Investigation of structural, morphological and magnetic properties of nanostructured strontium hexaferrite through co-precipitation technique: Impacts of annealing temperature and Fe/Sr ratio

Abstract

M-type strontium hexaferrite (SrM) were successfully synthesized from Sr²⁺ and Fe³⁺ precursor salt through co-precipitation technique. Different higher sintering temperatures (800–1000 °C) were used to get the desired SrM with variation of Fe³⁺/Sr²⁺ mole ratio as well. The characterization of SrM and its properties were investigated using modern instrumental techniques viz. X-ray diffraction (XRD), Fourier Transform Infrared Spectrometer, Scanning Electron Microscopy, Vibrating Sample Magnetometer, UV–Visible NIR Spectrometer, Impedance Analyzer and Thermal Conductivity Meter. The phase of the synthesized SrM were confirmed by comparing the XRD patterns with the standard ICDD data and Reitvelt Refinement for the SrM having Fe³⁺/Sr²⁺ ratio 10 and SrM with distinct annealing temperature were performed. The structural parameters, particle size (75 nm–318 nm) and shape of the as prepared samples were changed with calcination temperature as well as mole ratio. The saturation magnetization (73.77–24.27 emu/g), coercivity (3732.28–642.10 Oe) and remanant magnetization (39.15–8.86 emu/g) were varied with calcination temperature and composition. The dielectric properties, optical properties and thermophysical properties were measured for the SrM keeping Fe³⁺/Sr²⁺ ratio 10 calcined at 1000 °C. The synthesized SrM can be applied in magnetic recording media and as photocatalyst due to its low coercivity (2764.48 Oe), high saturation magnetization (73.77 emu/g) and low band gap energy (Eg-2.04 eV) respectively.

1. Introduction

Hexagonal Strontium ferrites (SrM) are commonly referred to as M-type, which have the complicated unit structure and representative to the magnetoplumbite structure type AFe₁₂O₁₉. The 12 iron (Fe) ions are arranged in the unit structure according to the following phenomena one ion in the hexagonal site, nine ions in the octahedral positions, and two ions in the tetrahedral positions [1,2]. Blocks made of rocksalt and spinel make up its structure [3,4]. The oxidation state of all iron cations is Fe³⁺. The oxygen ions coordinate the iron ions tetrahedrally (FeO₄), trigonal bipyramidally (FeO₅), and octahedrally (FeO₆) during the crystallization of the hexagonal ferrite with space group P63/mmc. On 11 different basic positions, there are 64 ions per hexagonal unit cell in the structure. According to the Wyckoff notation, it has a ferrimagnetic structure, 24 Fe³⁺ ions are arranged on five different crystallographic positions-with three octahedral positions (12k, 4f2, and 2a), one tetrahedral position (4f1), and one bipyramidal position (2b) for each formula unit. The Gorter model's description of the ferromagnetic structure includes two anti-parallel (4f1 and 4f2) and three parallel (12k, 2a, and 4f2) sites that are connected via super-exchange interactions with the O2 ions [5,6]. This specific ferrite has a strong magnetocrystalline anisotropy and a high coercive field as a result of its atomic structure.

The strontium hexaferrite is of special importance since it has drawn the attention of numerous researchers due to its unique electromagnetic behavior and applicability in a variety of fields. This is due to its high saturation magnetization [7], high coercive force [8,9], high Curie temperature (740 K) [10], strong magnetocrystalline anisotropy [11], great resistance to corrosion, and high chemical stability [12], poor electrical conductivity (∼10⁵–10⁹ Ω-cm) [13]. Hard ferrite production is currently dominated by hexaferrite, which is still by far the most applicable material for real-world uses. According to real-world uses, SrM-based materials have made their way into a variety of technological and industrial applications, including fuel cells, [14], batteries [15], solar cells [16], sensors [17], perpendicular magnetic recording media [18], generator rotors in electric vehicles [19], hydrogen energy [20], magnetic coatings [21], magnetic fluids [22], and microwave catalysts [23].

Shaterian et al. synthesized SrM by sol-gel method using starch as coating agent calcined at 900 °C for 1 h to obtain the SrM with Ms value13.28 emu/g and Hc value 1000 Oe and investigate the hydrogen energy storage performance [20]. They obtained the highest hydrogen storage capacity of SrM nanostructures is about 3100 mA h/g. Furrer et al. investigate the recording performance SrM and they were achieved areal density of 317 Gb/in² [24]. Wang et al. synthesized SrM by coprecipitation method and investigate the photocatalytic activity under visible light [25]. They reported that 90.2% of degradation efficiency against MB dye.

However, various studies over the last decades have sought to create unique strategies for improving the structural, electrical and magnetic characteristics of hard magnetoplumbite type ferrites. The typical method of producing SrM materials is to sinter and calcine a mixture of carbonates and oxides at high temperatures [[26], [27], [28], [29]]. The primary disadvantage of this technique is the greater particle size and massive energy consumption. Although the ceramic approach is frequently applied to make SrM, the ball-milling step utilized in this technique causes adulterations. There were other alternate synthesis methods such as mechanochemical [30,31], co-precipitation [[32], [33], [34], [35]], hydrothermal [36], sol-gel process [37,38], microwave-induced combustion processes [39], and ultrasonic aided synthesis [17]. In comparison to the other approach, co-precipitation synthesis has advantages due to higher chemical homogeneity, a lower calcination temperature needed to generate the ferrite phase, and ferrite with finer particle size [40]. The preparation of nanomaterials is particularly well suited for co-precipitation. To dispersed the starting materials and the precursors utilized in a reaction to fabricate a desired product, the co-precipitation technique is becoming more and more useful. Coprecipitation is the process of preparing multicomponent materials by producing intermediate precipitates [41]. Because of these benefits, SrM was produced for the current investigation using the co-precipitation method.

The change of mole ratio has great impact on the crystal structure, morphology and magnetic properties of materials as well as effectiveness on various field [42]. Fu and Lin prepared SrM through microwave-induced combustion process having a Fe/Sr ratio of 11.6 which calcined at 1000 °C for 2 h. They have got pure SrM having a magnetic saturation field of 62 emu/g and an intrinsic coercivity of 1950 Oe [39]. Lim et al. investigate the consequences of annealing temperature on phase formation of SrM through the conventional ceramic method, and achieved single phase SrM following annealing at 1260 °C [29]. The co-precipitation method also used by Rajabi et al. have synthesized SrM varying molar ratio, and they discovered that highest magnetic saturation of the SrM sample with 58 emu/g was attained at 950 °C temperature for Fe³⁺/Sr²⁺ ratio 12 [43]. Rashad et al. fabricated SrM using co-precipitation method at different pH values and found that the highest saturation magnetization secured at pH 10 [8]. In this article they have reported that a single-phase SrFe₁₂O₁₉ was enhanced by raising the pH value to 11.5 at a Fe³⁺/Sr²⁺ mole ratio of 9.23.

In our case, we have concentrated our research on SrM as an alternative permanent magnet of metallic magnets, which is magnetically hard. It is well known that the microstructure of a calcined body has a significant impact on the characteristics of ferrite. Studying the impact of various processing parameters on the densification behavior of ferrite is required in order to manage its microstructure. This research focused on how the variation of calcination temperature and Fe³⁺/Sr²⁺ mole ratio affected the physical properties and finally we got the ideal condition in the manufacturing process by analyzing the crystal structure, crystallite size, microstructure and magnetic characteristics of SrM. The single phase SrM obtained by the co-precipitation process having a Fe³⁺/Sr²⁺ ratio of 10 and calcined at 1000 °C for 4 h having high density 4.93 g/cm³ (∼96% of theoretical value), high saturation magnetization (67.45 emu/gm), coercivity (3732.28 Oe) and remanant magnetization (31.22 emu/gm).

2. Experimental

2.1. Synthesis of SrFe₁₂O₁₉ nanopowders

The co-precipitation technique was followed for the concocting of SrFe₁₂O₁₉ nanopowders using high purity ferric chloride hexahydrate (FeCl₃.6H₂O) strontium Chloride hexahydrate (SrCl₂.6H₂O), sodium hydroxide (NaOH) and sodium carbonate (Na₂CO₃). The chemicals purchased from Merck, Germany and the laboratory prepared deionized water (DI) were used to make the precursor solutions.

In typical procedure to synthesized SrM, 3.0281 g of strontium chloride (0.0113 mol) and 36.6161 g of ferric chloride (0.1355 mol) were dissolved in DI water separately using hot plate with magnetic stirrer for SrM with a mole ratio of 1:12. Then, the above solutions were added together with constant stirring to mix homogeneously and subsequently mixture of base solution (10% NaOH and 10% Na₂CO₃) were added as precipitating media dropwise from burette until the pH reach to 13 which indicates the completion of precipitation. Thereafter, the precipitate was washed DI water assisted with centrifugation for 10–12 times to decrease the pH at 7–8 and eliminate the undesirable ions from solution like Na+, Cl- etc. Then, the residue was dried at 110 °C for 16 h' in an oven (Model: ED 53/E2, Brand: Binder, Germany) and dried materials was grinded for homogeneity. Finally, the powder was calcined at different temperatures viz. 800 °C, 900 °C, and 1000 °C for 4 h with 5 °C/min heating rate in a heating chamber (Model: LHT-8/16, Brand: Nabertherm, Germany) to get the desired ferrites. It was also prepared strontium hexaferrite by varying the Fe/Sr mole ratio 10, 11 and 12 with a calcination temperature 1000 °C for 4 h by taking precursor salts of strontium and iron in a stoichiometric amount.

2.2. Characterizations

The synthesized materials have characterized and their different properties have been measured using distinct sophisticated techniques. Thermogravimetric and differential thermal analysis (TG- DTA) was carried out using a thermal analyzer (model: Exstar 6300 TG/DTA instrument, Brand: Seiko Instruments Inc., Chiba, Japan) with a constant 10 °C/min heating from 20 up to 1000 °C. The structural behavior of the ferrite was studied using powder X-ray Diffraction (XRD) (SmartLab SE, Rigaku, Japan) with Cu K-alpha radiation (λ = 1.540598 Å), at a scan rate of 30°/min, and step size of 0.02°. For Rietveld refinement, XRD powder data analysis with GSAS II software. Fourier transformed infrared (FT-IR) spectrophotometer (Model: Frontier, Brand: Perkin-Elmer, UK) was used to investigate the metal-oxygen (M − O) bond nature with the frequency range 400–4000 cm⁻¹ at a spectral resolution of 4 cm⁻¹.

The microstructure and morphology of calcined SrM was studied with the aid of scanning Electron Microscopy (SEM, Model: MA15 VP-SEM, Brand: Carl Zeiss Evo, UK). Average particle size was evaluated by image-J software using about 100 particles arbitrarily. The laser diffraction-based particle size distribution of the synthesized materials was done by particle size analyzer (Model: Mastersizer 3000, Brand: Malvern Instruments Ltd, UK). A distinct amount of powder was dispersed in DI water to measure the particle size as well as the particle distribution of the said SrM. The magnetic behavior of the calcined SrM powders were assessed at room temperature using vibrating Sample Magnetometer (VSM, Model: EV VSM System, Brand: Microsense, USA) at a highest applied field of 20 kOe. The dielectric and electrical properties of the SrM materials were measured by impedance analyzer (model: 65120B, Wayne Kerr Electronics, UK) in the frequency range from 100 Hz to 20 MHz at room temperature using dielectric measurement fixture. The powder was mixed homogenously with 3% polyvinyl alcohol (PVA) solution in water as binder by mortar-pestle. After drying, the powder was pelletized at 13 mm diameter by a press machine using 5 KN pressure. The pellet was then calcined at 500 °C for 1 h with a heating rate 90 °C/h to remove the PVA.

3. Results and discussion

3.1. Thermal analysis

Fig. 1 shows TGA curve of the intermediate dry powder derived from the co-precipitation route with mole ratio at Fe/Sr = 11 employing thermogravimetric analyzer at the ramping rate of 10 °C/min to monitor the structural alteration, such as the weight loss and phases transformation during heating process. The evaporation of the absorbed water that causes a significant mass loss is happened from RT to around 275 °C, where the sample displays a 4% of weight loss as shown in Fig. 1a. The DTA curve (Fig. 1b) shows an endothermic reaction at about 244 and 280 °C that is connected to water loss.

Fig. 1.

(a) DTG and %weight loss, and (b) DTA curve for un-calcined sample of SrM with Fe³⁺/Sr²⁺ mole ratios 11.

While heat treatment process was carried out, a steep declivity of weight loss (16%), which is likely related to the conversion of hydroxides to oxides, is seen from 300 to 1000 °C. The major weight loss that occurs after 964 °C may be a solid-state reaction connected with the progressive growth of the SrM phase. Weight loss occurs between 800 and 1000 °C as a result of precursor breakdown, and structural change from hematite to hexaferrite. The DTA study shows that the strontium hexaferrite can be produced in an endothermic peak at 964 °C. According to the formation of Fe₂O₃ simultaneously with SrM, the Fe₂O₃ phase begins to disintegrate around 964 °C [44].

3.2. X-ray diffraction

Fig. 2 depicts the XRD patterns of SrFe₁₂O₁₉. ICDD card no. 24–1207 was used to index the diffraction peaks of SrFe₁₂O₁₉ at different calcination temperatures and molar ratios (Fe³⁺/Sr²⁺ = 12, 11, 10). A series of studies were conducted at various calcination temperatures viz. from 800, 900 and 1000 °C for 4 h, to clarify the impact of annealing temperature on the synthesis of SrM samples. In Fig. 2a, the XRD patterns of the synthesized sample of SrM powders at various calcination temperatures keeping a Fe³⁺/Sr²⁺ ratio of 12 were displayed. The XRD peaks show the creation of a hexagonal magnetoplumbite structure and the lack of any impurities at calcination temperature 800 °C and 900 °C. The major diffraction peaks of SrM at 2θ = 32.3°, 34.1°, 34.5°, 37.1°, 40.3°, 55.1°, 56.8°, and 63.1° are assigned to the (107), (114), (200), (203), (207), (217), (2011) and (220) crystal planes, respectively [1,45]. However, a few peaks for α-Fe₂O₃ phases were seen in the diffraction pattern after the annealing at 1000 °C, although the strength of the peaks are less. Table 1 displays the structural properties of all SrM samples. Classical Scherrer formula Eq. (1) is used to calculate the crystallite size of all samples [46].

$$\tau =\raisebox{1ex}{$\mathrm{s}\lambda $}\!\left/ \!\raisebox{-1ex}{$\mathrm{\beta }\cos \mathrm{\theta }$}\right.$$ (1)

where τ signifies the crystallite size, s = 0.94 as indicates the shape factor, β denotes the full line width at half maximum intensity (FWHM), λ represents the wavelength of the X-ray radiation whose value is CuKα = 1.5406 Å and θ is the diffraction angle in radians.

Fig. 2.

XRD patterns of the SrM as (a) annealed at distinct temperature, and (b) different mole ratio.

Table 1.

Lattice parameters of SrM synthesized at distinct temperature and Fe/Sr ratio.

Calcination Temperature	800 °C	900 °C	1000 °C
Crystal Size (nm)	28	34	34
a = b (Å)	5.91195	5.91026	5.93451
c (Å)	23.66628	23.68258	23.61171
X-ray Density, Dx (g/cm3)	4.92	4.92	4.90
V (Å3)	716.345	716.429	720.1571
Mole Ratio (Fe3+/Sr2+)	10	11	12
Crystal Size (nm)	30	34	34
a = b (Å)	5.91437	5.91339	5.93451
c (Å)	23.60831	23.62178	23.61171
X-ray Density, Dx (g/cm3)	4.93	4.92	4.90
V (Å3)	715.1751	715.3474	720.1571

It is evident that, the lowest crystallite size obtained at 800 °C (28 nm), and the highest size was obtained at 1000 °C (34 nm), with Fe³⁺/Sr²⁺ ratio 12. The amalgamation of particles is the fundamental reason for this increase. Accordingly, the height of the peaks and crystallite size enlarge as a result of the atoms having adequate energy to move towards a stable position at higher temperatures [46]. The demonstration of enlargement in crystallite size that shows the samples enhanced crystallinity with a rise in calcination temperature is a steady increase in the height of diffraction peaks and a shrinking of the peak breadth. This is frequently attributed to the particle grain development in the nano-region to relatively low surface energy [47,48].

In comparison, XRD analysis was used to investigate the impacts of the Fe³⁺/Sr²⁺ ratio on the structure of SrM which were annealed at 1000 °C. The XRD patterns of SrM powders with distinct Fe³⁺/Sr²⁺ ratios (12, 11, 10) calcined at 1000 °C were shown in Fig. 2b The evolution of SrM pure phase was enhanced by reducing the Fe³⁺/Sr²⁺ mole ratio. The XRD patterns of the product at a mole ratio of Fe³⁺/Sr²⁺ of 10 revealed the presence of single phase which is matched with standard SrM (ICDD # 24–1207). The excellent crystalline structure of the synthesized SrM is shown by well-defined, sharp peaks, which supported previous work [49]. However, SrM keeping a Fe³⁺/Sr²⁺ ratio of 11 and 12 have α-Fe₂O₃ impurity phase. From XRD peak analysis, it can be concluded that the Fe³⁺/Sr²⁺ mole ratio has a significant effect on the SrM phase. This finding will be discussed further in the context of SEM micrographs. Classical Scherrer formula also was employed to determine the crystallite size of SrM by using the most intense peak.

The crystalline size was summarized in Table 1. The results showed that reducing the Fe³⁺/Sr²⁺ ratio lead to increase the crystalline size. The crystallite size is 30 nm when the mole ratio 10 which is increases to 34 nm for Fe³⁺/Sr²⁺ ratio 12. Based on XRD patterns of the SrM powder, the lattice constants ‘a’ and ‘c’ were determined from the major peaks obtained for (107), (114), (203), (205), (217), (2011), and (220) planes of the SrM-phase. The following Eq. (2) is used to calculate the lattice constants [50]:

$${\mathrm{d}}_{\mathrm{h}\mathrm{k}\mathrm{l}}={\left[\left(\raisebox{1ex}{$4$}\!\left/ \!\raisebox{-1ex}{$3{\mathrm{a}}^2$}\right.\right)\left({\mathrm{h}}^2+{\mathrm{k}}^2+\mathrm{h}\mathrm{k}\right)+\left(\raisebox{1ex}{${\mathrm{l}}^2$}\!\left/ \!\raisebox{-1ex}{${\mathrm{c}}^2$}\right.\right)\right]}^{1/2}$$ (2)

Table 1 listed the calculated lattice constants for magnetic SrM powders. The lattice parameters and unit cell volume are found to increase with calcination temperature. The lattice constant c rises from 23.666 to 23.68258 Å as an annealing temperature increases from 800 to 900 °C. The sample obtained at 900 °C has the most enhanced c value, while the sample obtained at 1000 °C have the lowest c values. The lattice constant c declined from 23.775 to 23.6085 Å as the mole ratio Fe³⁺/Sr²⁺ decreases from 12 to 10. Furthermore, the value of lattice parameter remains almost unchanged. On this matter of consequences, the magnetocrystalline anisotropy is influenced by c/a value that is relevant to c direction. All powders have a c/a value that is close to 4.0. As a result, it is revealed that there is no structural deformity with changing calcination temperatures [29]. The values of X-ray density (Dx) of M-type hexaferrite were estimated using the following Eq. (3) [6]:

$${\mathrm{D}}_{\mathrm{x}}=2\mathrm{Z}\mathrm{M}/\sqrt{3}{\mathrm{N}}_{\mathrm{A}}{\mathrm{a}}^2\mathrm{c}$$ (3)

Here, Z denotes the number of molecules contains each unit cell, N_A the Avogadro's number, a and c the lattice parameter which determined through XRD study, and M represents the molecular weight of the samples.

The Rietveld refinement of the XRD pattern of the SrM obtained at the calcination temperature 1000 °C with mole ratio 10vcarried out using GSAS II tool to investigate the crystal structural optimizations. Starting structural variables for SrM were derived following the researcher Muller and Collomb [51]. Scale parameters, zero point, 15 coefficients of the relocated Chebyschev assignment to adjust the framework, unit cell structure, location of the atoms, isotropic superseded factors, instrumental rearrangement coefficients (one Gaussian, GW, two Lorentzian components, Lx and Ly, and peak modification due to impacts of asymmetry), and preferred arrangements towards [001] of SrM using March-Dollase estimation were all refined. Unit cell size and accessory component phase fractions were also adjusted.

The output of the Rietveld refinement on the SrM XRD pattern at 1000 °C with a mole ratio of 10:1 is shown in Fig. 3a and b. With a c/a ratio of 3.916, the refined lattice parameters are a = 5.88203 Å and c = 23.0348 Å, and 6.76 g/cm³ is the estimated X-ray density based on unit cell volume as indicated in Table 2. Verstegen and Stevels claim that measuring the c/a ratio can be applied to determine the kind of structure because a magnetoplumbite structure can be presumed when the ratio is achieved less than 3.98 [52]. According to the standard, the pattern has been indexed. The soundness of the refinement is approximated by weighted residual error (wR_factor 0.01866 and wR expected 0.01535), and goodness of fit (GoF = 1.50). Average bond length Fe–Sr, Fe–O and Sr–O are 3.5711, 1.97622 and 2.9956 respectively. Average bond angle of O–Fe–O are 103.3.

Fig. 3.

(a) Rietveld refined PXRD pattern of SrM with Fe³⁺/Sr²⁺ = 10 mol ratio and (b) crystal structure visualization by Vesta.

Table 2.

Bond length, bond angle and structural parameter of SrM obtained at 1000 °C Fe/Sr with mole ratio 10.

Bond length		Interatomic angles
Sr–Fe2	3.4045	O1–Fe2–O3	098.439
Sr–Fe4	3.6706	O3–Fe2–O3	117.885
Sr–Fe5	3.6384	O4–Fe3–O4	106.493
<Sr–Fe>	3.5711 (19)	O3–Fe4–O5	090.902
Sr–O3	2.9420	O3–Fe4–O1	115.878
Sr–O5	3.0493	O1–Fe5–O4	100.271
<Sr–O>	2.9956 (17)	O1–Fe5–O5	093.274
Fe1–O4	2.2990	< O–Fe–O>	103.300 (21)
Fe2–O1	1.6379	Crystal Data and Figure of merits
Fe3–O2	1.7866	a = b (Å)	5.88203 (18)
Fe4–O3	2.0667	c (Å)	23.0348 (5)
Fe5–O1	2.0909	V (Å3)	690.190 (20)
< Fe–O>	1.97622 (6)	RF2 wR	0.69089
		wR_expected	0.01866
		X2	0.01535 1.50

The Rietveld refinement of the XRD pattern also carried out for SrM obtained at different calcination temperature as shown in Fig. 4a–c. The output of the Rietveld refinement result such as cell parameters, weighted residual error (wR_factor and wR expected), goodness of fit (GoF), average bond length (Fe–Sr, Fe–O and Sr–O), and average bond angle (O–Fe–O) were listed in Table 3.

Fig. 4.

Rietveld refined PXRD pattern of SrM at (a) 800 °C, (b) 900 °C, and (c) 1000 °C.

Table 3.

Bond length, bond angle and structural parameter of SrM obtained at(a) 800 °C, (b) 900 °C, and (c) 1000 °C.

Sample calcined at	800 °C	900 °C	1000 °C
a = b (Å)	5.8851 (7)	5.8845 (5)	5.8826 (9)
c (Å)	23.0577 (19)	23.0528 (13)	23.0373 (23)
V (Å3)	691.60 (8)	691.31 (5)	690.39 (10)
RF2	0.06146	0.06221	0.08521
wR	0.023461	0.03545	0.03661
wR_expected	0.01278	0.01385	0.01282
GoF	2.13	2.21	2.39
Average bond length (Å)
<Sr–Fe>	3.6459 (3)	3.4045 (5)	3.6694 (4)
<Sr–O>	2.9130 (11)	2.8833 (2)	2.9427 (4)
< Fe–O>	1.9355 (8)	2.0302 (5)	2.0051 (4)
Avg. Interatomic angles (deg.)
< O–Fe–O>	109.737 (16)	99.001 (5)	102.403 (6)

3.3. FT-IR

It is feasible to foretell the existence of several bonds in a material using FT-IR spectroscopy. The results of an FT-IR examination between 400 and 4000 cm⁻¹ that was conducted at room temperature are shown in Fig. 5. Octahedral metal-oxygen bond stretching is attributed to the peak at low wavenumber, whereas tetrahedral metal-oxygen bond stretching is responsible for the peaks at high wavenumber. The longer bond length of octahedral bonds—which will have lower stretching frequency than tetrahedral bonds—is the reason why they exhibit less stretching frequency than tetrahedral bonds [53].

Fig. 5.

FT-IR Spectrum of SrM with Fe³⁺/Sr²⁺ = 10 mol ratio.

The curve shows that the adsorption bands at 448, 548.5, and 594 cm⁻¹ matches to typical SrM adsorption bands [54,55]. These peaks are caused by the metal-oxygen (M − O) bond stretching vibrations. The vibration of Fe³⁺-O bond stretching located at tetrahedral and octahedral position are represented by the frequency absorption bands at 594 cm-1 and 448 cm⁻¹ respectively, while the characteristic peaks at 548.5 cm-1 are connected to the Sr–O stretching vibration band. The broad lump in the region of 3000–3400 cm⁻¹ because of O–H stretching vibrational mode and the band about 1643 cm⁻¹ corresponds to bending vibration of water molecules chemically bonded with SrM [56]. The vibrational band at about 1445 cm⁻¹ attributed to the presence of metal-oxygen-metal for Fe–O–Fe [57]. The large FT-IR peaks at 1200-2000 cm⁻¹ and 3500-4000 cm⁻¹ may also be attributable to H₂O decomposition [58].

3.4. Morphological studies

The SEM technique was applied to investigate the morphology of the synthesized SrM particles as presented in Fig. 6 and to estimate the average particle size of the said materials was determined using ImageJ software by counting about 100 particles in each case to draw the histogram as well. The SEM images of co-precipitated samples having a Fe³⁺/Sr²⁺ molar ratio of 12 calcined at distinct temperatures (800, 900, and 1000 °C) for 4 h were shown in Fig. 6a-c. The particles shape was enlarged hexagonal in structure and size of the particles are equal in nature at annealing temperature 800 °C with some particles having flower like shape (Fig. 6a) which were increased with calcination temperature 900 °C. The SrM powders had a plate-like hexagonal shape and the particles were higher in size as the calcination raised to 1000 °C (Fig. 6c). The average particles size was obtained 75 nm, 155 nm and 252 nm with heat treatment at 800, 900 and 1000 °C respectively. Therefore, using heat treatment to increase the crystallinity and obtain homogeneous nanopowders is challenging for SrM. The data demonstrate that when calcination temperature rises, the mean particle size increases due to agglomeration which comply with previous research [59]. The shape of the particles also changes with temperature.

Fig. 6.

SEM micrograph of SrM annealed at (a) 800 °C, (b) 900 °C, (c) 1000 °C with Fe³⁺/Sr²⁺ mole ratio 12 and Fe³⁺/Sr²⁺ mole ratio of (d) 11, (e) 10.

From Fig. 6d, it is evident that the SrM samples having a Fe³⁺/Sr²⁺ ratio of 11 had hexagonal particles that have average particles size 290 nm. The SrM samples having a Fe³⁺/Sr²⁺ ratio of 10 had a coarse, plate-like hexagonal form and a wide particle size distribution, as seen in Fig. 6e. When two or more oxide particles come together, they generate larger, differently sized oxide particles. At calcination temperature of 1000 °C, this phase transition of the oxide particles diffused and the SrM materials are obtained.

3.5. Particle size analysis

The particle size distribution of the prepared SrM were determined following laser diffraction technique as depicted in Fig. 7. SrM particles calcined at 800, 900, 1000 °C with mole ratio Fe³⁺/Sr²⁺ 12, and variation of Fe³⁺/Sr²⁺ mole ratio 10, 11, and 12 which calcinations at 1000 °C give maximum peak in the area about 12 μm and 150 μm. Particle size estimation by laser diffraction provides hydrodynamic diameter, therefore the findings do not accord with those from scanning electron microscopy. The diameter measured by hydrodynamics is always significantly bigger than the size predicted by SEM [60].

Fig. 7.

Particle Size distribution curve of SrM obtained from laser-diffraction analysis.

The specific surface area was 34.74, 33.06, 29.60 m²/kg for the sintering temperature 800, 900, 1000 °C with mole ratio 12 and 22.26, 22.19 m²/kg for the Fe/Sr mole ratio of 11, 10 with firing temperature 1000 °C. It is noted that the specific surface area was declined with rising particle size which comparable with literature [61,62]. The detailed report of particle size analysis has been visualized in Table 4.

Table 4.

Particle size of SrM obtained from laser diffraction and SEM analysis.

Calcination Temperature (°C)	10% Particles Size, ${\mathbf{D}\mathbf{v}}_{\mathbf{10}}$ (μm)	50% Particles Size, ${\mathbf{D}\mathbf{v}}_{\mathbf{50}}$ (μm)	90% Particles Size, ${\mathbf{D}}_{\mathbf{90}}$ (μm)	Particle Size, nm (from SEM)
800	6.83	31.6	163	75
900	13.8	96.2	296	155
1000	15.8	93.4	281	252
Mole Ratio Fe3+/Sr2+
11	21.8	114	394	290
10	20.8	118	376	318

3.6. Magnetic properties analysis

Fig. 8 portrayed typical magnetization hysteresis loops of SrM. The precursor obtained at Fe³⁺/Sr²⁺ ratio 12 with pH 13 and calcined at 800 °C for 4 h yielded a higher saturation magnetization (Ms) value of 73.77 emu/g, which is near to the theoretical value (74.3 emu/g) of the magnetic saturation of magnetoplumbite SrM [59,63].

Fig. 8.

Magnetic hysteresis loops of SrM nanoparticles (a) calcined at different temperature and (b) various mole ratio.

Magnetic domain size is directly correlated with particle size. The atomic spin numbers toward the applied magnetic field grow in proportion to the magnetic domain size, which enhances saturation magnetization. According to Okuyama et al., notwithstanding of the crystalline structure or shape of the particulates, the Ms increases with increasing particle size. After passing through the superparamagnetic boundary, the coercivity and magnetic remanence values are escalated with particle size, reaching a highest at the critical size of ∼76 nm. Exceeding this critical size, the Hc and Mr values falling off as the particle size rises, and nanoparticles transformation from a single- to multi-domain system. The Hc value decreases due to the multiple orientations of the crystallites within the multi-domain-structured nanoparticles [64].

The saturation magnetization obtained at 800 °C was 73.77 emu/g, which decreased to 24.27 emu/g when the annealing temperature was raised to 1000 °C. Correlation of the SEM analysis with the magnetic behaviors of co-precipitated sample having a Fe³⁺/Sr²⁺ ratio 12 calcined at various temperatures, it is clear that the calcination influenced considerably the magnetic behaviors of SrM materials as depicted in Fig. 8a [65]. SrM powders calcined at 800 °C with homogeneous particle size arrangement and hexagonal structure attained highest saturation magnetization. This is owing to the existence of very defined crystalline nature of SrM having single domain system and microstructure homogeneity with particle size 75 nm. The saturation magnetization dropped when the sintering temperature was raised to 900 °C and above. The generated SrM powders at 1000 °C with a prolonged branching hexagonal structure and porosity between the crystals with particles size 252 nm, which far more than critical particle size showed poorer magnetic characteristics than the synthesized powder calcined at 800 °C as reports in Table 5.

Table 5.

Effect of annealing temperature and mole ratio on saturation magnetization (Ms), retentivity (Mr), coercivity (Hc), magnetic anisotropy (K) and rectangular ratio (RS).

Effects of Calcination Temperature and Composition on Magnetic Properties
Temperature (°C)	Coercivity Hc (Oe)	Saturation Magnetization Ms (emu/gm)	Remanant Magnetization Mr (emu/gm)	Magnetic anisotropy K × 103(erg/cm3)	Rectangular ratio RS(Mr/Ms)
800	2764.48	73.77	39.15	203.90	0.53
900	1474.65	31.25	12.57	46.08	0.40
1000	642.10	24.27	8.86	15.58	0.36
Composition Fe3+/Sr2+
12	642.10	24.27	8.86	15.58	0.36
11	1028.76	32.03	12.87	32.95	0.40
10	3732.28	67.45	31.22	251.74	0.46

The Hc of the materials was also depend on particle size. It improved when particle size was augmented with increasing sintering temperature until single domain converted to multi-domain. The fact that these samples had higher calcination temperatures and lower remanence magnetization and coercivity suggests that they were multi-domain in nature. In case of our study, it is obvious that coercivity drops as calcination temperature rises. The multi-domain behavior of the grains is thought to be responsible for the abatement of Hc with rising calcination temperature. This multi-domain feature is suggested by the decrease in coercivity brought on by increasing the temperature over 800 °C.

For contrast, the consequence of mole ratio on the magnetic behaviors of the yielded SrM were investigated at Fe³⁺/Sr²⁺ ratio 10, 11, and 12 and displayed in Fig. 8b. The findings manifested that the Ms was increased from 24.27 to 67.45 emu/g, and the Hc was increased from 642.10 to 3732.28 Oe with mole ratio decreased from 12 to 10. The sample with the maximum Ms (67.45 emu/g) and Mr (31.22 emu/g), as well as a comparatively high intrinsic coercivity (3732.28 Oe), is one having a Fe³⁺/Sr²⁺ ratio of 10. High purity and crystallinity are to responsible for this, which is in accordance with the structural analysis. Moreover, a lower than stoichiometric Fe/Sr ratio can result in the formation of iron and oxygen vacancies, which could increase the magnetic characteristics of strontium ferrite powder by enhancing the diffusion of different ions [39]. As expected, the presence of the non-ferromagnetic intermediate phase, α-Fe₂O₃, accounts for the low saturation magnetization values as well as coercivity at high Fe/Sr ratios [27].

The increase in magnetic anisotropy of the SrM grains, which prevents the magnetic moment to orient in the direction of applied field, is responsible for the increase in saturation magnetization and coercivity [66].

Eq. (4) is used to compute the magnetic anisotropy (K) values [60], and listed in Table 5.

$$k=\frac{M_s\times H_c}{0.96}$$ (4)

The level of crystalline nature, the proportion of the surface area and volume, and the behavior of interconnection between the nanoparticles affect the magnetic anisotropy [60,65].

The reorientation of magnetization of the materials to its easiest axis determined from the rectangular ratio (R_S) as in Eq. (5)

$$R_s=\raisebox{1ex}{$M_r$}\!\left/ \!\raisebox{-1ex}{$M_s$}\right.$$ (5)

The rectangular ratio (RS) is attained between 0.36 and 0.53. The particles interact via magnetostatic interactions since the RS value is smaller than 0.5 [71,72]. The aggregation of SrM nanoparticles, which is visible in SEM pictures as well, is caused by magnetostatic interactions. Strontium ferrites are excellent candidates for magnetic recording applications because of their low coercive field and strong saturation magnetization. Along with a suitably low value of the coercive field, the squareness ratio that determines the squareness of the hysteresis loop, is also −0.46 for the sample calcined at 1000 °C [73]. A detailed comparison of magnetic properties of the current findings with previous literature are listed in Table 6 [39], [67], [68], [10], [69], [70], [39], [67], [68], [10], [69], [70],.

Table 6.

Magnetic properties of SrM reported in previous work.

Synthesis Routes	Temperature (°C)	Composition Fe3+/Sr2+	Coercivity Hc (Oe)	Saturation Magnetization Ms (emu/gm)	Remanent Magnetization Mr (emu/gm)	Ref
Microwave-combustion	1000	11.6	1950	62	–	[39]
Sol-gel	800	11.6	6267.5	51.92	30.33	[67]
co-precipitation	100	12	4625	68	41	[68]
Sonochemical	1100	12	2600	44	27	[10]
Sol-gel	1100	12	5500	60	–	[69]
Sol-gel	900	12	1318	58.83	30.40	[70]
co-precipitation	800	12	2764.48	73.77	39.15	Present work
co-precipitation	1000	10	3732.28	67.45	31.22	Present work

3.7. Dielectric properties

Fig. 9 displays the frequency dependent dielectric behaviors of SrM for Fe/Sr mole ratio of 10. The dielectric constant was determined using parallel plate capacitor relation as Eq. (6) [74]:

$${\epsilon }^{\prime }=\frac{Cd}{{\epsilon }_{0}A}$$ (6)

where C describes the capacitance of the test sample in farad, d signifies the thickness of the test sample in meter, A indicates the cross-sectional area of the test sample and ${\epsilon }_0$ denotes the dielectric constant of free space.

Fig. 9.

Frequency dependency survey of dielectric constant (ε′), dielectric loss (ε′′) and dielectric loss factor (tan δ) of SrM for mole ratio 10.

The following relation as shown in Eq. (7) can be used to determine the dielectric loss tangent factor [74]:

$$\mathit{tan}\delta =\frac{1}{{2\pi fR}_p{C}_p}$$ (7)

where, Rp and Cp are representing parallel resistance and capacitance respectively, and $\delta$ indicates the loss angle, f represents the frequency.

The tangent loss factor allows us to estimate the dielectric loss (${\epsilon }^{″}$) by following Eq. (8) [74]:

$${\epsilon }^{″}={\epsilon }^{\prime }\mathit{tan}\delta$$ (8)

It has been noted that dielectric constant (Ɛ′) has higher in magnitude at lower frequencies and Ɛ′ gradually declines with frequency then reaches a constant value. Koop’s (1951) used the Maxwell-Wagner (1913) two-layer model to describe how interfacial (inter granular) polarization could account for the drop in Ɛ′ with frequency [75,76]. These ferrites all contain grain borders that conduct electricity poorly, separating the well-conducting grains. In the polarization process, electrons are largely exchanged between Fe³⁺ and Fe²⁺ ions, and in order to do this, they must pass through poorly conducting grain borders that block their flow, creating electron buildup near the boundaries, as a result space charge polarization are increased [74].

There will arrive a point where the space charge conveyors fail to hold up with the field and the switching of the field, leading to a declined the materials dielectric constant. Space charge conveyors in a dielectric material need a definite time to orient their axes parallel to a fluctuating electric field. The space charge conveyors will eventually barely have begun to travel before the field overturns and has no impact on the polarization of the dielectric as the frequency of the field increases [77]. Fig. 9 shows the frequency dependence dielectric loss. Dielectric loss (${\epsilon }^{″}$) manifests when this polarization falls behind the applied ac electric field. The main cause of this dropping is the solids structural flaws or defects [78]. A rapid reduction in dielectric loss is seen as frequency rises. The reason for this decline is that at high frequencies, the dipole motions are unable to keep up with the changes in the external field. This is because of low conductivity through the grain limit at low frequencies. Because more energy is needed, a considerable amount of energy is lost during the exchange of electrons between Fe³⁺ and Fe²⁺ ions. Because less energy is needed for the exchange as frequency rises, dielectric loss rapidly reduces [77,79]. Due to same reason dielectric loss tangent also decrease as a result of frequency increases. In addition, it also noticed that dielectric loss tangent decrease as a result of frequency increases and then increases with frequency and again decreases to reach constant as shown in Fig. 9. It is clear that SrM displayed a slight aberrant behavior at various frequencies. Rezlescu's model suggests that both p and n type charge carriers may have contributed individually to the relaxation peak [80]. That behavior is also caused by the electrical interactions between Fe²⁺ and Fe³⁺ [81]. The dielectric loss tangent also depends on conductance, with higher value of conductance exhibits higher dielectric losses.

Impedance spectroscopy is an essential tool for understanding the electronic characteristics of ferrites because the impedance of grains can be distinguished from elsewhere impedance contributors, such as the impedance of electrodes and grain barriers. Fig. 10 depicts the real part (Z′) and imaginary part (Z″) of impedance as a function of frequency. With an increase in frequency, it is discovered that Z′ and Z″ values decrease. While discussing Z″ as a function of frequency, there are three general relaxation dissipation circumstances that may arise for solids: charge carriers associated with conductive systems, dielectric-system dispersion typically caused by the dipole circulation of lattice structures, and the coexistence of both kinds of dispersions within the determined frequency limit. It is taken for granted that crystalline plates stacked on top of one another make up ferrite material. A sample is considered to be a microstructure from the perspective of its parallel conducting plates (grains), which are disconnected by resistive plates (grain boundaries) [82].

Fig. 10.

Variation of real part (Z′) and imaginary part (Z″) of impedance of SrM for mole ratio 10.

3.8. Optical properties

Optical properties were determined by measuring the Diffuse reflectance of the sample SrM with a Fe/Sr mole ratio 10 calcined at 1000 °C for 4 h. Reflectance ($R_{\infty }$) was transformed to absorbance (K/S) by the Kubelka–Munk relation as presented in Eq. (9) [83,84]:

$$\raisebox{1ex}{$K$}\!\left/ \!\raisebox{-1ex}{$S$}\right.={\left(1-R_{\infty }\right)}^2/2R_{\infty }$$ (9)

Normally, when the substance absorbs the energy of light as photons, electrons are stimulated from the valence to conduction band. The diffuse reflectance spectrum and absorbance spectrum as a function of wavelength are shown in Fig. 11a. In addition to the peaks shown at 440 and 480 nm, which are reflection peaks, the spectra clearly show absorption peaks at 390,460 and 510 nm. Both an absorption peak (730 nm) and a reflection peak (700 nm) are visible at longer wavelengths.

Fig. 11.

(a) UV visible absorption spectra (b) Bandgap energy obtained Kubelka Munk function of SrM for mole ratio 10.

The optical band gap (E_g) was estimated depending on the absorption spectrum utilizing the Kubelka–Munk (K–M) function. In accordance with Kubelka–Munk (K–M) function, the ${\left[F\left(R_{\infty }\right)\times h\vartheta \right]}^n$ with ($h\vartheta$) curves was utilized to determine the Eg employing Eq. (10) as follows [84]:

$$\left[F\left(R_{\infty }\right)\times h\vartheta \right]=A{\left(h\vartheta -E_g\right)}^n$$ (10)

where $F\left(R_{\infty }\right)$ signifies absorbance, ($h\vartheta$) indicates the photon energy and n rely on the category of the movement of electrons from valence band to conduction band, and n = 1/2 used for the direct band gap. The plots of the obtained powders are shown in Fig. 11b. The observed value for the sample is 2.04 eV which is similar with the previous works [85,86].

3.9. Thermal conductivity

Fig. 12 shows the temperature dependent thermophysical behavior of SrM having Fe³⁺/Sr²⁺ ratio 10 calcined at 1000 °C for 4 h. Laser flash analysis (LFA) was utilized to investigate the thermal diffusivity, thermal conductivity and specific heat. LFA was carried out in a graphite furnace equipped with a Type S thermistor for measuring the temperature using an alumina shielding tube and alumina specimen chambers.

Fig. 12.

Temperature variance of thermal diffusivity, thermal conductivity and specific heat of SrM for mole ratio 10.

The LFA HyperFlash is an upright system that includes the light source at the lower portion, the sample in the middle, and the detector on the upper portion, where lower face of the sample is equally illuminated by a xenon light flash. Using an infrared InSb detector, the temperature rise on the back side is examined as a function of time. The thermal diffusivity (α) may be calculated using observed temperature/time curve by Eq. (11) [87].

$$\alpha =0.1388\times \raisebox{1ex}{$d^2$}\!\left/ \!\raisebox{-1ex}{$t_{1/2}$}\right.$$ (11)

where d refers to thickness of the pellets in mm, and $t_{1/2}$ refers the time requires to 50% of the heat rises. The thermal conductivity (λ) was calculated using Eq. (12) which associated with the bulk density (ρ) [87];

$$\mathrm{\lambda }\left(\mathrm{T}\right)=\alpha \left(T\right)\rho \left(T\right)C_p\left(T\right)$$ (12)

where $C_p$ signifies the specific heat of the sample which calculated by making comparison with a reference sample. In this case, pyroceramics was used as reference material having density 2.749 gcm⁻³, thickness 2.497 mm and diameter 12.56 mm.

From obtained result, it can be seen that thermal diffusivity and thermal conductivity decrease with increasing heat up to 100 °C and then reach a nearly constant value as listed in Table 7. The thermal diffusivity of the materials depends on the quantity of phonons present in the basic unit structure. When the temperature is raised, the number of phonons increases due to overtone combination excitation factor. As a result, the diffusivity inclined up to 100 °C. Above 100 °C temperature, the excitation factor of phonon become saturated and the diffusivity almost constant. On the other hand, as temperature increases induces thermal expansion and crack which reacting as scattering points for the heat transporting phonons made a contact resistance between materials grain [88,89]. That’s why, it was observed the decline of conductivity with rising temperature. Above 100 °C temperature, number of phonons are no longer increase with in a specific lattice and the path of phonon to move freely is about constant. As a result, the conductivity is near about same at elevated temperature. The specific heat capacity minute decrease with rises temperature and then increase again [88,90]. Overall, specific heat capacity has no regular significant change with temperature, whereas thermal diffusivity and thermal conductivity have a remarkable temperature dependence up to 100 °C.

Table 7.

Reports of thermal diffusivity, thermal conductivity and specific heat of SrM as a function of temperature.

Temperature (°C)	Thermal diffusivity (mm2/s)	Thermal conductivity (W/(m.K))	Specific heat (J/(g.K))
25	0.381	0.847	0.809
100	0.154	0.274	0.647
250	0.117	0.238	0.739

4. Conclusions

SrM were effectively synthesized through co-precipitation route using metal salts as starting materials applying distinct sintering temperatures viz. 800, 900 and 1000 °C as well as modification of Fe³⁺/Sr²⁺ mole ratio. Single phase SrM was yielded at sintering temperature 800 and 900 °C with mole ratio 12 and for mole ratio 10 with calcination temperature 1000 °C, which are confirmed by the XRD patterns. The particles size was increased with the increasing temperature and decreasing mole ratio as well as shape of the particles also changed. With increasing mole ratio and calcination temperature, the magnetic properties viz. saturation magnetization (73.77–24.27 emu/gm), coercivity (3732.28–642.10 Oe), remanant magnetization (39.15–8.86 emu/gm) and magnetocrystalline anisotropy (251.74 $\times$ 10³–15.58 $\times$ 10³ erg/cm³) were decreased. The improved magnetic properties were found for SrM with the lowest molar ratio 10 (Ms- 67.45 emu/gm, Hc-3732.28 Oe) as well as calcination temperature 800 °C (Ms- 73.77 emu/gm, Hc-2764.48 Oe). The structural and magnetic properties of SrM through co-precipitation technique are a strongly dependent on the synthesis conditions. The dielectric properties were change with frequency, band gap (Eg = 2.04 eV) and the change of thermophysical properties were investigated for the SrM having a Fe³⁺/Sr²⁺ ratio 10 calcined at 1000 °C which are comply with literature. The synthesized SrM can be applied in magnetic recording media and as photocatalyst due to its low coercivity, high saturation magnetization and low band gap energy respectively.

Author contribution statement

Md. Lutfor Rahman: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper.

Salina Rahman: Performed the experiments.

Bristy Biswas, Md. Farid Ahmed: Contributed reagents, materials, analysis tools or data.

Moksodur Rahman: Analyzed and interpreted the data; Wrote the paper.

Nahid Sharmin: Contributed reagents, materials, analysis tools or data; Wrote the paper.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement

No data was used for the research described in the article.

Declaration of interest’s statement

The authors declare no conflict of interest.