Characterization and optimization of the coercivity-modifying nitrogenation and re-calcination process for strontium hexaferrite powder synthesized conventionally

Strontium hexaferrite powder synthesized conventionally in-house from strontium carbonate and hematite (Fe2O3) without using additives has been treated in a static nitrogen atmosphere and subsequently calcined in static air. The phase identification studies by means of X-ray diffraction (XRD) and thermal magnetic analysis (TMA) indicated the decomposition of the strontium hexaferrite and the reduction of the resultant iron oxide (Fe2O3) during the reaction with nitrogen. High-resolution scanning electron microscopy (HRSEM) studies show that the reduction occurring during nitrogenation results in the conversion of some of the large grains into much finer sub-grains. Strontium hexaferrite, Fe3O4, and Sr7Fe10O22 were the main phases obtained after reduction. However, weak traces of other phases, such as Fe2O3, were also detected. The hexaferrite phase re-formed on subsequent calcination. The magnetic measurements indicated a significant decrease in the intrinsic coercivity during nitrogenation due to the formation of Fe3O4. However, after a re-calcination process, the remanence and maximum magnetization (i.e., magnetization at 1100 kA/m) exhibited values close to the initial values before treatment, but the value of the intrinsic coercivity was higher than that prior to nitrogenation. Examination of the re-calcined microstructure showed that this could be attributed to the fine grains that originated from the fine sub-grain structures formed in the powder particles during nitrogenation.The optimum time, initial gas pressure, and temperature of nitrogenation and the optimum temperature of re-calcination were investigated using a vibrating sample magnetometer (VSM), XRD, and HRSEM. The optimum temperature for nitrogenation was 950 and 1000 °C for re-calcination. The optimum time and initial nitrogen pressure were 5 h and 1 bar, respectively. The highest intrinsic coercivity obtained after re-calcination was 340 kA/m.     

Appreciable Magnetic Moment and Energy Density in Single-Step Normal Route Synthesized MnBi

We study the structural and magnetic properties of the MnBi intermetallic compound. The LTP (Low Temperature Phase) MnBi compound is successfully synthesized in single step by vacuum encapsulation technique and rapid quenching from phase formation temperature. The phase purity and the magnetic moments of MnBi are highly dependent on heat treating schedule. The best phase purity and the magnetic moment are found for a heat-treated sample at 310 °C for 48 h. Rietveld fitted X-ray diffraction (XRD) patterns revealed that the studied MnBi compound is crystallized in hexagonal P63/mmc space group with minute presence of unreacted Bi and Mn phases. The scanning electron microscopy (SEM) study is carried out to visualize the grains morphology and phase identification. The bulk MnBi powder showed appreciable magnetic moment of ～62 emu/g at 6 Tesla and maximum energy product BH _max of 4.01 MGOe at 6 Tesla. The magnetic properties of synthesized MnBi show that it could be a potential candidate for rare earth free permanent magnets.     

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.     

纳米晶复合SrM永磁铁氧体的制备和交换耦合作用

采用sol-gel方法制备M型六角锶铁氧体。利用X光衍射、透射电子显微镜和VSM对纳米晶样品进行了研究。当热处理温度小于 80 0℃ ,样品存在复相。在同样条件下 ,压成薄片的样品存在硬磁与软磁SrFe12 O19/γ Fe2 O3 的纳米复合相的磁性交换耦合作用。温度为 80 0℃的薄片样品 ,比饱和磁化强度σS 为 75 .6emu/g ,内禀矫顽力Hcj 为6 0 15Oe ,最大磁能积 (BH) Max 为 1.87MGOe ,而粉末样品相应的分别为 75 .9emu/ g ,6 40 0Oe和 1.5 2MGOe。当热处理温度大于 85 0℃时 ,只有单一M相     

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.     

Effect of Ho3+ substitution on magnetic and microwave absorption properties of Sr(ZnZr)0.5Fe12O19 hexagonal ferrite nanoparticles

Sr_1?x Ho _x (ZnZr)_0.5Fe₁₁O₁₉ (x = 0.03, 0.06 and 0.09) hexaferrite nanocrystallites of average sizes in the range of 46–60 nm are synthesized by the citrate sol–gel method. Crystalline structure, morphology, magnetic properties, and microwave absorption properties of powders were studied via X-ray diffraction, field emission scanning electron microscope vibrating sample magnetometer, and vector network analyzer, respectively. The magnetic properties such as saturation magnetization (M _s ) and coercivity (H _c ) were calculated from hysteresis loops. The XRD patterns show that the main phase is M-type strontium hexaferrite without other impurity phases. Microwave absorption properties of hexaferrite (70 wt%)–acrylic resin (30 wt%) composites were measured by the standing-wave-ratio (SWR) method in the range from 12 to 20 GHz. Results showed that substitution of Ho³⁺ ions for Sr²⁺ ions in Sr(ZnZr)_0.5Fe₁₁O₁₉ resonance frequency moves to higher frequency. For samples with x = 0.03, a minimum reflection loss of ?42 dB was obtained at 16.6 GHz for a layer of 1.7 mm in thickness. It was concluded that the prepared composites could be good candidates for electromagnetic compatibility and other practical applications at high frequency.     

Influence of Co<Superscript>++</Superscript> ion concentration on magnetic and microwave properties of Ba<Subscript>(4−X)</Subscript>Co<Subscript>(2+X)</Subscript>Fe<Subscript>36</Subscript>O<Subscript>60</Subscript> (Ba-M-Y) hexaferrite

The present paper reports the influence of cobalt content on the structural, electrical, magnetic and microwave properties of barium hexaferrite synthesized via chemical co-precipitation method. The samples were characterized for their structural, electrical, magnetic and microwave characterizations using XRD, SEM, TEM, VSM etc. The transmission electron microscopy results showed that stacking of nanoparticles of size?~?50 nm. In addition, the highest saturation magnetization of 29.82 emu/g was observed for composition x?=?0.2. The microwave permittivity and permeability decreases with frequency and it varies with the cobalt concentration. Cobalt concentration strongly affects the microwave and magnetic properties of hexaferrite.     

Study of morphological and magnetic properties of microwave sintered barium strontium hexaferrite

Magnetic materials are important electronic materials that have a wide range of industrial and commercial applications. Barium strontium hexaferrite (Ba_0.5Sr_0.5Fe₁₂O₁₉-BSF) were prepared by a sol–gel method using d-Fructose as fuel and the heat treatment was carried out in a microwave furnace. The effects of the sintering temperature on the morphology, crystalline structure and magnetic properties are studied. Sintering temperature affected the grains in compact samples. The sintered product possessed dense microstructure with clear micro grains and is in consistence with the XRD analysis based on the peak intensity of the (107) plane. Magnetic measurement shows that the barium strontium hexaferrite sample sintered at 1,150?°C has the coercive field of 1,998 Gauss, remnant magnetization of 38.87?emu/g and the saturation magnetization of 53.44?emu/g.     

Synthesis and magnetic properties of conventional and microwave calcined barium hexaferrite powder

Single phase nanoparticles of barium hexaferrite (BaFe₁₂O₁₉–BaF) were synthesized by sol–gel method using metal nitrates as source and d-Fructose as a fuel. The prepared precursors were calcined by two different calcination techniques, using conventional furnace and microwave furnace. The samples are characterized using powder X-ray diffraction, theromogravimetric analysis and vibration sample magnetometer. Thermal analysis studies showed exothermic and endothermic reaction peaks from room temperature to 1,200 °C. X-ray diffraction studies established the formation temperature of single phase BaFe₁₂O₁₉. HR-SEM results showed the dispersed particles of hexagonal structure in platelet form. The broad hysteresis loop showed that the barium hexaferrite powder was in good crystalline nature.     

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.     

Effect of Mn doping concentration on structural,vibrational and magnetic properties of NiO nanoparticles

The Ni_1?xMn_xO (x?=?0.00, 0.02, 0.04 and 0.06) nanoparticles were synthesized by chemical precipitation route followed by calcination at 500?°C for 4?h. The prepared samples were characterized by energy dispersive analysis of X-rays (EDAX), powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR) and vibrating sample magnetometer (VSM). Rietveld refinement of XRD data confirms the structural phase purity and XRD patterns are well indexed to NaCl like rock salt fcc crystal structure with Fm-3m space group. The particle size of Mn doped samples is found to be less than that of pure NiO sample. However, the particle size increases slightly on increasing the Mn concentration due to surface/grain boundary diffusion. The vibrational properties of the synthesized nanoparticles were investigated by Raman and FT-IR spectroscopy. The results of room temperature magnetization (M-H) and temperature dependent magnetization (M-T) measurements are explained with a core-shell model. The synthesized nanoparticles show weak ferromagnetic and super-paramagnetic like behavior at room temperature.     

Structural and Magnetic Properties of Cobalt and Manganese Doped Ni-Ferrite Nanoparticles

This study reports that NiCoMn ferrite [Ni_(1?x)Co _x Mn _y Fe_(2?y)O₄ with (x=y=0.01,0.02)] powders are prepared by using the sol-gel combustion method. The effect of various calcination temperatures on their structural and magnetic properties is also investigated. Structural properties of the powders are carried out by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). According to XRD analysis, all samples of two compositions have cubic spinel structure, with an enlargement in crystalline size is observed with increasing of calcination temperature. The crystallite size of the nanopowders is estimated from (311) peaks using Scherrer’s formula. Spherical particles of nanocrystalline ferrite powders are shown in TEM photographs. The room temperature magnetic properties of particles are studied by using a vibrating sample magnetometer (VSM). The magnetization measurements also indicated that the saturation magnetization (M _s) increases as the calcinations temperature increases for both A and B samples in the range of 31.69 to 47.77 and 21.81 to 48.89 emu/gr, respectively. The value of coercivity fields (H _c) decrease with increasing the calcinations temperature. Furthermore, the properties of two samples synthesis at the optimum calcinations temperature (800 ^°C) compared together.     

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.     

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.     

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.     

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。     

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.     

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.     

Effect of calcination temperature on the physical properties of LiFe5O8 nanostructures

The structural properties of LiFe₅O₈ nanostructures, which were synthesized using a thermal treatment method, were investigated using different characterization methods. The XRD, FESEM and TEM results showed a phase transition from uncompleted α-LiFe₅O₈ and β-LiFe₅O₈ phases to completed α-LiFe₅O₈ phase when the growth calcination temperature shifted from 873 to 973?K. The crystallization was completed at 973?K, revealed by the absence of organic absorption bands in the FT-IR spectra. The results of band gap energy which were studied by UV–vis spectroscopy indicated that when calcination temperature increased, the appraised band gap energy of LiFe₅O₈ nanostructures decreased. Laser Raman analysis was used to determine the peaks of the synthesized LiFe₅O₈ nanostructures accurately and to differentiate between the α-LiFe₅O₈ and β-LiFe₅O₈ phases around 217?cm^?1. The results of a vibrating sample magnetometer (VSM) indicated that the magnetic properties differed between these nanostructures so that saturation magnetization and coercivity increased when the calcination temperature increased. The obtained results from electron paramagnetic resonance (EPR) spectroscopy demonstrated that as the growth calcination temperature shifted from 673 to 873?K, the results of g value and ΔH_pp increased up to the maximum value and then reduced for calcined sample at 973?K.     

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.