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.     

Comparative effects of the hydrogen and nitrogen gas treatment and re-calcination (GTR) routes on the composition, microstructure, and magnetic properties of conventionally synthesized Sr–hexaferrite

Optimized static hydrogen treated and recalcined (HTR) and static nitrogen treated and recalcined (NTR) Sr–hexaferrite powders synthesized conventionally in-house are compared with one another. The phase identification studies and lattice parameter measurements showed first that the Sr–hexaferrite decomposed, forming iron oxide (Fe2O3), which was then reduced during the static hydrogen or nitrogen treatment, and, second, that the hexaferrite phase was recovered albeit with a small change in the composition (as indicated by the lattice spacings) after the re-calcination treatment in static air. These effects were more pronounced in the hydrogen process than in the nitrogen process. The main effect of this gas-treatment and re-calcination (GTR) process on the microstructure of the Sr–hexaferrite was the transformation of the single-crystal particles into particles with a very fine sub-grain structure during the gas treatment, which resulted in the formation of polycrystalline hexaferrite particles with a much finer grain size during subsequent recalcination, compared to that of the initial hexaferrite powder. This finer structure was responsible for the higher coercivities observed after re-calcination. With regard to the hydrogen and nitrogen processes, the former resulted in a higher degree of oxide reduction and hence a higher coercivity on re-calcination. The coercivity of the initial Sr–hexaferrite increased from 310 kA/m (3.9 kOe) to 400 kA/m (5 kOe) after HTR and to 342 kA/m (4.3 kOe) after NTR. The initial magnetization behavior was also different for the HTR- and NTR-processed powders, with the former exhibiting behavior characteristic of single domains. This was consistent with the grain size being significantly less than the single-domain size (1 ).     

Optimization of the coercivity-modifying hydrogenation and re-calcination processes for strontium hexaferrite powder synthesized conventionally

Strontium hexaferrite powder, synthesised conventionally in-house from strontium carbonate (SrCO3) and hematite (Fe2O3) without additives, has been treated in a static hydrogen atmosphere and subsequently calcined in static air under different conditions. The optimum time, temperature, and initial pressure of hydrogenation and the optimum temperature of re-calcination for a fixed time of 1 h were determined using a combination of X-ray diffraction, vibrating sample magnetometer, and high-resolution scanning electron microscope techniques.Increasing the temperature, initial pressure, and time of hydrogenation up to the determined optimum values resulted in the decomposition of the strontium hexaferrite into Fe2O3 and Sr7Fe10O22, together with a more marked reduction of the resultant Fe2O3 to Fe. This was accompanied by the conversion of the initial single-crystal particles into very fine sub-grains, which is the reason for the higher coercivities obtained after re-calcination. Increasing the hydrogenation and re-calcination parameters beyond the optimum values, however, generally resulted in grain growth, which decreased the final magnetic properties. Increasing the re-calcination temperature to 1000 °C resulted in completion of the hexaferrite reformation. Beyond this temperature, however, the coercivity decreased due to grain growth.The optimum conditions were as follows: hydrogenation at 700 °C for 1 h under an initial pressure of 1.3 bar and then re-calcination in air at 1000 °C for 1 h. The highest coercivity obtained after re-calcination was around 400 kA/m. The remanence and saturation magnetization values were very similar to their initial values before the hydrogen treatment.     

The influence of the coprecipitation conditions on the low-temperature formation of barium hexaferrite

We have investigated the formation of barium hexaferrite via the coprecipitation method. Various reagent salts and solvents were tested, and the coprecipitates were calcined at 300–800 °C for 1–50 h. The samples were characterized with X-ray powder diffraction, electron microscopy and magnetometry. The coprecipitation conditions had a significant influence on the formation time of the barium hexaferrite, which started to form at as low as 500 °C. The optimum coprecipitation conditions were: ethanol as a solvent and chlorides as reagent salts. Powders with optimum magnetic properties, saturation magnetization 60–63 emu/g and coercivity 4–5 kOe, were obtained by calcining at 600–700 °C.     

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.     

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     

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.     

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.     

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.     

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.     

Synthesis, stability against air and moisture corrosion, andmagnetic properties of finely divided looseNd2Fe14BHx, x⩽5, hydride powders

Nd₂Fe₁₄BH_x, x⩽5, hydride powders, with particle size as small as 1 μm, have been successfully prepared using a chemical method derived from the well-known oxide-reduction diffusion (ORD) method. In this method, the raw materials (Nd₂O₃, iron and boron) are mixed with calcium metal or hydride powder (in excess) and additions of anhydrous CaCl₂ and NaCl, and finally sintered at 1170-1270 K for a few hours under an argon atmosphere. This yields finely divided Nd₂ Fe₁₄B crystallites embedded in the byproducts. The material is then washed with water at room temperature, where the excess Ca in the mixture reacts with water and produces nascent hydrogen, which reacts with the alloy particles embedded in the byproducts, and finally yields a well-separated Nd₂Fe₁₄BH_x, x⩽5, hydride powder. Thermal stability, crystalline structure, and magnetic properties of several hydrided powders are studied systematically. These studies show that the interstitial hydrogen atoms led to 1) an increase in the lattice volume by as much as 4.2%, 2) a decrease in the coercivity to almost zero, 3) a dramatic improvement in T_C from 593 to 642 K, and 4) a substantial modification of the magnetization process, showing magnetic saturation at lower fields of ≈60 kOe (against ≈150 kOe in anhydride)     

Microwave properties of polymer composites containing combinations of micro- and nano-sized magnetic fillers

We investigated the microwave absorbing properties of composite bulk samples with nanostructured and micron-sized fillers. As magnetic fillers we used magnetite powder (Fe3O4 with low magnetocrystalline anisotropy) and strontium hexaferrite (SrFe12O9 with high magnetocrystalline anisotropy). The dielectric matrix consisted of silicone rubber. The average particle size was 30 nm for the magnetite powder and 6 micro/m for the strontium hexaferrite powder. The micron-sized SrFe12O19 powder was prepared using a solid-state reaction. We investigated the influence of the filler concentration and the filler ratio (Fe3O4/SrFe12O19) in the polymer matrix on the microwave absorption in a large frequency range (1 / 18 GHz). The results obtained showed that the highly anisotropic particles become centers of clusterification and the small magnetite particles form magnetic balls with different diameter depending on the concentration. The effect of adding micron-sized SrFe12O19 to the nanosized Fe3O4 filler in composites absorbing structures has to do with the ferromagnetic resonance (FMR) shifting to the higher frequencies due to the changes in the ferrite filler's properties induced by the presence of a magnetic material with high magnetocrystalline anisotropy. The two-component filler possesses new values of the saturation magnetization and of the anisotropy constant, differing from those of both SrFe12O1919 and Fe3O4, which leads to a rise in the effective anisotropy field. The results demonstrate the possibility to vary the composite's absorption characteristics in a controlled manner by way of introducing a second magnetic material.     

High performance Sm-Co powders obtained by crystallization from ball milled amorphous state

Isotropic Sm-Co nanoparticle powders with high coercivity were prepared by high-energy ball milling followed by optimal annealing at different temperatures. The covercivity increased monotonically with increasing of the annealing temperature and a highest coercivity of 31.2 kOe was obtained. The sample with an optimal energy product of 17.0 MGOe still had a coercivity of 18.2 kOe. The evolution of phase,particle size, mechanism of coercivity and other related magnetic properties were analyzed. The excellent performance is attributed to nanoscale size grains below 15 nm and good exchange coupling between nanoparticles.     

Coercivity and superparamagnetic evolution of high energy ball milled (HEBM) bulk CoFe2O4 material

Ball milling (BM) of bulk CoFe₂O₄ powder material carried out in order to study its structural stability and attendant property changes with respect to coercivity enhancements and superparamagnetic behaviors, showed that drastic crystallite size reduction occurred within the first 1 h of ball milling. Crystallite size dropped from 74 nm for the as-received material to a value of 11.6 nm for 600 min of ball milling. Combined X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses confirmed crystallite size reduction with corresponding increase in interparticle agglomeration/pores with increasing milling time. The maximum coercivity of 0.46 T and the crystallite size of 15.6 nm were recorded with 20 min, while peak residual strain of 0.0066 mm/mm was for 180 min of BM. Material with peak coercivity value did not have peak residual strain, or minimum crystallite size, thereby suggesting that other structural defects contributed to coercivity enhancement. The saturation magnetization (M_s) value decreased continuously with increasing milling time, while remanence magnetization (M_r) and coercivity decreased with increasing BM time, after an initial increase. Mössbauer spectroscopy (MS) measurements confirmed both particle size distribution and decomposition/disordering of the material together with superparamagnetism as BM time increased. The degree of inversion ranged from 41% to 71.7% at different milled states from Mössbauer spectroscopy. The internal magnetic fields of the Fe sites associated with the tetrahedral and octahedral sites were 507.4 kOe and 492 kOe respectively in the unmilled state, while 484 kOe and 468.5 kOe in the 600 min milled state correspondingly.     

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.     

Magnetic and Morphological Properties of Electrodeposited Thick FePt Films on Metallic (Au, Ag, Cu) Underlayers

Electrodeposited thick films of FePt (with the nominal composition 50 % Fe/50 % Pt) on three metallic (Au, Ag, Au) underlayers were annealed at various temperatures. The magnetic and morphological properties of the resulting films were then monitored. The Au and Ag underlayers promoted the growth of the (bct) L1₀ FePt phase. The greater growth of this phase in the films deposited on the Ag underlayer led to the crystallographic texturing in the (001) direction. This was accompanied by a significant magnetic anisotropy and a negative shift of the remanent magnetization in the presence of an applied field. The coercivity of the Ag underlayer films increased to 18 kOe while the coercivity of the Au underlayer films decreased to ～2 kOe when the annealing temperature was increased to 800 ^°C.     

Fine particles of lanthanum-and cobalt-doped strontium hexaferrite prepared through glass crystallization

Glass of nominal composition Sr_0.6La_0.4Fe_11.6Co_0.4O₁₉ + 12SrB₂O₄ was prepared by rapidly quenching an oxide melt and was then heat-treated at temperatures from 550 to 900°C to give glass-ceramics containing fine lanthanum-and cobalt-doped strontium hexaferrite particles and microcrystalline SrB₂O₄. The materials were characterized by x-ray diffraction, scanning electron microscopy, electron probe x-ray microanalysis, and magnetic measurements. The coercivity of the glass-ceramic samples was shown to increase up to 427 kA/m with increasing heat-treatment temperature. The saturation magnetization of the samples increases up to 25.0 A m²/kg as the heat-treatment temperature is raised to 750°C, and decreases slightly at higher temperatures. Dissolving the nonmagnetic matrix of the glass-ceramic prepared at 900°C, we obtained submicron powder of composition Sr_0.88La_0.12Fe_10.74Co_0.47O_y, as determined by x-ray microanalysis.     

Enhancing absorption properties of Mg–Ti substituted barium hexaferrite nanocomposite through the addition of MWCNT

M-type barium ferrite with Mg–Ti substitution and MWCNT addition was synthesized using high-energy ball milling. The prepared sample was further analyzed using X-ray diffraction, field emission scanning electron microscope (FESEM), vibrating sample magnetometer and vector network analyzer. The results showed that the particle size had a wide range of distribution, and a hexagonal structure was formed in the sample. The sample was observed to have lower saturation magnetization and coercivity after Mg–Ti was substituted with MWCNT and added into the barium hexaferrite. Reflection loss was studied as a function of frequency and thickness of the sample. For Mg–Ti substituted barium hexaferrite composite with a thickness of 2.0 mm, the reflection loss peaked at ?28.83 dB at a frequency of 15.57 GHz with a bandwidth of 6.43 GHz at a loss of less than ?10 dB. The microwave absorption primarily resulted from magnetic losses caused by magnetization relaxation, domain wall resonance, and natural resonance. FESEM micrograph demonstrated that carbon nanotubes were attached to the external surface of the ferrite nanoparticles. The investigation of the microwave absorption indicated that with an addition of carbon nanotubes, the real and imaginary parts of permittivity and reflection loss had enhanced to ?34.16 dB at a frequency of 14.19 GHz with a bandwidth of 5.72 GHz.     

Structural and magnetic properties of BaFe12-2xCoxTixO19 powders prepared by the glasscrystallization method

The glass crystallization method is shown to be suitable for preparing BaFe_12-2xCo_xTi_xO₁₉ powders with a nearly perfect crystallographic structure and a narrow particle size distribution. The dependences of the specific magnetization, the coercivity, and the particle size distribution on both the annealing temperature and the Co-Ti content correlate in a characteristic way. Furthermore, the lattice parameters, the specific surfaces and the thickness of the effective nonmagnetic surface layer were determined. The coercivity of the hexaferrite magnetic tapes is more than 8×10³ A/m higher than for the corresponding powders