally, Table 2 includes the weights percentage (w%) of the secondary phase Fe2O3 which it has very small amounts and decreases gradually with increasing calcined temperature, it is clear that this secondary phase disappears totally at 1100oC, and this is led to the formation of the pure single phase of Sr Hexaferrite. In summary, at calcined temperature higher than 1000oC, a comparatively neat M-type hexagonal ferrite phase can be determined [22]. 
The refined lattice parameter values of the intrinsic M-type Sr hexaferrite and Hematite Fe2O3 are listed in the Table 3. And, the values of lattice constant (a) and (c) are obtained in the (a = 5.88121–5.88174 Å) range as well as (c = 23.0592–23.0637 Å) range, correspondingly. Such values are similar to the standard values of (a = 5.88640 A), (c = 23.050 A), in accordance with the ICSD code (98.004-3590) and this in virtuous covenant with the obtained values via [1, 23, 24].
The cells volume was obtained in the (V = 691.45– 691.18 Å3) range which is clear that their values decrease with increasing calcined temperature for all samples. The values of cell parameters (a and c) are decreased with increasing calcined temperature as shown by the Fig. 3, also the c/a values decreased as the calcined temperature is increased and their values vary from (3.9212 Å) to (3.9208 Å). Such values are similar to the standard value (3.9800) of the M-type hexagonal structure [25].
Further studies have been carried out through the present studies, since the process of fitting has been conducted without refining the atomic locations of the SrM structure, due to the difficulties to find an accurate interatomic distances of the samples [26]. The structure profile of hexaferrite SrFe12O19 is shown in Fig. 4, the atomic sites, bond types, and bond lengths (Å) were obtained and analyzed by applying VESTA software-program for crystal structure model, as well as the results are listed in Table 4 and are in good agreement with literature study [1, 10, 27, 28]. 

FE-SEM and EDS analysis 
The surface morphology and elemental composition of the SrFe12O19 hexaferrite nanostructure were revealed using electron microscopy in conjunction with energy dispersive X-ray spectroscopy. The samples were prepared at 275oC (as burnt) and calcined at 1000oC and 1100oC, as depicted in the Fig. 5. It’s clear from this figure that the particles possess a distinct form. And, it can be observed that the whole samples have no exact hexagonal shapes, but the structures with increasing calcined temperature aggregate to show a certain geometrical shapes, as plate-like as well as polyhedral shapes. The uneven dispersion and inhomogeneous way in the size particles may be owing to the fabrication technique influence. And, the fast growth in the crystalline hexaferrite development employing sol-gel auto burning method has resulted in the change of the distribution of the size of particles that are connected with the time length. The adequate elevated temperature being needed for completing the preparation [29]. The existence of certain agglomerated particles collected in the images of FE-SEM is ascribed to the magnetic interactions presence among the particles [30]. And, the substantial reductions in the samples’ voids are attributed to the big masses collecting with increasing calcined temperature up to 1100oC, it is obvious that the voids of sample are disappeared at this temperature, and the crystal size was enlarged very dramatically demonstrating that the size of particle and the morphology were subtle to the calcined temperature, which is coincide with increasing the particle size from 79.84 nm to 295.5 nm with raising the calcined temperature up to 1100oC, as shown in Fig. 6.
Typically, the corresponding EDS spectrum is presented in Fig. 7, which expressly confirms that the material composition contain Strontium (Sr), Iron (Fe) and Oxygen (O) which is the elements that compose the strontium hexaferrite. The elements percentage is in a virtuous covenant with the SrFe12O19chemical composition, which verifies the contaminations nonexistence [31], except there exists a small amount of carbon element(C), as a contamination, which probably comes from the electrically conductive glue upon the holder of specimen or could be connected to the instrument sample carrier [1, 10]. 
And, the theoretical composition of elements percentages have been computed employing the this equation [1]:

 Where: 
Ms: The element molar mass 
MT: The total molar mass 
Z: The elements no. 
And, the theoretical as well as the experimental weight percentages of the elements of strontium hexaferrite are tabulated in Table 5, for three different temperatures. The elements experimental weight percentage is in a good covenant with the theoretical values. Actually, the crystal structure manifested a uniform chemical composition.

Raman Measurements 
Raman spectroscopy is a significant description process to analyze the M-type hexagonal ferrite structure. It’s of a high significance to study the materials’ lattice distortion [22]. And, in Raman spectroscopy, the incident phonons either lose quanta or gain quanta via interacting with the material vibrational modes. When it acquires energy, it becomes blue-shifted, and when it misplaces, it’s red-shifted. And, the shift quantity obtains the phonon energy in material [9]. The Raman spectra peaks are chiefly linked to the atomic bond vibration. Thus, a slight quantity of contaminations can be noticed. In this paper, Raman spectroscopy was has been utilized for studying the composition as well as the phases’ uniformity in the system of SrFe12O19. Raman spectra of the neat SrM are shown in Fig. 8. 
The strontium hexaferrite have been performed via comparing the noted outcomes with the chosen rules as well as the mode assignments debated via Kreisel et al. [32]. And, from the researches, it was documented that the (42) Raman-active modes (17E2g 11A1g + 14E1g + 17E2g) and (30 IR) active modes (13A2u + 17E1u) being anticipated for the system of hexaferrite. Also, the spectra of Raman have been obtained at the room temperature. The M-type strontium hexaferrite’s hexagonal structure has been constructed of (5) layers: Three cubic blocks of S* and S with a spinel structure, and two hexagonal blocks of R* and R that contain the ion of Sr2+. Such (5) layers make a single molecule, and (2) molecules make a single unit cell. And, the (24) ions of Fe3+ are dispersed above (5) various crystallographic locations, (3) octahedral locations (12k, 2a and 4f2), (1) tetrahedral location (4f1), and (1) trigonal-bipyramidal (2b) location, correspondingly. Spectra of the Raman of all samples in Fig. 8 reveals the sturdiest ions peak at almost (686 cm-1), which being ascribed to the bipyramidal set motions (A1g) of the ions of Fe–O (site 2b) [33, 34]. A feeble peak was noted at a (725 cm-1) frequency, which can be ascribed to the Fe–O ions movement (A1g) at the (4f1) tetrahedral locations. And, the 618 cm-1 as and 532 cm-1 bands are owing to the (A1g) as well as the modes of the (E1g) vibration of the bonds of Fe–O at the (4f2) octahedral location. Also, the 531 cm-1 and 469 cm-1 bands are owing to the modes of the (A1g) vibration of the bonds of Fe–O at the (2a) octahedral location. Besides the overhead peaks, the samples’ measured Raman spectra evinced that, other than the different bands widening, no fresh band was noted. And, this verifies that the samples being in a single-phase shape. The observed Raman spectra for most prominent peaks were indexed, and the relative condition of the noticed modes of vibration is shown in the Table 6. Such outcomes display slightly the shifts in some Raman modes of the (SrM) bands occurring toward the higher frequency values for Raman sites (12k.A1g , 4f2.A1g , and 2b.A1g). Such discrepancies are connected to the length of chemical bond. The perceived values of peak frequency in spectra have been compared with those in the researches for the sole crystals, [33] nanoparticles [35], and polycrystalline [36]. It was been seen that they’re in too virtuous covenant with those related with the sole crystals as well as nanoparticles. In Raman spectra, the peaks shifting toward the higher or lower wave no. is linked to the molecules’ chemical bond length. And, the shorter length of bond reasons to move the higher wave no. or vice versa. When the length of the chemical bond of molecules varies owing to any interior or exterior influences, then it may result in a movement to the wave no. toward higher or lower frequencies.  

Magnetic properties 
Fig. 9 demonstrates the magnetic hysteresis loops of SrFe12O19 for as-burnt and calcined at temperatures (1000°C, 1100°C). The materials have been measured at a (±15 KOe) magnetic field at the room temperature. The whole investigated samples indicated a famous magnetic hysteresis with a ferromagnetic conduct noted. The of saturation magnetization (Ms) values for every sample was computed from the magnetic hysteresis loops utilizing the model of Stoner-Wolfforth (S-W), which being practicable for a sole domain [37, 38]. And, the magnetic factors, like coercivity (Hc), saturation magnetization (Ms), and remnant magnetization (Mr) have been obtained from the (M-H) loops and are listed in the Table 7. Moreover, the ratio of squareness (S) has been computed employing the subsequent equation [39, 40]:

 
Form the results listed in Table 7 show the saturation magnetization (Ms) highest value is (66.2854 emu/g) performed at (15 KOe) for SrFe12O19, calcined at 1000°C, while the lowest value is (46.2680 emu/g) determined in the similar implemented field for the as-burnt temperature. It was confirmed that the calcination temperature has major influence on the saturation magnetization. The value of remnant magnetization (Mr) of the whole fabricated samples is ranged from (26.40 emu/g) to (34.57 emu/g), and such a behavior displays an increase of remnant magnetization with the increasing of calcined temperature of strontium hexaferrite. The squareness ratio (Mr/Ms) at calcined temperature of 1000°C for (SrM) was computed to be (0.496); the value under 0.5 is linked to the multi-magnetic domains. And, the obtained squareness value of (0.544) for a sample calcined at 1100oC, which being over a (0.5) theoretical value, signifying that the materials being a sole magnetic domain. In this paper, the noted value of Mr/Ms is too near to (0.5), proposing that the fabricated samples being in the sole magnetic domain [1, 41]. Fig. 9 manifests the Ms and Mr of as-burnt and calcined SrFe12O19 nanopowder at 1000°C and 1100°C.The obtained values of coercive field (Hc) were decreased in the range (4610.0-3995.0 Oe) with increasing calcined temperature of the synthesized MSr powder sample. In order to investigate the influence of calcined temperature on the behavior of saturation magnetization (Ms) and remnant magnetization (Mr), and Fig. 9 has been plotted for as-burnt and calcined temperatures at (1000 and 1100oC) of SrM nanopowders ferrite. Higher saturation and remnant magnetization value were found about (66.285 emu/g) and (38.90 emu/g), respectively for SrM calcined at 1000oC. The variation in magnetic characteristics can be ascribed to the crystalline size variation of the manufactured ferrite powders [13]. Furthermore, decreasing the (Fe/Sr) weight ratio as in Table 7 showed increasing the saturation magnetization (Ms) to (66.285 emu/g) and remnant magnetization to (38.9 emu/g) for calcined temperature 1000oC. And, this is owing to the decrease of the existence of Fe2O3 (0.7 wt.%) non-magnetic species. Also, the coercive force (Hc) of SrFe12O19 for as-burnt specimen has a higher value of (4610 Oe), and this lowered to (3995 Oe) with a decreased ratio of Fe2O3 up to zero at the calcined temperature of 1100oC. These results might be related to the residual Fe2O3 at lower calcined temperature having a higher intrinsic coercive force [42]. 

CONCLUSION 
In this current investigation, the technique of sol-gel auto-combustion was obtained to be a simple, efficient and convenient route for preparing the M-type Sr hexaferrite nanopowder. And, the X-ray diffraction outcome manifested that the manufactured powder was a neat phase hexagonal crystal system with space group P63/mmc. The powders formed were investigated using Rietveld refinement (full-prof) program which evinces the decrease of lattice constants (a and c) with the calcined temperatures. FE-SEM, EDX, Raman spectroscopy and VSM technique were utilized for further investigation. And, the work elucidated that an increment in the calcination temperature resulted in the considerable rise in the crystallite size as well as the last product crystallinity. Also, the calculated crystallite size is increased up to (76.107 nm) for calcination temperature 1100oC. The obtained data confirmed that the prepared materials were SrM with different surface morphologies and particle sizes in the range (79.84 nm-295.5 nm) with increasing calcined temperature up to 1100oC. The magnetic properties including, Ms, Mr, SQR, and Hc were investigated. Although the saturation magnetization (Ms) does not reveal the clear calcination temperature dependence at all, while the magnetic remnant (Mr) and coercivity (Hc) depict the clear calcination temperature dependence. 

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.