Effect of microwave sintering on microstructural and magnetic properties of strontium hexaferrite using sol–gel technique

The sol–gel method is used to prepared hexaferrite using d-Fructose as a fuel. The effect of sintering temperature on the microstructure of SrFe₁₂O₁₉ ceramics is analyzed. The observed XRD results indicate a well-formed crystalline phase of dense hexagonal SrFe₁₂O₁₉ ceramics. From this analysis, no secondary phases are identified. The microstructure of the sintered single phase M-type ferrites ceramics displays a hexagonal-platelet like morphology. Sintering temperature can markedly affect the grains in sintered ferrite. The sintered product is shown to be dense microstructure with relatively small grains. The maximum sintered density 95 % was obtained at lower temperature of 1,150 °C. In addition, saturation magnetization (50.43 emu/g) and the coercivity (H_c) 5,594.53 Gauss were observed.     

Hydrothermal Synthesis of SrFe12O19 and Its Characterization

We have synthesized strontium hexaferrite particles in an alkaline medium using a hydrothermal process at 180?°C. Crystalline phase of samples were determined by XRD and spectroscopic, morphological, and magnetic investigation of the sample were FT-IR, SEM, and TG analysis, respectively. XRD analysis revealed few impurity phases in the as-made powder; upon calcinations, the material is converted to desired hexaferrite phase. As synthesized powder exhibits agglomerates with rather smooth facets, in the form of thick platelets. Upon calcination, all these structures were observed to transfer to rod-like structures. The As calcined sample has high specific saturation magnetization (M _s ) values of 65?emu/g that is close to its theoretical value of 74.3?emu/g but the hydrothermally synthesized sample does not. This is in agreement with the observations from XRD analysis where few impurity phases observed in the as-made powder cause a weak magnetic response. Upon calcination, the material is converted to a desired hexaferrite phase with better magnetic properties.     

Preparation of magnetic coatings from strontium hexaferrite on tin and cardboard by cold rolling

A finely dispersed powder of strontium hexaferrite doped with aluminum of the composition SrFe_12?x Al _x O₁₉ with an aluminum content x = 0.6 ± 0.1 is prepared through crystallization of oxide glasses. The powder is characterized by a saturation magnetization of 60.2 A m²/kg and a coercive force of 550 kA/m. The hexaferrite particles predominantly have the shape of thick hexagonal platelets with a diameter ranging from 300 to 500 nm and a thickness-to-diameter ratio varying from 0.3 to 0.5. Magnetic coatings on tin and cardboard substrates are produced by cold rolling of strontium hexaferrite powders. It is shown that hexaferrite particles in the magnetic coatings have the preferred orientation of the well-developed facets along the rolling plane, which manifests itself in anisotropy of the magnetic properties of the coatings. The degree of texturing in the strontium hexaferrite coatings on cardboard and tin substrates is equal to 44 and 66%, respectively.     

A novel approach for synthesis of M-type hexaferrites nanopowders via the co-precipitation method

M-type hexaferrites; barium hexaferrite BaFe₁₂O₁₉ and strontium hexaferrite SrFe₁₂O₁₉ powders have been successfully prepared via the co-precipitation method using 5 M sodium carbonate solution as alkali. Effects of the molar ratio and the annealing temperature on the crystal structure, crystallite size, microstructure and the magnetic properties of the produced powders were systematically studied. The results indicated that a single phase of barium hexaferrite was obtained at Fe³⁺/Ba²⁺ molar ratio 12 annealed at 800–1,200 °C for 2 h whereas the orthorhombic barium iron oxide BaFe₂O₄ phase was formed as a impurity phase with barium M-type ferrite at Fe³⁺/Ba²⁺ molar ratio 8. On the other hand, a single phase of strontium hexaferrite was produced with the Fe³⁺/Sr²⁺ molar ratio to 12 at the different annealing temperatures from 800 to 1,200 °C for 2 h whereas the orthorhombic strontium iron oxide Sr₄Fe₆O₁₃ phase was formed as a secondary phase with SrFe₁₂O₁₉ phase at Fe³⁺/Sr²⁺ molar ratio of 9.23. The crystallite sizes of the produced nanopowders were increased with increasing the annealing temperature and the molar ratios. The microstructure of the produced single phase M-type ferrites powders displayed as a hexagonal-platelet like structure. A saturation magnetization (53.8 emu/g) was achieved for the pure barium hexaferrite phase formed at low temperature 800 °C for 2 h. On the other hand, a higher saturation magnetization value (M _s = 85.4 emu/g) was obtained for the strontium hexaferrite powders from the precipitated precursors synthesized at Fe³⁺/Sr²⁺ molar ratio 12 and thermally treated at 1,000 °C for 2 h.     

Magnetic properties of Ni–Co doped barium strontium hexaferrite

A series of Ni–Co substituted barium strontium hexaferrite materials, Ba_0.5Sr_0.5Ni _x Co _x Fe_12–2x O₁₉ (x = 0.0, 0.2, 0.4, 0.6, 0.8 mol%) was synthesized by the sol–gel method. X-ray diffraction analysis has shown that the Ni–Co substitutions maintain in a single hexagonal magnetoplumbite phase. The room temperature magnetic properties and the cation site preferences of Ni–Co substituted ferrite were investigated by VSM. Substitutions led to decrease in coercivity while saturation magnetization remains the almost same. It indicates that the saturation magnetization (52.81–59.8 Am²/kg) and coercivity (69.83–804.97 Oe) of barium strontium hexaferrite samples can be varied over a very wide range by an appropriate amount of Ni–Co doping contents.     

Facile synthesis of SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nanoparticles and its photocatalyst application

The strontium hexaferrite (SrFe₁₂O₁₉) nanoparticles have been successfully synthesized by co-precipitation route. The effect of various parameters such as calcination temperature and chelating agents were screened to achieve optimum condition. Different chelating agents such as amino acids (proline, alanine, aspartic acid) and surfactants (SDBS, PVP, and EDTA) were used. Compared with the amino acids, the surfactants increase the particle size and the best result was observed for alanine. The SrFe₁₂O₁₉ nanoparticles showed enhanced photocatalytic activity in the degradation of methyl orange under visible light irradiation (λ?>?400 nm). The degradation rates of the methyl orange were measured to be as high as 95% in 220 min. The nanoparticles were also characterized by several techniques including FT-IR, XRD, SEM, and VSM. The VSM measurement showed a saturation magnetization value (Ms) of 32 emu/g. The SEM images proposed that the particles are almost spherical with an average particle size of 90 nm.     

Preparation of M-Ba-ferrite fine powders by sugar-nitrates process

In this paper, fine M-type barium hexaferrite (M-Ba-ferrite) particles were synthesized from sugar and nitrates by simple route, which revealed the feasibility of using sugar as chelating agent in forming solid precursors of BaFe₁₂O₁₉. The effects of factors, such as the molar ratio of Fe/Ba, calcination temperature and time, on the morphology, the phase component and the magnetic properties of M-type barium hexaferrite particles were studied by means of X-ray diffraction, infrared spectroscopy, transmission electron microscopy and physical property measurement system. The results showed that the molar ratio of Ba²⁺ to Fe³⁺ influenced significantly on the formation of the single phase barium ferrite. The hexagonal platelet barium ferrite particles with a specific saturation magnetization of 64.48 emu/g, remanence magnetization (Mr) of 33.84 emu/g, and coercive force (Hc) of 1848.85 Oe were obtained when the molar ratio of Fe/Ba was 11.5 and the calcination temperature was 1100 °C for 2 h.     

Magneto-Structural Characterization of Strontium Substituted Lead Hexaferrite

In this work, the magnetic and structural properties of the system Pb_1?x Sr _x Fe₁₂O₁₉ (x=0.1,0.3,0.5,0.7 and 0.9) are reported. The samples were prepared by the traditional ceramic method. All the compounds are isostructural with the strontium hexaferrite (SrFe₁₂O₁₉). X-ray powder diffraction was used to carry out the quantitative analysis of phases and to determinate the crystallographic parameters. It was found that the compound consists of only one phase and that the coercivity, remanence and saturation increased with the strontium content. The initial susceptibility was also obtained and results are discussed in terms of the magnetization mechanisms produced by the effect of the substitution on the hexaferrite. Furthermore, Néel temperature measurements indicate a strengthening of the exchange interactions with increasing strontium content.     

Fabrication,Characterization, and Magnetic Properties of BaAlFe<Subscript>11</Subscript>O<Subscript>19</Subscript> Particles Via Auto Combustion Method

Al-substituted barium hexaferrite particles have been successfully synthesized via sol-gel auto combustion method in the presence of citric acid as fuel. Thermal decomposition, phase evolution, and the microstructure of products were characterized by DTA/TG, XRD, and SEM. Magnetic measurements were carried out on a VSM. To investigate the effects of citric acid to metal nitrate (CA/MN) molar ratios and combustion temperatures on the morphology, phase structure, and magnetic properties of products and finding the optimal condition, several experiments were carried out. The results revealed that the formation temperature, crystallite size as well as magnetic properties are significantly influenced by these parameters. A saturation magnetization of 56.96 emu/g and a coercivity of 7279 Oe were obtained in BaAlFe₁₁O₁₉ powders with CA/MN = 1.0. High Ms and Hc values make them particularly suitable for hard magnetic applications.     

Effect of Fuel on the Synthesis and Properties of Poly(methyl methacrylate) Modified SrFe12O19 Nanoparticles

Nanosized strontium hexaferrite (SrFe₁₂O₁₉) has been synthesized by citrate, urea, oxalic, and glycine precursor via a sol-gel route with poly(methyl methacrylate) (PMMA) as a templating agent. Crystal structure, morphology, and magnetic properties of as-synthesized nanoparticles were characterized by XRD, SEM, FT-IR, and VSM techniques. The formation of strontium hexaferrite and its crystallite size in presence of different fuels were compared. The influence of different fuels was reflected on the phase purity, morphology of the final powders as well as the magnetic properties. Magnetic measurements revealed that samples prepared by citric acid and glycine as fuel have high specific saturation magnetization and moderate coercivity, while urea and oxalic acid fuels resulted in low phase purity, and thus inferior magnetic properties.     

Preparation and characterization of various morphologies of SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nano-structures: investigation of magnetization and coercivity

Hard magnetic SrFe₁₂O₁₉ (SrFe) nanostructures were synthesized by a facile chemical precipitation procedure. The influence of temperature, concentration and different capping agents on the particle size and morphology of the magnetic nanoparticles was investigated. The synthesized ferrites were characterized by X-ray diffraction pattern, scanning electron microscope, and Fourier transform infrared spectroscopy. Ferromagnetic property of the hexaferrite nanostructures was determined by vibrating sample magnetometer. The results show hard magnetic ferrite with a high coercivity about 2800–4000 Oe and saturation magnetization around 11–14 emu/g were synthesized.     

Controlling the structural,microstructure and magnetic properties of barium W-type hexaferrite elaborated using tartaric acid precursor strategy

In this study, barium W-type hexaferrite (BaCo₂Fe₁₆O₂₇) nanopowders have purposefully fabricated through tartaric acid precursor method using inexpensive starting materials. In this regards, the impact of the synthesis conditions namely the annealing temperature and the Ba:Co molar ratio on the crystal structure, crystallite size, microstructure and magnetic structure was explored using X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer. For instance, well crystalline W-type hexaferrite was realized for the precursors annealed at a low temperature of 1100 °C for 2 h using two different Ba:Co molar ratios of 1.1:2.2 and 1.2:2.4. The crystallite size, the lattice constant, the aspect ratio as well as the unit cell volume were substantially affected with the Ba:Co molar ratio and the annealing temperature. Remarkably, the morphology of hexaferrite powders can be controlled by adjusting the annealing temperature and the Ba:Co molar ratio. Clearly, the microstructure of the formed powders was improved to a hexagonal platelet-like structure by raising the annealing temperature. Eventually, maximum saturation magnetization Ms?=?72.3 emu/g was accomplished for W-hexaferrite particles obtained with Ba:Co molar ratio 1.1:2.2 annealed at 1350 °C for 2 h. Wide coercivities (196–1097 Oe) were achieved at the different synthesis conditions.     

Mössbauer and magnetic study of Mn, Zr and Cd substituted W-type hexaferrites prepared by co-precipitation

BaCo_2−xMn_xFe_16−2y(Zr–Cd)_yO₂₇ (x = 0–0.5 and y = 0–1.0) hexaferrite nanocrystallites of average sizes in the range of 33–42 nm are synthesized by the chemical co-precipitation method. The synthesized materials are characterized using different techniques including X-ray diffraction (XRD), energy dispersive X-ray florescence (ED-XRF), scanning electron microscope (SEM), Mössbauer spectrometer and vibrating-sample magnetometer (VSM). Based on analysis of the data obtained from Mössbauer spectral studies, doping is believed to have occurred preferably in the vicinity of 12k sub-lattice, i.e. f_IV (4e, 4f_IV), 2b (6g, 4f) and 2d site. Variations in the saturation magnetization (77.1–60.9 emu g⁻¹), remanent magnetization (22.08–31.23 emu g⁻¹) and coercivity (1570.1–674.7 Oe) exhibit tunable behavior with dopant content and therefore can be useful for application in various magnetic devices.     

Magnetic Properties of Nanosized Ba2Mg2Fe12O22 Powders Obtained by Auto-combustion

We studied the magnetic properties of nanosized Ba₂Mg₂Fe₁₂O₂₂ powder obtained by citrate auto-combustion synthesis. The powder consists of agglomerates with mean crystallite size of 100?nm. The magnetic properties of the powder were investigated at 4.2?K and at room temperature. The values measured of the magnetization M at a magnetic field of 60?kOe are 22.78?emu/g and 30.47?emu/g at room temperature and 4.2?K, respectively. The magnetic phase transition at 183 K is related to the ferromagnetic-to-spiral spin order and is a precondition for this material??s exhibiting multiferroic properties.     

Heat treatment of strontium hexaferrite powder in nitrogen,hydrogen and carbon atmospheres: a novel method of changing the magnetic properties

Strontium hexaferrite powder has been treated in nitrogen, hydrogen and carbon atmospheres. The results show that the phase composition and morphology, and hence, the magnetic properties of the strontium hexaferrite are affected significantly by these gas/vapour treatments. Generally, the coercivity decreased to below 0.8 kOe (regardless of the initial coercivity) and the magnetization at 14 kOe increased significantly, when strontium hexaferrite powder had been treated in a nitrogen, hydrogen or carbon atmosphere. However, it was found that a post-gas treatment of calcination in air, under appropriate conditions, resulted in a recovery of the hexaferrite structure (i.e. it is a reversible reaction). However, the particle/grain sizes of the calcined samples were significantly smaller than those of the non-treated samples, and it is believed that they were single domain particles/grains. In some cases, the coercivity increased by about 400%. The magnetization at 14 kOe and the remanence were either not affected or sometimes increased; magnetic measurements indicated a preferred orientation of the grains.     

Magnetic properties of hydrothermally synthesized strontium hexaferrite as a function of synthesis conditions

Fine particles of strontium hexaferrite, SrFe₁₂O₁₉, with a narrow size distribution have been synthesized hydrothermally from mixed aqueous solutions of iron and strontium nitrates under different synthesis conditions. The relationship between the synthesis variables (temperature, time and alkali molar ratio) and the magnetic properties has been investigated. The results have shown that, as the synthesis temperature increases, the saturation magnetization of the particles increases up to a plateau and the coercivity decreases. As the alkali molar ratio R(=OH^–/NO ₃ ^– ) increases, the coercivity decreases and goes through a local minimum, while the saturation magnetization increases and goes through a local maximum. Increasing the synthesis time from 2 h to 5 h has no significant effect on the saturation magnetization, but decreases the coercivity. An anisotropic sintered magnet with a high saturation magnetization value of 67.26 e.m.u g^–1 (4320 G) has been fabricated from the hydrothermally synthesized powders.Relationship between the c.g.s and S.I.units which are used in this paper are as follows: 1 erg = 10^–7 J, 1 e.m.u. cm^–3 = 12.57×10^–7 Wom^–2 (tesla), 1 oersted (Oe) = 79.6 A m^–1, 1 G = 10^–4 tesla (T).     

Bulk Nanocrystalline SmCo6.6Nb0.4 Sintered Magnet With TbCu7-Type Structure Prepared by Spark Plasma Sintering

We prepared bulk nanocrystalline SmCo_6.6Nb_0.4 sintered magnet material by spark plasma sintering technique. X-ray diffraction patterns show that the magnet exhibits a stable TbCu₇ structure. Transmission electron microscopy indicates that the microstructure of the magnet is composed of SmCo_6.6Nb_0.4 single-phase grains with an average grain size of 30 nm. Magnetic measurement shows that under a 7 T magnetic field, the coercivity of the magnet reaches as high as 2.8 T; the saturation magnetization and the remanence are 69.6 and 51.4 emu/g, respectively. The magnet exhibits good thermal stability with the coercivity of 0.48 T at 773 K, and the coercivity temperature coefficient beta of -0.169%/K.     

Synthesis and properties of magnetoresistive (La,Sr)MnO<Subscript>3</Subscript>-based glass-ceramic borate-matrix composites

Magnetic glass-ceramic borate-matrix composites containing micron-sized lanthanum strontium manganite grains have been prepared by ceramming amorphous La₂O₃-SrO-MnO _x -B₂O₃ materials at 800 and 900°C. The glass-ceramics had a magnetization of up to 48.7 A m²/kg in a magnetic field of 400 kA/m. Their relative magnetoresistance reached 6.2% at 290 K in a magnetic field of 80 kA/m and 16% at 77 K in a field of 160 kA/m.     

Hydrothermal synthesis and characterization of monodisperse α-Fe2O3 nanoparticles

Monodisperse α-Fe₂O₃ nanoparticles have been successfully prepared by hydrothermal synthetic route using FeCl₃, CH₃COONa as reagents and reacted at 200 °C for 12 h. The morphology and structure of products were characterized by powder X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results showed that the α-Fe₂O₃ nanoparticles were single-crystalline hexagonal structure and average diameters were about 80 nm. Magnetic properties have been detected by a vibrating sample magnetometer at room temperature. The nanoparticles exhibited a ferromagnetic behavior with the coercive force (Hc), saturation magnetization (Ms) and remanent magnetization (Mr) was 185.28 Oe and 0.494 emu/g, 0.077 emu/g.     

Size variation and magnetic dilution effects on the structure and magnetic properties of cobalt zinc ferrite

Nanocrystalline cobalt zinc ferrites Co_1?xZn_xFe₂O₄ (x?=?0.0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0), have been prepared by employing a precursor combustion method via decomposition of the metal carboxylato hydrazinate precursors. This synthesis technique yields nanoparticles with particle size between 12 and 15 nm as determined from transmission electron microscopy (TEM) studies. The nanoferrites were then sintered at 1000 °C for 15 h to obtain micrometer size ‘bulk’ ferrites in the range of 0.3–0.8 μm. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) Spectroscopy confirmed the formation of the mixed ferrites without any impurities. Addition of non-magnetic ion like Zn²⁺ into the crystal structure of cobalt ferrite leads to a prominent change in the size, structure and properties. The saturation magnetization values (M_S) increases upto x?=?0.4 and then decreases with further increase in Zn concentration. A maximum M_S value of 90.85 emu/g and 79.59 emu/g for x?=?0.4 was obtained for the sintered and nanoferrite sample, respectively. The lower M_S and higher coercivity (H_C) values for nanoferrites than the sintered ferrites exhibited a strong dependence on the particle size due to the cation distribution and surface effects. The Curie temperature (T_C) was found to decrease appreciably with the reduction in particle size and with increasing concentration of Zn. The room temperature Mössbauer spectra showed a transition from ferrimagnetic to a paramagnetic state with increasing zinc concentration along with superparamagnetic features which was in corroboration with VSM studies.