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.     

X-ray diffraction studies on aluminum-substituted barium hexaferrite

Effect of aluminum substitution in barium hexaferrite was studied following the hydrothermal precipitation–calcination techniques. It was attempted to prepare aluminum-substituted barium hexaferrites with compositions BaAl_xFe_12−xO₁₉ having x=2,4, 6, 8 and 10. The precursors were prepared by using stoichiometric amounts of Ba, Al and Fe³⁺ nitrate solutions with urea as the precipitating agent. The hydrothermally prepared precursors were calcined at temperatures in the range of 800–1200 °C. The detailed work carried out on identification of crystalline phases through XRD revealed that, after the hydrothermal treatment, the samples showed barium carbonate, hematite and boehmite as the crystalline phases (except that boehmite was not identified for Ba/Al/Fe ratio as 1:2:10). Irrespective of the Al content, none of the 1000 and 1200 °C calcined samples showed any crystalline phase of Al. The 1200 °C calcined samples showed that Al-substituted barium hexaferrite is formed only with the Ba/Al/Fe atomic ratio of 1:2:10. With increase in the Al content, formation of hexaferrite was not observed. BaCO₃ was found be present even at 1200 °C in all the samples except for the one having Fe/Al ratio 5. The normal decomposition temperature of BaCO₃ is between 950 and 1050 °C. It is difficult to explain the increased stability of BaCO₃, which is perhaps responsible for hindering the formation of aluminum-substituted barium ferrite having Fe/Al ratio ≤2. The Al substitution in barium hexaferrite was confirmed through magnetic measurements.     

单相钡铁氧体磁性材料的制备及磁性能研究

以硝酸钡、硝酸铁和柠檬酸为原料,采用溶胶-凝胶法制备了单相钡铁氧体(BaFe12O19)纳米粉体,并进一步研究了n(Fe)/n(Ba)、热处理温度对产物组成、形貌以及磁性能的影响。用X射线衍射仪(XRD)、扫描电子显微镜(SEM)和振动样品磁强计(VSM)分别对样品的组成、形貌和磁性能进行了表征。实验结果表明,当煅烧温度不变时,样品的晶粒尺寸随着n(Fe)/n(Ba)的增大而变大,磁性能随n(Fe)/n(Ba)的增大而增强;当n(Fe)/n(Ba)不变时,样品的晶粒尺寸随着煅烧温度的升高而变大。当n(Fe)/n(Ba)=12时,在800℃煅烧2h得到单一晶型的钡铁氧体粉体。1000℃时样品的磁性能最佳,饱和磁化强度(Ms)为70.88A.m2/kg,矫顽力(Hc)为372.89kA/m。     

Synthesis of barium hexaferrite by the co-precipitation method using acetate precursor

Fine particles of barium hexaferrite were synthesised by a chemical co-precipitation method using acetate-nitrate (barium acetate + iron nitrate) precursors. The thermal properties, phase composition and morphology of hexaferrite powders were studied. Simultaneous DTA/TG results confirmed by those obtained from XRD and VSM, indicated that the formation of barium hexaferrite occurs at a relatively low temperature of 710°C. This temperature is affected by the Fe³⁺/Ba²⁺ molar ratio. The SEM investigations revealed that the mean particle size of barium hexaferrite increases with increasing calcination temperature. In this system the Fe³⁺/Ba²⁺ molar ratio of 12 (stoichiometric ratio) is favourable.     

Solvent-free synthesis of hexagonal barium ferrite (BaFe12O19) particles

M-type barium ferrite (BaFe₁₂O₁₉) particles, from a mixture of barium nitrate, ferric nitrate, cetyltrimethylammonium bromide (CTAB), and ammonium carbonate, have been successfully prepared through simple grinding and calcination in the absence of any solvent. The products are characterized by X-ray diffraction, scanning electron microscope, and vibrating sample magnetometer, whose results indicate that they have well crystalline phase of BaFe₁₂O₁₉, typically hexagonal platelet-like structure, large saturation magnetization, even submicrometer particle size under the optimum condition. Meanwhile, the effects of Fe/Ba ratio, CTAB, and ammonium carbonate are also investigated. It has been found that the proper Fe/Ba ratio could suppress the intermediate phase such as α-Fe₂O₃ and BaFe₂O₄, CTAB could promote the crystallinity of BaFe₁₂O₁₉ and produce hexagonal crystal structure, and ammonium carbonate was the key for forming BaFe₁₂O₁₉ phase. This facile method may be helpful for the preparation of other multicomponent functional materials.     

Influence of synthesis variables on the properties of barium hexaferrite nanoparticles

Barium hexaferrite (BaFe₁₂O₁₉) nanoparticles were synthesized by sol–gel auto-combustion route. Prepared samples were sintered at 950 and 1100 °C with Fe³⁺/Ba²⁺ = 12 and 20 mol ratio. The formation mechanism of barium hexaferrite was investigated by using X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analyses. In addition, the effect of temperature and Fe³⁺/Ba²⁺ mole ratio on BaFe₁₂O₁₉ formation and magnetic properties, and the effect of increasing the Fe³⁺/Ba²⁺ upon gel ignition and subsequent phase development were investigated. Finally the magnetic behavior was monitored with VSM. DSC studies showed that pure barium hexaferrite phase was formed from maghemite (γ-Fe₂O₃), without the formation of hematite (α-Fe₂O₃). Also, XRD results confirmed the formation of barium hexaferrite phase in non stoichiometric Fe/Ba ratio. VSM results showed that the saturation magnetization was decreased and coercivity increased with decreasing the grain size.     

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.     

Synthesis and characterizations of Nd doped SrFe12O19 nanoparticles

This is the first report ever on Nd³⁺ doped M-type hexaferrite nanoparticles: SrNd_xFe_12−xO₁₉ (0 ≤ x ≤ 1) prepared by citrate precursor using the sol–gel technique followed by gel to crystallization. The influence of the Nd³⁺ substitution, Fe³⁺/Sr²⁺ molar ratio and the calcination temperature on the crystallization of ferrite phase have been examined using powder X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), inductance capacitance resistance meter bridge (LCR) and vibrating sample magnetometer (VSM). The structural analysis reveals that the Nd³⁺ ions rearrange themselves in the host lattice without disturbing the parent lattice and Fe³⁺/Sr²⁺ molar ratio less than 12 is more favorable to achieve single phase hexaferrite at calcination temperature 900 °C for 4 h. Mid-IR analysis confirms that Nd³⁺ occupies the octahedral site. Detailed studies of electrical properties of prepared materials have been investigated in the frequency range 100–1000 Hz at room temperature by LCR meter and two probe technique. The result shows that the electrical properties strongly depend upon the frequency of applied field and dopant concentration. The magnetic measurements showing a considerable improvement in coercivity with the substitution of Nd³⁺ on iron sites, while the unsubstituted hexaferrites have highest value of specific saturation magnetization.     

Preparation of high quality,single domain BaFe12O19 particles by the citrate sol–gel combustion route with an initial Fe/Ba molar ratio of 4

BaFe₁₂O₁₉ particles have been synthesized by citrate sol–gel combustion route in a wide temperature range between 800 and 1200 °C with initial Fe/Ba molar ratios between 12 and 2. Structural, morphological and magnetic properties of the powders have been investigated by XRD, FT-IR, SEM and magnetization measurements. It was observed that both coercivity and specific saturation magnetization increase with annealing at temperatures up to 1100 °C, where a transition from single to multi domain structure occurs. To prevent formation of the hematite phase (α-Fe₂O₃), samples with different Fe/Ba molar ratios between 12 and 2 have been prepared and an intermediate phase, BaFe₂O₄, which may occur in Ba-rich samples has been removed by etching the powders in diluted hydrochloric acid. In this way, it was shown that single domain barium hexaferrite particles having high saturation magnetization, close to the theoretical value, and high coercivity can be synthesized with the initial Fe/Ba molar ratio of 4 in the sol–gel method. The chemical composition of this sample was determined as BaFe_11.80O_19.45 by the EDS analysis and Ba_1.05Fe_11.54O_18.4 using an ICP-MS device. Both are very close to the theoretical formula.     

Solvothermal Synthesis of SrFe12O19 Hexaferrites: Without Calcinations

In this study, the effect of temperature on the hydrothermal synthesis of single-phase SrFe₁₂O₁₉ hexaferrite (SrM) was investigated. For this synthesis, annealing or calcination process was applied. The Fe/Ba molar ratio was taken as 8:1. In this study, single-phase SrM NPs were synthesized via hydrothermal method. XRD patterns showed the presence of the hard (SrM) phase in the samples treated at 200 and 220 ^°C. Besides, formation of hexagonal plate-like samples was observed in SEM micrographs. Despite the low magnetization and coercive field values, the presence of the SrM phase was also shown in magnetization measurements. A reduced magnetization was explained by the existence of SrCO₃ and Fe₂O₃ phases, and a high shape anisotropy is probably the reason of low coercivity.     

Synthesis and Characterization of Nano-crystalline Calcium Hexaferrite Particles by Mechano-Thermal Method

Calcium hexaferrite was synthesized using high-energy planetary ball milling of a mixture of CaCO₃ and Fe₂O₃ powders for 10 hours followed by calcination at 1175 ^°C for 1 h. Effects of Fe₂O₃/CaCO₃ molar ratio, mechanical activation and dopant addition on the phase evolution, morphology, and magnetic properties of the products were studied. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) technique equipped with an energy dispersive spectrometer (EDS), and vibrating sample magnetometer (VSM). XRD results showed that addition of La₂O₃ in the samples lead to the formation of stabilized calcium hexaferrite particles with a mean crystallite size of 54 nm. Moreover, the formation of calcium hexaferrite was acknowledged at Fe₂O₃/CaCO₃ molar ratio of 5. SEM micrographs showed agglomerated particles of calcium hexaferrite with a mean particle size of 1.5 μm. Nano-crystalline Ca_0.8La_0.2Fe₁₂O₁₆ particles show saturation magnetization of 14 emu/g and coercivity of 500 Oe.     

Hexagonal ferrite particles for perpendicular recording prepared byion exchange

Particles of nonstoichiometric M-type barium hexaferrite with the chemical formula BaFe_10.5M_0.25O_17.05 (M=Mg, Cd, Co, Ni, Nd Zn) have been prepared by ion exchange in Ba containing molten salts from precursor ferrites with the β- or β"-alumina structure. The particles possess a plateletlike shape with diameters as low as 0.2 μm and as diameter-to-thickness ratio between 5 and 10. The Curie temperature is close to 470°C. Saturation magnetization values σ_s up to 64 emu g and coercive fields jH_c ranging from 0.5 to 1.5 kOe were found     

Synthesis of barium hexaferrite for magnetic recording media using the KCl flux system

Individual and fine crystals of barium hexaferrite were prepared by a modified flux method using the KCI flux system. Co²⁺-Ti⁴⁺-substituted barium hexaferrite with a homogeneous composition was synthesized at 950° C for 5 h or at 1000° C for 1 h from a mixture of BaCO₃, Fe₂O₃, CoO and TiO₂ with 30 wt% KCI added. Laboratory-prepared fine Fe₂O₃, was preferred because it gave ferrite particles with diameters of 0.2 to 0.4m. Magnetic properties were controlled by the Co-Ti content in hexaferrite crystals. Coercive force and Curie temperature decreased with the degree of Co-Ti substitution with saturation magnetization held at high value. The present process, from which individual and fine barium hexaferrite crystals can be prepared by using the KCI flux system, is recommended as a means of mass-production of ferrite powders with controlled magnetic properties for use in magnetic recording media.     

Synthesis and characterization of nano crystalline BaFe12O19 powders by low temperature combustion

Nano crystalline BaFe₁₂O₁₉ powders have been prepared at a relatively low calcination temperature by a gel combustion technique using citric acid as a fuel/reductant and nitrates as oxidants. The effects of processing parameters, such as Ba/Fe ratio, citric acid/nitrates ratio, reaction temperature on the powder characteristics and magnetic properties of the resultant barium ferrites were investigated. By controlling the molar ratio of citric acid to metal nitrates, nano crystalline BaFe₁₂O₁₉ powders with different particle sizes have been obtained. Phase attributes, microstructures and magnetic properties of the powders were characterized using X-ray diffraction analysis, X-ray line-broadening technique, Fourier transform infrared spectroscopy measurements, transmission electron microscopy and vibrating sample magnetometer. The maximum saturation magnetization value and intrinsic coercivity value for the obtained barium hexaferrites are 59.36 emu/g and 5540 Oe.     

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.     

Phase formation and magnetic property evolution processes of the hexaferrite with composition BaO⋅0.9Sc<Subscript>2</Subscript>O<Subscript>3</Subscript>⋅5.1Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>

The phase formation and magnetic property evolution processes of the hexaferrite with composition BaO?0.9Sc₂O₃?5.1Fe₂O₃ have been investigated. Results show that when the calcination temperature is lower than 1000 °C, the spinel phase BaFe₂O₄ and M-type hexaferrite phase BaFe₁₂O₁₉ dominate. The M-type hexaferrite BaFe_12?xSc_xO₁₉ (0?<?x?≤?1.8) appears above 1050 °C and becomes a single phase BaFe_10.2Sc_1.8O₁₉ above 1200 °C. A two-step decrease of both the coercivity and remanence ratio is observed above 1050 °C, which agrees well with the appearance of soft magnetic phase BaFe_12?xSc_xO₁₉ (0?<?x?≤?1.8). The saturation magnetization of the sample increases with calcination temperature until 1100 °C and then decreases. Raman spectra results show that the above magnetic property evolutions can be explained by a temperature dependent incorporation of Sc³⁺ into the lattice sites nearby the magnetic blocks’ interfaces. This weakens the local magnetic exchange interactions between Fe³⁺ and thus leads to a change in the magnetic structure.     

Complex Permittivity and Permeability of Co$_2$U (Ba$_4$Co$_2$Fe$_36$O$_60$) Hexaferrite Bulk and Composite Thick Films at Radio and Microwave Frequencies

We measured dielectric, magnetic, and microwave properties of Co ₂U hexaferrite (Ba₄Co₂Fe₃₆O ₆₀) polycrystalline bulk and composite thick film samples, and studied the effect of annealing temperature on phase formation and microstructure. We synthesized the bulk samples from a precursor prepared by the citrate method. The values of dielectric constant at radio frequencies (50 Hz-1 MHz) of the bulk Co₂U12 sample sintered at 1200degC are much higher, and the resistivities are lower, compared to M-type barium hexaferrites. Coercivity is also low, having a value of 5 G for the Co₂U12 sample, whereas the saturation magnetization value is 59 emu/g, which is comparable to that of M-type hexaferrites. We also measured the complex permittivity and permeability for Co₂U12 samples at microwave frequencies and found the values high compared to M-type barium hexaferrite at these frequencies. Thick composite films were prepared from a ferrite-polymer mix, and all the above properties were studied for these films. We observed that these thick films have lower values of dielectric and magnetic parameters both at low and microwave frequencies. We measured microwave-absorbing properties for the ferrite-polymer sample (ferrite to polymer ratio 70/30) which showed a maximum reflection loss of -37.5 dB at 11.5 GHz for the 3.5-mm-thick sample     

Structural and Magnetic Properties of Nano-structured Eu 3+ Substituted M-Type Hexaferrites Synthesized by Sol-Gel Auto-combustion Technique

Nano-structured M-type hexaferrites having the nominal composition Sr_0.8Ca_0.2Eu _x Fe_12?x O₁₉ (x=0.0, 0.05, 0.1, 0.15, 0.2, 0.25) have been synthesized by a sol-gel auto-combustion technique. The aim of the present study is to investigate the effect of rare-earth Eu³⁺ ions substitution at Fe³⁺ site on the structural and magnetic properties of M-type hexaferrites that might have not been previously explored especially using the sol-gel auto-combustion technique. The samples have been characterized by Differential Scanning Calorimetry (DSC), Fourier Transform Infra-Red (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray fluorescence (ED-XRF) and vibrating sample magnetometer (VSM). The XRD analysis confirms the formation of single M-type hexaferrite phase. The ratio ‘c/a’ lies in the expected range of 3.946–3.951 for M-type hexaferrites phase. The crystallite size was found to be in the range of 15–45 nm, which is sufficient to obtain the suitable signal to noise ratio in the high density recording media. Scanning Electron Microscopy (SEM) analysis exhibits the morphology of grains to be hexagonal platelet. The values of remanence (M _r ) and maximum magnetization (M) lie in the range 31–68 emu/g and 47–90 emu/g, respectively. The coercivity (H _c ) values lie in the range of 2412–4046 Oe and enhancement in the coercivity may be due to increase in the shape anisotropy. The magnetic properties such as coercivity (H _c ), magnetization (M), and retentivity (M _r ) make the synthesized materials useful for applications in the recording media.     

Extraordinary role of Ce–Ni elements on the electrical and magnetic properties of Sr–Ba M-type hexaferrites

The structural, electrical and magnetic behavior of Sr_0.5Ba_0.5−xCe_xFe_12−yNi_yO₁₉ (where x = 0.00–0.10; y = 0.00–1.00) hexaferrite nanomaterials are reported in this paper. The structural analysis indicates that the Ce–Ni doped Sr–Ba M-type hexaferrite samples synthesized by the co-precipitation method are stoichiometric, single magnetoplumbite phase with crystallite sizes in the range of 35–48 nm. The dc-electrical resistivity of the pure Sr–Ba hexaferrite is enhanced to almost 10² times by doping with Ce–Ni contents of x = 0.06; y = 0.60. The dielectric constant and dielectric loss tangent decrease to values 14 and <0.2, respectively, by increasing the frequency up to 1 MHz. Small relaxation peaks at frequencies >10⁵ Hz are observed for the samples with Ce content of x > 0.06. The values of saturation magnetization increase from 66.32 to 84.33 emu/g and remanance magnetization from 42.64 to 56.01 emu/g but coercivity decreases from 2.85 to 1.59 kOe by substitution of Ce–Ni. Sharp ferri-paramagnetic transition is observed in the samples, which is confirmed by DSC results. Ce–Ni substitution acts to reduce the electron-hopping between Fe²⁺/Fe³⁺ ions and also improves the magnetic properties. These characteristics are desirable for their possible use in microwave and chip devices.     

Towards Further Improvements of the Magnetization Parameters of B2O3-Doped BaFe12O19 Particles: Etching with Hydrochloric Acid

In the present study, we investigate influence of HCl-etching on magnetic parameters of B₂O₃-doped M-type barium ferrites. Our studies have shown that magnetization parameters; the remanence magnetization M _r and the specific magnetization M _s at 1.5?T, increase significantly with HCl-etching. The best magnetic parameters were observed in the sample of 0.1?wt% B₂O₃-doped and HCl-washed one after calcination at 1000?°C (M _r =34.9?emu/g, M _s =0.63.3?emu/g). Increments up to 50% in magnitudes could be achieved with HCl washing. Exchange interactions between particles were also examined by Wohlfarth model. It was observed that magnetizing-like interactions between particles become stronger but ,on the other hand, demagnetizing-like interactions becomes weaker with HCl-etching.