Comprehensive study of structural, physical, and spectroscopic properties of Co-Ni substituted BaMg2Fe16O27 W-type hexaferrites

Cobalt and nickel substituted BaMg2Fe16O27 W-type hexaferrite nanoparticles were synthesized via citric acid-assisted sol–gel auto-combustion technique. XRD results revealed the single-phase W-type hexagonal structure of these nanomaterials. The decrease in lattice parameters and unit cell volume is accredited to smaller ionic radii of substituted ions as compared to that of host ions. The variations in Raman spectra as a function of substitution content (x, y) are ascribed to the induction of strain in the unit cell. The PL spectra of synthesized samples revealed all emissions invisible (red) regions with PL intensity near 661 nm. Furthermore, the semiconducting nature of synthesized materials has been assessed based on bandgap energy Eg  = 1.875 eV. FTIR investigations endorsed the successful settlement of substituents at tetrahedral and octahedral sites. SEM images exhibit non-homogenous hexagonal platelet-shaped particles. The research outcomes suggest the promising applications of synthesized compounds with suitable cationic substitution for microwave devices.


Introduction
Owing to ferrites unique structural and magnetic characteristics, these are immensely used in numerous applications of technological importance [1,2] such as permanent magnets, magnetism based recording media, electronic appliances, communication gadgets, RAMs, stealth combat aircraft, and high frequency (HF) applications [3]. Hexagonal ferrites have been attracted huge attention due to their high resistivity, high magnetization, large anisotropy, superior stability, which in turn confirm their potential use in microwave applications as well as electromagnetic absorbers [4]. As an electromagnetic absorber, hexaferrite materials are highly recommended. The presence of Fe 3+ ions in hexagonal ferrite decreases dielectric loss, which is a crucial aspect in improving electromagnetic wave material absorptivity. The high Curie temperature and total magnetic moment of ferrites are significant benefits. Environmental safety and chemical resistance are additional undeniable advantages of these substances. Ferrites in nanoform have better qualities than their micro-sized counterparts and are significantly more promising [5,6]. Barium-based hexaferrite has been designated as the most efficient magnetic compound with outstanding structural and magnetic properties [7][8][9]. Due to dissimilar crystal structures, these ferrites have been categorized into seven classes namely: M-, W-, R-, X-, U-, Y-, and Z-type. The magnetic characteristics of hexagonal ferrites are owing to the super exchange contact, and they are all ferrimagnetic in nature. W-type hexaferrite has the general formula: AMe 2 Fe 16 O 27 , where A = (Sr, Ba), and Me represent divalent cation (e.g. Co 2+ , Ni 2+ , Mg 2+ , Mn 2+ , Zn 2+ , Cu 2+ or any other suitable divalent cation), the combination of divalent cations can also be used [10].
The crystal structure of W-type hexagonal ferrites comprises of alternate stacking of S (Fe 6 O 8 ) and R (BaFe 6 O 11 ) blocks along the c-axis in the sequence RSSR * S * S * , where the asterisk ( * ) symbol designates 180°rotation of the respective block. Here, S stands for spinel block that comprises two oxygen layers and R designates rhombohedral block containing three oxygen layers [11]. Seven different sub-lattices are located in these blocks and are denoted by 12k, 4f VI , 6g, 4f, 4e, 4f IV , and 2d. Of these seven sub-lattices, the first four (12k, 4f VI , 6 g, 4f) own octahedral site, sub-lattice 2d has special bipyramidal site, and the remaining two (4e, 4f IV ) possess tetrahedral site [12].
The simultaneous existence and distribution of trivalent as well as divalent cations amid seven lattice sites fascinate the researchers to investigate W-type hexaferrite for various scientific applications [13] because their properties can be tuned by appropriate divalent and trivalent cationic substitution. Various factors like preparation technique, type of substitution, substitution content, and sintering temperature greatly affect the structural, electrical, and magnetic characteristics of W hexaferrites [14]. EM materials, a combination of magnetic filler and polymer, can decline electromagnetic interference (EMI) effectively, and W hexaferrite can be utilized as high-frequency magnetic fillers because of their substantial magnetic loss at FMR point [15,16]. Because, un-substituted ferrites do not have the characteristics needed for a specific application, the replacement of divalent or trivalent metal ions in hexaferrites has a lot of promise. Such replacements assist a lot with resistivity, permittivity, permeability, and magnetic properties including saturation magnetization (M s ) and magnetic coercivity (H c ), which are important for highfrequency applications. The magnetic, electrical, and dielectric characteristics of microwave absorbers may be affected by the chemical composition and crystal structure of ferrites. W-type hexaferrites are prospective microwave absorbent materials due to their soft magnetic character. Many individuals have examined Wtype hexagonal ferrites because they have good magnetic and electromagnetic characteristics [17,18].
Raman spectroscopy is a non-destructive characterization tool that has been used by many researchers for vibrational analysis of various spinel ferrites as well as M-type hexaferrite with powdered, pellet, or filmshaped samples [19][20][21][22][23]. However, to the best of our knowledge, not any noteworthy Raman spectroscopic work has been reported for W-type hexaferrite except R. Sagayaraj et al. [24].
In BaMg 2 Fe 16 O 27 , the divalent magnesium ions are positioned at tetrahedral and octahedral sites of spinel block, while barium ions are located in R-block [25]. The effect of Ni substitution on magnetic and dielectric properties of BaCo 2 W-type hexaferrite has been investigated by A. M. Abo El Ata et al. [26]. With Ni substitution, the optimized X-band microwave characteristics of ZnFe 2 O 4 have been reported [27]. Recently, antibacterial activity of Zn substituted BaCu 2 W-type hexaferrite nanoparticles has been reported and published [24]. M. H. Won and C. S. Kim mentioned the fabrication of material through the replacement of Co by Ni in Ba 2 Co 2 Y-type hexaferrite, that can be used in antenna applications (GHz range) [28]. Imran Khan et al., probed the Nd-Ni substitution effect in SrCo 2 W hexaferrite showing the potential of their fabricated materials for high-density recording media [29]. Majid Niaz Akhter et al. found the enhancement in saturation magnetization, remanence, and coercivity with the increment in Ce 3+ substitution level in Ce-substituted Cu nanoferrite materials. Additionally, the maximum value of saturation was measured using law of approach (LOA). They looked at several features of Ce-substituted Cu nanosized ferrites having different characteristics as potential material for core, security, sensing, switching, multilayer chip inductor, microwave absorption and biomedical applications [30]. Majid Niaz Akhter and M. Azhar Khan [31] prepared Ce substituted Ni-Zn spinel ferrites. According to them, the reduction in saturation magnetization, remanence, initial permeability, and Bohr magneton was observed whereas the enhancement in coercivity with Ce 3+ substitution. The increment in Yafet and Kittle (Y-K) angles were found with Ce substituted NiZn nanoferrites from x = 0 to x = 0.04 correspondingly. The evaluation regarding switching field distribution (SFD) for Ce substituted NiZn nanoferrites was calculated by taking first derivative of the demagnetization data accordingly. M. N. Akhter et al. [32] proposed synthesis of spinel ferrites with Pr substitution, and observed decrement in magnetic saturation, coercivity, remanence as well as anisotropy constant with Pr concentration in Cu spinel ferrite. However, Y-K angles were enhanced with Pr substitution level in Cu spinel nanoferrites. The evaluation of microwave frequency response was done that established the application of these Pr substituted Cu spinel nanoferrites in the range of 5.2-9.5 GHz correspondingly. These Pr doped Cu spinel nanoferrites' characteristics suggested that they may be employed in microwave devices, memory devices, and recording media. M. N. Akhter et al. [33] prepared Co-Ce based spinel ferrites with Cu-substitution. They found reduction in the magnetic parameters including saturation magnetization, remanence, coercivity, Bohr magnetic moment and anisotropy constant of the Cu doped Co-Ce nanoferrites were decreased. However, Yafet-Kittel (Y-K) angles were found to be enhanced with Cu doped Co-Ce nanoferrites. The saturation data of the samples was also used to examine the high frequency response of the Cu substituted spinel nanoferrites in the microwave region. All the Cu doped Co-Ce nanoferrites samples were suggested to be used in microwave for X-band regime. M. N. Akhter et al. [34] proposed the preparation of Rare earths (Res) substituted Mn spinel nanoferrites having chemical composition MnR 0.2 Fe 1·8 O 4 (REs = Tb, Pr, Ce, Y and Gd) using sol gel method. They found the enhancement in saturation magnetization from 1.332 to 38.097 emu/g whereas increment in remanence was observed from 1.096 to 25.379 emu/g correspondingly. Other magnetic characteristics including initial permeability, magnetic anisotropy, and magnetic moments were also improved. Furthermore, with REs doping in Mn ferrites, Y-K angles exhibited a substantial reaction.
In this research article, by keeping iron content constant, we have substituted Mg 2+ ions with Ni 2+ ions, and replaced Ba 2+ ions by Co 2+ ions. It was also established that the addition of Co 2+ and Ni 2+ cations enhanced and changed the structural, spectroscopic, electric, and magnetic characterizations. Different methods have been utilized for the preparation of ferrites, such as micro-emulsion, aerosol pyrolysis, rotary evaporation, sol-gel autocombustion, and co-precipitation [35]. The sol-gel auto-combustion method is applied for the synthesis of W-type hexaferrite with the composition Ba 1−x Co x Mg 2−y Ni y Fe 16 O 27 because of its various advantages including excellent chemical homogeneity, low synthesis temperature, energy efficiency, atomic level precursors' mixing, low cost, facile operation, narrow-size distribution, short reaction time, and extraordinary purity of samples as well as the simplicity of method, highest crystallinity, and high surface area of particles [17,[36][37][38]. The key motivation of the present study is to synthesize Wtype hexaferrite nanoparticles substituted by Co-Ni that were not reported earlier with best of our knowledge. A comprehensive structural study was carried out using the XRD technique. Moreover, the substitution effect of (Co, Ni) ions on spectroscopic and optical properties of BaMg 2 W-hexaferrite have been investigated by FTIR, Raman, and PL spectroscopy. (PRS Panreac; 98%) were used. The aqueous solutions using stoichiometric molar amounts of these metallic and nonmetallic nitrates, and citric acid (Chem Lab; 99.5%), were prepared separately with the help of deionized water. From these prepared solutions, stoichiometric volumes were taken and five mixtures were prepared as per samples' compositions. The presence of citric acid in the ratio 1:1 with metal ions supports the binding of metal ions, lowers the reaction temperature as well as acts as fuel for self-burning of the material. Each resultant solution was stirred and heated to 40°C to accomplish the chelation process of metallic ions that results in their segregation as well as unvaried distribution. Ammonia liquor is added dropwise to increase solution pH to 7. After neutralization of the solution, each solution is heated to 80°C along with continuous stirring. After continuous heating plus stirring for 2.5 h, each solution is converted into a viscous gel that undergoes auto-combustion to yield fine powders as the final product. The samples were placed in the laboratory microwave oven at 200°C for two hours to eradicate any moisture. The powdered samples were grinded in an agate mortar and pestle, and then annealed in a muffle furnace at 1250°C for 6 h. These powders were then cooled to ambient temperature. Each sintered powder was again grinded for further characterization. Crystallographic probing of samples was done by XRD apparatus (Bruker D8) equipped with CuKα source emitting monochromatic radiations of 0.154 nm wavelength. Optical characterizations were accomplished by employing a Raman spectrometer (MN STEX-PR1100), and a PL spectrometer (MN STEX-PR1100 Dangwoo optron). FTIR analysis of prepared materials was done by FTIR (Bruker Tensor27) spectrometer at room temperature in order to probe the stretched atomic vibrations, and structure as well. To investigate the morphology, shape, and grain size of unsubstituted and substituted BaMg 2 WHFs, scanning electron microscopy (SEM) (Emcrafts; cube 1100; SK) was employed.

X-ray Diffraction (XRD) analysis
To investigate the structural characteristics of prepared W-type hexaferrite samples having chemical composition Ba 1−x Co x Mg 2−y Ni y Fe 16  , respectively. Thus, at a higher substitution level, the switching of maximum intensity peak to different angles as well as the appearance of new peaks confirmed the required Co-Ni substituting effect in the structure of these materials. Moreover, the variations in peak intensities for synthesized samples can be attributed to substitutions Co-Ni cations [39]. The effective use of X-ray Diffraction (XRD) data yielded various important crystallographic details of powdered hexaferrite samples such as lattice constants (a, c), cell volume (V), the crystallite size (D), X-ray density (d x ), bulk density (d B ), porosity (%P), dislocation density (δ) and lattice strain ( ) that are represented in tabular form in Table 1.
Lattice parameters (a, c) were calculated with the help of cell software (version 5.0, Copyright (C) 1986 by K. Dwight) using [h k l]. The measured lattice parameters are displayed in Table 1, and their values are closely related to those reported in the previous literature. The values of lattice parameter "a" decreased with enhancement in substitution contents (x, y) as shown in Figure  2. This variation can be attributed to the comparatively smaller ionic radius of Co 2+ (0.74 Å) [40] as compared to Ba 2+ (1.35 Å) [41] ions, and the slightly smaller ionic radius of Ni 2+ (0.69 Å) [42] replacing Mg 2+ (0.72 Å) ions [43,44]. The greater variations in lattice parameters "c"   than "a" can be justified based on the fact that in Wtype hexaferrite, the c-axis; being perpendicular to the basal plane bears the easy axis character causing an easy orientation of spin directions in its direction. Because of the varying ionic radii of the substituents, the crystal structure usually undergoes a distortion (shrinking or stretching) after the cation replacement. For all samples, the calculated crystallographic axis ratio c/a is presented in Table 1. As per Verstegen & Stevels, the assessment of structure type may be carried out by calculation of the c/a ratio. If this ratio is less than 5.585 then the structure can be of W-type, and the c/a ratio of the currently synthesized materials is less than 5.585 [3,45].
The inter-planar spacing (d hkl ) and cell volume (V cell ) of each sample were measured by using relations (1) and (2), respectively. The variation in cell volume is attributed to the variation in lattice parameters a and c as displayed in Table 1. With the increase in substitution level (x, y), a decreasing trend in cell volume ( Figure 3) was observed that might be due to the abovementioned decreasing behaviour of lattice parameters. The reduction in cell volume might be due to the compression of unit cell. Moreover, the tabulated values of d hkl (Å) and V cell (Å 3 ) are in good agreement with earlier reported literature for W-type hexaferrite.
Here, h, k, and l are the miller indices of the most intensive peak of each material, while "a" and "c" represent the lattice parameters. For each synthesized sample, the crystallite size (D) was determined by applying Gaussian fit to the highest intensity peak and resultant D values are obtained from Scherrer's relation (3); which was found to be in the range of 14-22 nanometres. When the crystallite size is  tiny, the surface area is bigger, indicating that the number of atoms at the surface is greater, i.e. the covered surface by atoms at the material's surface is more relevant when the crystallite size is small. The ferrimagnetic materials with D < 50 nm can be potentially employed for the reduction of signal-to-noise ratio in numerous high density recording, and microwave applications [3,[46][47][48]. So, we can made conclusion that the prepared samples are potential candidate for microwave applications. The deviation of d-spacing and crystallite size with substituted contents are shown in Figure 4.
The microstrain was calculated utilizing equation (4), and its increasing behaviour might be due to the difference in ionic radii of substituent ions versus host ions causing strain in the crystal structure. Moreover, an increase in dislocation density (δ) and stacking fault coefficient (α) is because of the decrement in crystallite size caused by the difference of ionic radii regarding the host and substituted ions. Physically, dislocation density describes the linear defects in the crystal lattice, while the stacking fault coefficient designates the planar defects. Stacking faults are common in closely packed structures like WHFs, and mainly arise during crystal growth. The variations in strain ( ), dislocation density (δ), and stacking fault probability (α) as a function of substituted contents (x, y) are depicted in Figure 5.
In the above relations, the parameter "K" called shape factor; accounts for the difference in hkl values and crystallite shapes. For W-type hexagonal ferrite, its value is fixed and is equal to 0.89. While, λ, β (rad), and θ (rad) denotes the wavelength of used X-rays ( = 1.5406 Å), full width at half maximum, and Bragg's angle, respectively. With the help of X-ray Diffraction (XRD) data, the physical properties like X-ray density (ρ X ; Equation (5)), bulk density (ρ B ; Equation (6)), and porosity (%P; Equation (7)) were also measured by the following relations, and are given in Table 1.
Here Z denotes the effective number of molecules in a unit cell, and in the present case, its value is 2. M represents the molecular weight; N A (6.023 × 10 23 per mole) stands for Avogadro's number and V cell denotes the cell volume. The obvious increasing behaviour of X-ray density can be linked with a decrease in cell volume; as X-ray density varies inversely with cell volume. The bulk density was found to be increase from 1.61 to 3.00 g cm −3 with the enhancement of substituted contents (x, y). This variation can be justified, because the density of substituent elements Co (8.9 g/cc) and Ni (8.9 g/cc) is fairly higher than those of host elements Ba (3.59 g/cc) and Mg (1.738 g/cc).
In the above relation, m = mass of pellet, r = radius of pellet, and h = height (thickness) of pellet. A comparison of X-ray density and bulk density for all compositions exhibits that bulk densities are smaller as compared to the X-ray densities. This difference can be attributed to the formation of grains of smaller size with composition. The greater value of X-ray density as compared to that of bulk-density is because of the tiny pores formation during the synthesis of materials as well as during sintering process [49]. A decrease in porosity (68-40%), measured by Equation (7), was also observed for studied materials, and can be linked with the increase of apparent density with increment in substitution level (x, y).
The porosity decreased as a result of the substituted ions, demonstrating that the replacement of Co-Ni improved the densification process [49]. The specific surface area (S) of crystallites calculated by employing Equation (8), indicates an increasing trend with (x, y). The increase in S may be due to the reduction in crystallite size. The smaller the crystallite size, the greater will be the number of surface atoms, and the larger will be the surface area.

Raman spectroscopic analysis
Raman spectroscopy is an attractive technique that is employed for spectroscopic probing of substituted hexaferrite materials [50,51]. It yields information about vibrational as well as rotational energy states provided that the effective derivative of polarizability (∂α/∂Q) should be non-zero. In hexaferrite, the use of D 6h symmetry-based group theory suggests the presence of 42 active Raman modes (i.e.14E 1g + 11A 1g + 17E 2g ), where the letters A and E represent one and twodimensional motions correspondingly [51]. The existence of all mentioned Raman modes is accredited to the activity of metal-oxygen ions or only O −2 ions [52][53][54]. Many researchers have probed the phonon modes of barium hexaferrites (BHFs) in thin-film or pellet shapes. However, the detailed investigation of barium hexaferrites in powder form via the Raman spectroscopic method has not been extensively reported in the literature survey [55]. Moreover, to the best of our knowledge, not any noteworthy Raman work has been carried out for W-type hexaferrites. Therefore, in this article Raman spectroscopy of our synthesized W-type nanoferrite powders (Ba 1−x Co x Mg 2−y Ni y Fe 16 O 27 ) has been carried out for a better understanding of structural variations of substituted metal cations. Raman spectroscopy not only provides information about crystal structure but also stipulates impurity concentration as well as crystal deformations [56][57][58][59].

Photoluminescence (PL) spectroscopic analysis
Owing to its contactless and non-destructive character, photoluminescence (PL) spectroscopy can be considered as the workhorse among various optical probing tools. This technique produces information regarding intrinsic and extrinsic transitions as well as imperfections and impurities [62,63]. The principle of PL (photoluminescence) spectroscopy is "photoluminescence" that corresponds to light emission of specific wavelengths from a material through optical excitation. If material is exposed to the light of appropriate energy (wavelength), the electronic excitations took place within the material by the absorption of photons' energy. The relaxation (radiative) of these electronic excitations to the ground state led to the emission of photons of a specific wavelength that are characteristics of the material. This phenomenon, photoluminescence, is highly dependent on the energy of exciting light that ultimately govern the number of excited electrons as well as holes [64]. In this work, PL (photoluminescence) spectroscopy is carried out under an excitation wavelength of 536 nm. The measurement of Photoluminescence spectra for all synthesized W-type hexaferrite samples was carried out at ambient temperature to probe their optical behaviour. The resultant intensity (a. u.) versus wavelength (nm) graphs for different substitution contents  Figure 7 revealing the existence of emission peaks in the range 620-700 nm. Eight clear peaks in the invisible (red) region corresponding to wavelengths 633, 639, 646, 650, 652, 655, 658, and 661 nm were observed in all samples. Very small shifting of peaks towards shorter wavelength was detected that can be linked with the decrease in crystallite size as revealed from XRD (X-ray Diffraction) data. The most

Fourier Transform Infrared (FTIR) spectroscopic analysis
Infrared spectroscopic studies enable to probe the structural and chemical changes as well as the development of the required phase in the synthesized materials. The Fourier Transform Infrared (FTIR) spectra of prepared W-type hexaferrite nanoparticles having composition Ba 1−x Co x Mg 2−y Ni y Fe 16 O 27 were measured in the wavenumber range of 600-4000 cm −1 , and are depicted in Figure 8. The occurrence of more than five peaks in all spectra exhibits the complex nature of the investigated materials [67]. The IR absorption band marked at 1622 cm −1 occurred due to the absorption of Co 2 present in the environment by the highly porous nature of materials [68]. The broad absorption band at 3124-3554 cm −1 is because of O-H stretching like vibrations of water due to the presence of moisture in  powders [69]. The broadness of the 3124-3554 cm −1 peak is associated with hydrogen-bonded chains. The vibrations of inorganic metallic ions located in the crystal lattice are responsible for IR bands in the range 600-1000 cm −1 [70,71].  to ammonia traces; that was used during the synthesis process for pH maintenance [73,74]. The variations in the FTIR (fourier transform infrared) spectrum confirm the successful substitution of Co 2+ and Ni 2+ in the lattice and these variations can also be due to the change in lattice parameters extracted from X-ray Diffraction (XRD) data. So, the above Fourier Transform Infrared (FTIR) results are completely in agreement with our Xray Diffraction (XRD) findings, and thus justify the synthesis of nano-ferrites with a W-type hexagonal structure.

Scanning Electron Microscopic (SEM) analysis
Microstructural analysis has vital importance in determining the desired characteristics of synthesized Wtype hexaferrites in the field of microwave absorption [75]. Figure 9(a-e) represents the Scanning Electron Microscope (SEM) images of (Co, Ni)-substituted BaMg 2 W-hexaferrites. Micrographs clearly indicate that cosubstitution of Co and Ni in the hexagonal lattice has modified the grain morphology. The presence of pores in all images indicates the synthesis of highly porous materials. Figure 9(a) belongs to an un-substituted compound, and shows the growth of condensed grains with sharp edges about mixed morphology like hexagonal, rectangular, and triangular shapes. For substituted compound (Figure 9(b)), the particles with hexagonal shape are dominant, and are stacked irregularly. Figure  9(c) show that the particles are aggregated possessing intra-granular pores. Figure 9(d,e) representing samples with maximum substitution level contain hexagonal platelet-like grains but their orientation is random. The presence of hexagonal morphology of grains in SEM micrographs satisfied our X-ray Diffraction (XRD) predicted results, and hence signifies the importance of the sol-gel self-combustion route for synthesizing pure hexaferrites. The assessed values of average grain size by ImageJ software were found to be in the range of 1.6-3.2 micron and are depicted in Figure 10. The development of these quite large grains confirms the polycrystalline nature of investigated compounds, and is because of higher sintering temperature and duration [68]. Also, larger grains can lead to greater absorption of microwaves [76]. Moreover, the synthesized materials contain hexagonal-shaped platelet-like grains and thus can be efficiently used in microwave absorbing coatings [77,78]. Raman spectra indicate the existence of ten active vibrational modes. Moreover, the differences in intensity and wideness of peaks were attributed to the substitutioninduced local strain inside the unit cell. Eight emission peaks in the visible region are obvious from PL spectra in all materials. Very small shifting of PL peaks towards shorter wavelength because of a decrement in crystallite size is observed. The calculated energy bandgap of 1.875 eV from all samples signifies that the synthesized materials exhibit semiconducting behaviour. The presence of large grains of average size 1.6-3.2 μm suggested their usage in the manufacturing of microwave absorbing materials (MAMs).

Disclosure statement
No potential conflict of interest was reported by the author(s).

Data availability statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.