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In the present investigation, the distribution of iron ions at octahedral and tetrahedral sites in BaFe 12 O 19 prepared by employing four different synthesis techniques, namely, solid-state reaction, oxalate precursor route, sol–gel and wet chemical methods, have been examined using Mossbauer studies and compared with magnetization data. It was observed that the iron ions distribute in different preferential order at various sites for hexaferrites prepared using different synthesis methods, which is confirmed by Mossbauer spectroscopy. Prepared samples were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and Field emission scanning electron microscopy. Rietveld refinement of all samples revealed an M-type hexagonal structure confirming P63/mmc space group along with a minor peak belonging to the α-Fe 2 O 3 phase, except for the sample synthesized by sol–gel route. A uniform spherical shape with a small grain size was observed in sol–gel prepared samples and the Williamson–Hall method was adopted to estimate the average crystallite size, which varies between 72 and 129 nm. The room temperature magnetization studies reveal that the sample synthesized via sol–gel route shows high coercivity and saturation magnetization values due to their smaller grain sizes. Mossbauer spectra of all BaFe 12 O 19 samples were fitted with five sexets assigned to the hexagonal crystal structure of 4f 2 , 4f 1 , 2a, 12k and 2b sites, where the variation in their relative areas confirms the redistribution of iron ions at these sites. Graphical abstract

Ca-doped barium hexaferrite (Ba 1− x Ca x Fe 12 O 19; x = 0.1–0.6) samples were synthesized successfully using solid-state reaction route. Further, the limit of the Ca solubility in BaFe 12 O 19 has been investigated in the 900–1200°C temperature range. X-ray analysis reveals maximum Ca solubility in BFO at 1200°C (≈50%). The emergence of a hematite secondary phase at Ca concentrations ≈50 and 60 at% was observed. The XRD analysis also confirmed a gradual reduction in the c -parameter from 23.195 Å (at x = 0.1) to 23.147 Å (at x = 0.5). The structural data further suggested an enhancement of sample density with Ca substitution. Field-emission transmission electron microscopy micrographs reveal (a) distinct grain morphology at lower Ca concentration ( x = 0.1) and (b) enhanced secondary grain growth and grain amalgamation at higher Ca concentration ( x = 0.5). The M–H studies reveal that M r and M s almost stay constant up to x = 0.2, beyond which they start rising rapidly. M r ≈ 42.65 emu g −1 and M s ≈ 92.19 emu g −1 , exhibit a maximum at x = 0.4. The coercivity first decreases rapidly from 4168 Oe (at x = 0.0) to 2326 Oe (at x = 0.1) followed by a marginal decrease to 2045 Oe (at x = 0.4). An H c value ≥1200 Oe in a majority of our samples makes them potentially suitable for perpendicular recording media and permanent magnet applications.

Cu 2+ ion substituted nanocrystalline BaFe 12 O 19 [Ba 1 − x Cu x Fe 12 O 19 (0.0 ≤ x ≤ 0.5)] hexaferrite powders were synthesized by sol–gel combustion route and its effects on structure, morphology and magnetic properties of barium hexaferrite (BaFe 12 O 19 ) were presented. X-Ray Powder Diffraction (XRD), Scanning Electron Microscopy (HR-SEM), Transmission Electron Microscopy (HR-TEM) and Fourier Transform Infrared (FT-IR) analyses revealed the M-type hexagonal structure of all samples. Vibrating sample magnetometer (VSM) analyses showed that all samples have strong ferromagnetic behavior at room temperature. The crystallite size varies in a range of 23.30–35.12 nm. Both HR-SEM and HR-TEM analyses confirmed the hexagonal morphology for products. A minimum of 40.49 and a maximum of 54.36 emu/g estimated specific saturation magnetization (σ s ) were observed for Ba 0.5 Cu 0.5 Fe 12 O 19 and Ba 0.9 Cu 0.1 Fe 12 O 19 NPs, respectively. The remnant magnetization (σ r ) has a minimum value of 21.27 emu/g belonging to Ba 0.5 Cu 0.5 Fe 12 O 19 and has a maximum value of 28.15 emu/g belonging to Ba 0.7 Cu 0.3 Fe 12 O 19 NPs. The coercive fields are between 1726 Oe and 2853 Oe. K eff (calculated effective anisotropy constants) is changing from 2.31 × 10 5 to 3.23 × 10 5  Ergs/g. It was observed that the strong magneto-crystalline anisotropy fields, ( H a ) above 11.0 kOe for all samples which confirmed that all samples are hard magnet. Due to their small crystallite size (smaller than 50 nm) and high saturation magnetization, Ba 1 − x Cu x Fe 12 O 19 (0.0 ≤ x ≤ 0.5) nanoparticles can be employed as magnetic recording materials.

Barium hexaferrites (BaFe 12 O 19 ) nanoparticles are prepared using γ-ray irradiation assisted polyacrylamide gel method (RIAPGM). The experiment parameters of as-prepared samples are investigated in detail, such as polymerization initiation mode and annealing temperature. The thermal decomposition behavior of BaFe 12 O 19 xerogels were measured by thermogravimetric and differential scanning calorimetry (TG–DSC). The structure, optical, magnetic and electrochemical properties of BaFe 12 O 19 nanoparticles were examined by various characterization techniques. The optical and magnetic properties of BaFe 12 O 19 nanoparticles are emphatically researched by changing the annealing temperature. The BaFe 12 O 19 nanoparticles prepared by RIAPGM exhibits uniform particle size distribution, excellent magnetic and electrochemical properties due to the effect of facet-dependent behaviour.

Rapid industrial growth causes considerable environmental havoc, adversely affecting human and aqueous life. It becomes a significant concern to deal with adequate wastewater treatment strategies by converging on water scarcity. This research work explored the synthesis of titanium-substituted Y-type barium hexaferrite (Co 2 -Y), having a general formula of Ba 2 Co 2 Fe 12-x Ti x O 22 ( x  = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5), using a facile nitrate-based sol–gel auto-combustion route and its suitability was investigated as a heterogeneous catalyst within the photo-Fenton-based degradation of methyl orange (MO), one of the significant pollutants generated from textile industries. Developing a thermochemically stable and magnetically separable heterogeneous catalyst for photocatalytic decomposition of nonbiodegradable organic dye from wastewater was also emphasized. The as-prepared nanocrystalline Co 2 -Y powders were analyzed using XRD, FTIR, DLS, UV–visible spectroscopy, SEM, VSM, and XPS. Furthermore, the photocatalytic degradation performance of pristine and titanium substituted Ba 2 Co 2 Fe 11.6 Ti 0.4 O 22 ferrite, having the lowest bandgap value among all samples, was quantified and compared in terms of apparent rate constant (k arc ) value and turnover frequency values. The enriched photocatalytic performance was correlated with the existence of multi-valance states of transition metal cations and the availability of oxygen vacancy, confirmed by the surface chemistry using the XPS analysis. The modified (enhanced thermal and chemical stability) hexaferrite catalyst was magnetically separable and reusable without significant losses to its catalytic performance. This promising catalyst may be considered as a replacement for soft ferrite materials to catalyze the degradation of several other nonbiodegradable organic pollutants from wastewater in large-scale industries. Graphical Abstract

The study of manganese-substituted barium hexaferrite BaFe 12– x Mn x O 19 with a degree of substitution x from 0 to 2 is presented. The samples are prepared by the solid-phase synthesis at a temperature of 1275 °C and isothermal holding for 5 h. The elemental composition of the samples is determined using energy dispersive spectroscopy, which shows good correspondence with the specified calculated compositions. Monophasicity of all synthesized samples is confirmed by powder X-ray diffraction. In addition, the effect of manganese substitution on the unit cell parameters is estimated. In the differential scanning calorimetry study of the properties, the effect of manganese substitution for iron on the Curie points of the samples obtained, which are caused by changes in the magnetic structures of the produced materials, is determined. The results indicate a significant effect of manganese substitution on the properties of barium hexaferrite and confirm the possibility to control the substitution process for the production of materials with adjustable magnetic characteristics.

In the present study, a heterostructure of Ni doped M-type hexaferrite i.e., BaNi 0.2 Fe 11.8 O 19 (BNFO) and reduced graphene oxide (RGO) has been prepared to understand its effect on the optical and magnetic properties of BNFO. Here, the preparation of both the components was carried out through sol–gel (followed by heat treatment) and modified Hummer’s method, respectively. The confirmation of phase formation was confirmed with XRD results and spectroscopic techniques (FTIR and Raman) were used to monitor the structural variation occurred as a result of attachment of BNFO and RGO. Further, vibrating sample magnetometry (VSM) and absorbance spectroscopy (UV–visible) were carried out to understand the effect of attachment. VSM study suggested the decrement of saturation magnetization, coercivity, and retentivity as a result of RGO addition in BNFO which might be associated to the soft magnetic characteristics of RGO. While, such synergistic heterojunction induced excellent visible absorption of prepared samples enabling its applicability in optical devices. Highlights A successful synthesis of rGO attached Ni-doped barium hexaferrite. Raman and FTIR spectroscopy confirmed the attachment. The optical analysis suggested the applicability of prepared samples as optical sensors and photocatalysts.

Cobalt-samarium (Co-Sm) doped M-type barium hexaferrite nanocrystalline materials (BaFe12-2ZCoZSmZO19) of various compositions (Z = 0.0, 0.2, 0.4, 0.6) were prepared via without water and surfactants (WOWS) sol–gel approach. X-ray diffraction (XRD) studies were employed to elucidate the structural properties. For each composition, the porosity, theoretical density, crystallite size and lattice constant were obtained from XRD data. The dielectric parameters were measured by LCR meter. A scanning electron microscope (SEM) was used to investigate the morphology of materials. DC electrical resistivity (ρdc) calculations were performed in a broad range of temperature (from 100°C to 400°C). The activation energy (ΔE) was also calculated from electrical resistivity data. The vibrating-sample magnetometer studies were employed to investigate hysteresis loops and magnetic properties were estimated from hysteresis curves. It was observed that the materials were of high magnetocrystalline anisotropy, large Curie temperature, high value of magnetization and excellent dielectric characteristics. The XRD spectrum analysis of materials disclosed that the structure of materials was hexagonal. The SEM investigations revealed that the materials are uniform in both shape and size, with a normal size ranged from 295 nm to 440 nm. Electrical resistivity of materials disclosed their dependence on the temperature, which suggests the semiconducting nature of M-type hexaferrites. The magnetic characteristics were evaluated at 25°C and they showed typical hysteresis loop demonstrating ferromagnetic nature of the compounds. The results of Sm3+ and Co2+ ions substitution expose the large values of coercivity (Hc), which indicates the nanocrystalline nature of the materials.Graphical Abstract

Barium hexaferriteBaFe12O19single crystals hexagonal platelet shape and sizes of up to 8 mm were grown of lead oxide and sodium oxide based fluxes at 1260 °C. The unit cell parameters of single crystals grown using different fluxes is in good agreement with literature data. Substitution of Ba by Pb was detected, but with only negligible influence on unit cell parameters and Curie temperature.

This study is aimed at the manufacture and the magnetic properties of polycrystalline M-type hexaferrites BaFe12O19 (barium ferrite, or BaM) materials of different magnetic texturing grades, going from a random distribution of the BaM crystallites to their almost complete stacking. Our target is to optimize the value of reduced-remanence magnetization MR/MS, which is among the most significant features of the self-polarized materials. In this study, we focus on the role played by the precursors hematite (isotropic spherical shape) and goethite (anisotropic lath shape). Therefore, 11 samples with a flat cylinder shape are fabricated, with an increasing hematite to goethite ratio. We demonstrate that this ratio drives the texturization of the samples by producing self-polarized materials with different MR/MS from the simple green compaction of the precursors, followed by a heat treatment. Most importantly, our study reveals the orientation of BaM particles after compaction; therefore, MR/MS, is strongly influenced by the aspect ratio of the lath-shaped goethite crystallites. Additionally, we show that finer goethite crystallites yield higher-value MR/MS. We optimize the aspect ratio of the goethite crystallites for an improved BaM texture. The optimization of the morphology of the goethite crystallites leads to an increase in the BaM particles’ orientation and stacking. The salient outcome of this work, which distinguishes it significantly from recent works, is that the particles stacking increases with the value of the shape factor η (defined as the ratio of the diameter of the laths to their length) of the goethite, evidenced by XRD results. The Rietveld refinements of powder diffractograms and the measured magnetic properties reveal a particle-stacking enhancement caused by not only the ratio of hematite: goethite but mainly by an optimal aspect ratio of the goethite crystallites. Based on this study, the BaM materials are further manufactured with a controlled magnetic texture; thus, they are partly self-polarized. They show reduced-remanence magnetization MR/MS varying from 0.5 and 0.81, while the angular dispersion of the BaM particles’ easy axis of magnetization varies from 60° to 10°. The magnetic properties of the samples are further studied in microwave experiments, from which the value of the magnetocrystalline anisotropy field HK = 16.6 kOe is deduced. The first magnetization curves of each sample are obtained using a VSM. A law of approach to the saturation suitable for the case of the uniaxial polycrystalline materials, and for which the particle stacking is only partial, is proposed for the fitting of the magnetization process. It is suggested that by using the proposed law with a known magnetocrystalline anisotropy constant K1, the angular grain-dispersion can be found.