M-type Strontium Ferrite Research Articles

Enhanced magnetic properties of Sr0.7Ce0.3Fe11.7Zn0.3O19 by tuning pre-sintering temperature

In this work, pre-sintering temperature was tuned to investigate its effect on the structural and magnetic properties of oriented M-type hexagonal strontium ferrite Sr0.7Ce0.3Fe11.7Zn0.3O19. X-ray diffraction pattern shows that Fe2O3 and CeO2 impurity phases exist in all samples. As pre-sintering temperature increased, both saturation magnetization (Ms) and remanence ratio (Mr/Ms) first increased and then decreased. In contrast, coercivity (Hc) and magnetocrystalline anisotropy field (Ha) first decreased and then increased as pre-sintering temperature went up. The average grain size (D) increases monotonously with pre-sintering temperature. At pre-sintering temperature of 1200 ℃, Sr0.7Ce0.3Fe11.7Zn0.3O19 achieved optimal magnetic properties of Ms = 58.24 emu/g, Mr/Ms = 0.95, Hc= 1816 Oe, Ha= 10433 Oe and K1 = 1.51 × 105 J/m3. This work shows that the magnetic properties of M-type hexagonal strontium ferrite could be significantly improved by optimizing the pre-sintering temperature, which helps the design of high-performance and self-biased circulator.

Simultaneous improvement of coercivity and saturation magnetization of M-type strontium ferrite by Nd3+-Co2+ unequal co-substitution

We synthesized Sr1-xNdxFe12-yCoyO19 (x = 0–0.25, y = 0–0.1) using a conventional ceramic route. The crystal structures were analyzed using X-ray diffraction. With increasing Nd3+ and Co2+ contents, lattice constant a increases, while lattice constant c decreases. In terms of magnetic properties, the saturation magnetization and coercivity are simultaneously increased when x = y = 0.1 (equal co-substitution). This is mainly because the bivalent Co2+ has a smaller magnetic moment and unquenched orbital moments. When substitution amount x is further increased (unequal co-substitution), the magnetic properties are further improved and reach the optimum values of Ms = 76.4 emu/g and Hc = 5115 Oe owing to the occupation of the 2a site by divalent Fe2+. The occupancies of Co2+ and Fe2+ are further verified using Raman spectroscopy.

Effect of Ca2+ on the Microstructure, Morphology and Magnetic Properties of La-Co Substituted M-Type Strontium Ferrite at Low Fe Content

Effect of Ca2+ on the Microstructure, Morphology and Magnetic Properties of La-Co Substituted M-Type Strontium Ferrite at Low Fe Content

Study on Electrical and Magnetic Properties of Nd-Co Co-Substituted M-Type Strontium Ferrite Suitable for Microwave Devices

Study on Electrical and Magnetic Properties of Nd-Co Co-Substituted M-Type Strontium Ferrite Suitable for Microwave Devices

Study on the Effect of Ca Content and Fe Content on the Structure and Magnetic Properties of M-Type Strontium Ferrite

Study on the Effect of Ca Content and Fe Content on the Structure and Magnetic Properties of M-Type Strontium Ferrite

Tailoring the magnetic properties of M-type strontium ferrite with synergistic effect of co-substitution and calcinations temperature

ABSTRACT SrFe12O19 samples were initially prepared using sol-gel auto-combustion method and calcined from 500 to 1100°C (for 6 h). XRD data established the phase formation temperature of SrM phase to be ≈1000°C. Thereafter, the effect of Zn2+-Zr4+ co-dopant on magnetic and structural properties of SrZnxZrxFe12-2xO19 (x = 0.00–1.00) samples calcined between 900–1100°C was investigated. XRD data shows that the cell volume increases (from 690.45Å3 (x = 0.00) to 705.72Å3 (x = 1.00)) while crystallite size decreases (from 123.9 nm (x = 0.00) to 62.3nm (x = 1.00)) calcined at 1000°C. The Rietveld refinement of XRD data confirms a) x = 0.00 sample to be phase pure and b) the presence of ZnFe2O4 (≈2.25) & SrZrO3 (≈2.93%) impurity phases in x = 1.00 sample, calcined at 1000°C. FESEM micrographs revealed that a) grain size decreases for x > 0.20 with `x’ at 900°C and b) increases from 122 nm at 900°C to 443 nm at 1100°C, for x = 0.00. Broadening of Raman bands reveals generation of strain in the lattice with enhancement of Zn2+-Zr4+ concentration. The magnetic studies reveal that the coercivity monotonically decreases (from 5358(x = 0.00) Oe to 690(x = 1.00) Oe at 1000°C) with enhancement of Zn2+-Zr4+ concentration. A maximum Ms value ≈ 70.89 emu/g was observed in x = 0.20 sample at 1100°C. Ms value also decreases for x > 0.20 at each calcination temperature.

Correction to: Effects of Al and Ca ions co-doping on magnetic properties of M-type strontium ferrites

Correction to: Effects of Al and Ca ions co-doping on magnetic properties of M-type strontium ferrites

Effects of Al and Ca ions co-doping on magnetic properties of M-type strontium ferrites

M-type strontium ferrite is a kind of magnetic functional material with wide application prospect. In this work, SrFe12−xAlxO19 (0 ≤ x ≤ 1) and Sr1−yCayFe8Al4O19 (0 ≤ y ≤ 0.4) have been prepared by a hydrothermal method, and the effects of Al3+ doping and Al3+ and Ca2+ co-doping on the magnetic properties have been investigated. X-ray diffraction analysis indicates all the samples are a hexagonal structure, and the shift of the diffraction peaks suggests that Al3+ and Ca2+ are successfully doped in SrFe12O19. Field emission scanning electron microscope images show the particles have the hexagonal shape with the size of 1.1–2.9 µm. Vibrating sample magnetometer reveals that the doping of Al3+ and the co-doping Al3+ and Ca2+ can enhance magnetic properties of strontium ferrite. For example, the values of coercivity and the saturation magnetization of SrFe8Al4O19 are 914.1 Oe and 44.0 emu/g, which are 3% and 117% enhancement compared with SrFe12O19, respectively. For Sr1−yCayFe8Al4O19, coercivity and saturation magnetization increase from 914.1 Oe and 41.8 emu/g to 1047.2 Oe and 50.2 emu/g when the Ca2+ doping concentration increases from 0 to 0.4. Moreover, the effects of acid washing also on phase composition, morphology and magnetism are researched. The as-synthesized doping strontium ferrite will be useful for the applications in security switching and recording media.

Effect of Zn substitution on the structure and magnetic properties of Sr0.1La0.45Ca0.45Fe11.7−xZnxCo0.3O19 hexagonal ferrites

M-type strontium ferrites Sr0.1La0.45Ca0.45Fe11.7−xZnxCo0.3O19 (x = 0, 0.05, 0.1, 0.15 and 0.2) were synthesized by ceramic process. The effects of Zn substitution on phase composition, microstructure, sublattice occupation and magnetic properties of the ferrites were systematically investigated using X-ray diffraction, field emission scanning electron microscopy, 57Fe Mossbauer spectroscopy and magnetic properties test instrument, respectively. The results showed that all the samples were single hexagonal ferrite phase with no observation of other phases, while the lattice constant a did not vary basically and the lattice constant c increased continuously with increase of the Zn substitution amount. Simultaneously, the FESEM micrographs showed that more platelet shaped grains were observed. Mossbauer spectra have revealed that Zn2+ ions occupied both 4f1 and 4f2 sites, but Zn2+ ions would prefer to occupying 4f2 site with increasing Zn substitution. In addition, the remanence (Br) of the sintered ferrites increased firstly and then decreased, while the intrinsic coercivity (Hcj) decreased continuously. The sintered ferrites with the optimal Zn doping level of x = 0.05 exhibited high magnetic properties, including Br= 439.7 mT, Hcj= 371.8 kA/m and (BH)max= 36.88 kJ/m3.

Experimental results and analysis of Yb-Co doped strontium ferrites prepared by the coprecipitation

M-type strontium ferrites Sr1-xYbxFe12-xCoxO19 (x = 0, 0.05, 0.1, 0.15, 0.2) were co-doped with equivalent Yb-Co ions by the co-precipitation method. When the doping amount was 0 ≤ x ≤ 0.1, the synthesized products shown a single phase M-type strontium ferrite. When x > 0.1, the phase of α-Fe2O3 and CoFe2O4 appeared in the sample, and the most of grains were the six angle flake structure. When x = 0.1, the saturation magnetization (Ms), the remanent magnetization (Mr) and the coercive force (Hc) of strontium ferrites Sr0.9Yb0.1Fe11.9Co0.1O19 reached to the maximum values of 60.89 emu/g, 31.96 emu/g and 6846 Oe, respectively.

Magnetic properties of M-type strontium ferrites with different heat treatment conditions

The effects of heat treatment conditions on the magnetic properties and microstructure of M-type strontium ferrite according to calcination temperature were analyzed. Strontium ferrite Sr0.06Ca0.52La0.52Fe11.68Co0.22O19 magnetic powder was prepared by a standard ceramic process. During experiments, the calcination temperature was varied from 1180 to 1260 °C, and sintering temperature was fixed. While the M-phase (SrFe12O19) existed with hematite (Fe2O3) in the powder calcined at below 1220 °C, the pure M-phase was observed in the powder calcined at over 1240 °C. With an increase in the calcination temperature, the magnetization of the calcined powder increases, meanwhile, the coercivity decreases. The magnetization is improved by decreasing the lattice constant c and activating the Fe3+–O–Fe3+ superexchange interaction, and the coercivity decreases by the large particle sizes due to the grain growth.

Two-step sintering of M-type strontium ferrite with high coercivity

Two-step sintering (TSS) was applied on the M-type Sr-hexaferrite to control grain growth and form a desirable microstructure during the final stage of sintering. The process involved T1 (temperature in the first sintering stage), t1 (holding time at T1), T2 (temperature in the second sintering stage), and t2 (holding time at T2). The effects of each setting parameter on the density, grain growth, degree of orientation and magnetic properties were systematically investigated. The results revealed that T1 was the main factor that primarily controlled microstructure and magnetic properties. Compared with conventional sintering (CS) process, the magnets with smaller average grain size were achieved by TSS, which suggested that two-step sintering could improve the coercivity of Sr-hexaferrites. It was shown that the high integrated magnetic properties for Sr-hexaferrites, including Br = 440.4 mT, Hcj = 328.2 kA/m and (BH)max = 37.6 kJ/m3, were obtained under the optimum processing condition: T1 = 1200 °C, T2 = 1000 °C, t1 = 5 min, and t2 = 2 h.

The design and the preparation of mesoporous Ag3PO4 nanorod/SrFe12O19 hexagonal nanoflake heterostructure for excellent microwave absorption

Due to the high cut-off frequency induced by strong magneto-crystalline anisotropy, M-type strontium ferrite has been commercially used in GHz frequency. However, M-type strontium ferrite is unable to meet the performance requirements (strong absorption ability, thin absorbing thickness and broad effective bandwidth) of advanced absorbing materials. In order to overcome the above defects, mesoporous Ag3PO4 nanorods/SrFe12O19 hexagonal nanoflake heterostructure has been designed and prepared in this work. Being evaluated as microwave absorbents, this heterostructure exhibits the superior EM absorbing performances, such as the optimal reflection loss value of −63. 18 GHz at 5.1 mm, effective absorption bandwidth of 6.24 GHz at 2.3 mm and the EM absorption efficiency of 46.45 dB GHz/mm at 2.0 mm. The theoretical simulations are applied to explain the EM loss sources through the adjustment of frequency. This work provides a strategy to design effective microwave absorbers based on the dielectric-magnetic heterostructure from two aspects of theoretical simulation and experimental testing.

Preparation and magnetic properties of Sm–Co doped strontium ferrite

:In this work, Sm–Co doped strontium ferrite, SrFe12-xSmxCoxO19 (x = 0·1–0·5) composites were prepared by adding Sm3+ and Co2+ cations into M-type strontium ferrite using one-step chemical coprecipitation method. The composites were characterized by FTIR and XRD. The magnetic properties of the samples were measured and analyzed by VSM. The results indicated that the average crystallite sizes located in a range of 29·7–32·9 nm. Lattice constants, a and c were close to 5·88 and 23·05 Å for the M-type strontium ferrite phase. The analyses of magnetic properties demonstrated that when x = 0·2 and 0·4, Sm3+ and Co2+ ions substituted Fe3+ ions at 12 k (↑) or 2a (↑) site. While, as x = 0·1, 0·3, and 0·5, Fe3+ ions at 12 k (↑) or 2a (↑) site were replaced by Sm3+ ions and Fe3+ ions at 4f2 (↓) site were substituted by Co2+ ions. Meanwhile, the proposed substitute mechanism in this work was expected to provide a good concept to modify directionally M-type strontium ferrite in order to extend its application field.

Effect of oxygen potential on Co solubility limit in La–Co co-substituted magnetoplumbite-type strontium ferrite

La–Co co-substituted magnetoplumbite-type (M-type) strontium ferrite is typically used as a base material for high-performance hard ferrite magnets. Generally, the unquenched orbital moment of Co2+, substituted for Fe3+, is thought to enhance magnetic anisotropy and then coercivity. La3+ is substituted for Sr2+ to compensate for the charge unbalance associated with Co2+ substitution. Even if the synthesis starts with equal amounts of Co and La, the Co contents of the resulting samples are generally lower than the La contents because of the partial reduction of Fe3+ to Fe2+. We studied how the oxygen potential suppresses the formation of Fe2+ and expands the Co solubility range in this system. Polycrystalline samples, synthesized under oxygen partial pressures of 0.2, 1.0, and 387 atm, were characterized by powder X-ray diffraction, wavelength-dispersive X-ray analysis, and magnetization measurements. We reveal that the application of higher oxygen pressure dramatically expands the Co solubility range, resulting in the enhancement in magnetic anisotropy. We successfully synthesized Sr-free LaFe11CoO19 with high magnetic anisotropy using the hot isostatic pressure (HIP) technique.

Structural and Magnetic Properties Evolution of Co-Nd Substituted M-type Hexagonal Strontium Ferrites Synthesized by Ball-Milling-Assisted Ceramic Process

M-type hexagonal Sr1−x Co x Nd x Fe12−x O19 (x = 0, 0.08, 0.16, and 0.24) has been synthesized by ball milling, followed by calcination in air. The calcined products have been characterized by x-ray powder diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectra, and vibrating sample magnetometry. XRD and SEM analyses confirm the formation of M-type Sr hexaferrite with platelet-like morphology when Sr1−x Co x Nd x Fe12−x O19 (x = 0, 0.08, 0.16, and 0.24) precursors are calcined at 950°C in air for 2.5 h. Lattice parameters “a” and “c” values of Sr1−x Co x Nd x Fe12−x O19 reflect a very small variation after doping of Nd3+ and Co2+ ions. Average crystallite size of Sr1−x Co x Nd x Fe12−x O19 sample, calcined at 1150°C, decreased obviously after doping of Co2+ and Nd3+ ions. This is because the bond energy of Nd3+-O2− is much larger than that of Sr2+-O2−. Magnetic characterization indicates that all the samples exhibit good magnetic properties. Substitution of Sr2+ and Fe3+ ions by Nd3+ and Co2+ ions can improve the specific saturation magnetizations and remanence of Sr1−x Co x Nd x Fe12−x O19. Sr0.84Co0.16Nd0.16Fe11.84O19, calcined at 1050°C, has the highest specific saturation magnetization value (74.75 ± 0.60 emu/g), remanence (45.15 ± 0.32 emu/g), and magnetic moment (14.34 ± 0.11 μ B); SrFe12O19, calcined at 1150°C, has the highest coercivity value (4037.01 ± 42.39 Oe). These magnetic parameters make this material a promising candidate for applications such as high-density magnetic recording and microwave absorbing materials.

Influence of Y3+ substitution on the structural and magnetic properties of Sr0.7La0.3 Fe11.75−x Y x Co0.25O19 hexagonal ferrites

In this study, the Y3+ ion-substituted M-type Sr0.7La0.3Fe11.75−x Y x Co0.25O19 (0 ≤ x ≤ 0.2) hexagonal ferrites were synthesized by the traditional ceramic method. The structural, morphological, and magnetic properties were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and vibrating sample magnetometer techniques. The results show that a single M-type strontium ferrite was obtained with the Yttrium content (x) from 0 to 0.08, and the impure phase appeared when x is above 0.08. SEM images indicate the hexagonal platelet-like particles, and the size of the materials is about 3–5 μm. The saturation magnetization (M s) and coercivity (H c) of the magnetic powders both increased with the increase of x from 0 to 0.12, then decreased with the increase of x from 0.16 to 0.2.

Characteristics Improvement of Low-temperature Sintered SrFe 12 O 19 Ferrites in LTCC System Applications

In order to improve the sintering characteristics of the low-temperature sintered M-type hexagonal strontium ferrites (SrFe 12 O 19 ) prepared by a solid phase method, the Bi 2 O 3 additive was used as a sintering aid. The crystal structure, electrical properties, and magnetic properties of the ferrites were investigated. The results show that the single phase structural SrFe 12 O 19 ferrites can be obtained even with a low sintering temperature of 900 °C. The Bi 2 O 3 additive is found to possess a significant effect on the electrical properties and magnetic properties for the ferrites sintered at low temperatures enhancing resistivity ρ , saturation magnetization M s , intrinsic coercivity H ci , and magnetic anisotropy field H a . It is suggested that the low-temperature sintered SrFe 12 O 19 ferrites with ρ of 0.42×10 8 Ω·cm, M s of 285.6 kA·m −1 , H ci of 347.3 kA·m −1 , and H a of 1546.6 kA·m −1 provide an important potential application in nonreciprocal LTCC (low temperature co-fired ceramics) ferrite devices

Inter-grain effects on the magnetism of M-type strontium ferrite

By using films as model systems, a combined study based on experiments and micromagnetic simulations has been performed to get further insight into the magnetic behaviour of M-type SrFe12O19 (SrF) system in the nanosized range. Both nanocrystalline SrF quasi-isotropic and highly anisotropic films have been obtained by sputtering. Tuning of the preferred c-axis orientation, either normal or in the film plane, has been possible by varying the growth parameters and, importantly, with no need of any underlayer. Small changes in the oxygen ratio (up to 2%) and in-situ heating lead to different microstructure, which determine the strong variations of the magnetic behaviour observed, including both magnetization curves, easy axis directions, and their different coercive fields, which vary from 1 kOe to 5 kOe within the sample series. Micromagnetic simulations reproduce the experiments with a model based on a mixed population of randomly distributed SrF grains with in-plane and out-of-plane uniaxial anisotropies. The simulations show that intergranular interactions play the major role on the coercive field behaviour of the SrF system. A microstructure comprising largely uncoupled grains leads to improved magnetic performance of highly c-axis oriented SrF nanostructures.

Thermomagnetic stability of M-type strontium ferrite (SrFe12O19) particles with different shapes

M-type strontium ferrite (SrFe12O19: SrM) with two different shapes were synthesized by a simple and benign process. One is peanut-like shape, and the other is hexagonal platelet. These shapes were controlled by the shape and size of precursor Fe3O4 particles. A mixture of submicron- or nanometer-sized Fe3O4 particles and SrCO3 nanoparticles was calcined to transform to SrM, followed by acid-washing to remove secondary phase from SrM particles. Static magnetic properties, magnetic interactions, and thermomagnetic stability of the SrM particles were studied. The measured saturation magnetization and intrinsic coercivity are 74.2 emu/g and 4431 Oe, respectively, for the peanut-like SrM particles and 73.6 emu/g and 5360 Oe for the hexagonal SrM platelets. The saturation magnetization is close to the theoretical value of 76 emu/g. Both types of SrM show dipolar interactions and good thermomagnetic stability, i.e. α = Δσs/σs = −0.16%/K and β = ΔHci/Hci = 0.15%/K for the peanut-like SrM particles and α = −0.12%/K and β = 0.12%/K for the hexagonal SrM platelets. The temperature coefficient of intrinsic coercivity (β) is positive and magnetization still remains high at 400 K; 60 emu/g for the hexagonal SrM platelets and 50 emu/g for the peanut-like SrM particles.