M-type Barium Hexaferrite Research Articles

Structural and magnetic properties of barium-gadolinium hexaferrites

A series of Gd-substituted M-type barium hexaferrites has been prepared by the ceramic route, according to the formula (Ba 1− x Gd x )O·5.25Fe 2O 3 ( x = 0 – 0.30 ). XRD analysis revealed that all the samples present primarily an M-type structure. Samples x = 0 and x = 0.05 are single-phase. Hematite (Fe 2O 3) and GdFeO 3 were detected in the remaining samples. Coercivity ( H c) shows remarkably high values, ∼293 kA/m for x = 0.20 and 0.30 with a maximum of 322 kA/m for x = 0.25 . Specific saturation magnetization ( σ sat) of the samples presents a small increase up to x = 0.10 . The microstructure examination indicates that Gd may act as a grain growth inhibitor.

Dielectric properties of cobalt substituted M-type barium hexaferrite prepared by co-precipitation

Barium hexaferrite was synthesised via the co-precipitation method using high purity nitrates, oxides and carbonates of iron (III), barium (II) and ammonium hydroxide. Once a phase pure sample of barium hexaferrite was obtained, it was doped, by weight, with 1, 2, 3, 5, 10, 15, 20 and 30% cobalt oxide (Co3O4). The addition of cobalt to the BaM had the effect of reducing the permittivity and loss tangent until a doping of 10% whereupon it remained constant at around 9. Thermogravimetric (TG) studies of green bodies showed decarboxilation to occur at around 825°C and the transformation of residual Co3O4 to Co2O3 at around 900°C. The X-ray diffraction (XRD) studies confirmed the Co ions substituting in the iron sites until a doping level, of 10–15% where the structure underwent a transition to one more closely resembling the W-type hexaferrite. The measured densities were found to vary with doping levels, with a maximum of 4.45 g/cm3 at 1% Co doping.

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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> U hexaferrite (Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">36</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sub> ) 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 Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> U12 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 Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> U12 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 Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> U12 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

Magnetic and structural properties of M-type barium hexaferrite prepared by co-precipitation

Stoichiometric and single-phase barium hexaferrite (BaM) has been synthesized by co-precipitation and an additional solid state preparative method using high-purity nitrates, oxides and carbonates of iron (III), barium (II) and ammonium hydroxide. The isochronally and isothermally measured permeability and magnetic loss tangents over 1 MHz–1 GHz in frequency remained moderately constant until a sintering temperature of 1300 °C where relative permeability of 1.3 and loss tangent of 0.06 at 1 GHz, indicate high-frequency magnetic characteristics for BaM. TGA/DTA studies showed the hexaferrite formation temperature was found to be 1050 °C. The apparent densities before and after each firing cycle showed a monotonic increase. X-ray diffraction studies established the presence of a single phase with the theoretical space group P63/mmc, and computed cell parameters of a= b=5.895 Å and c=23.199 Å. The samples exhibited a Curie point ( T c) of 452 °C and displayed ferromagnetic resonance characteristics centred on a frequency of 500 MHz.

Structure and Magnetism of Ba-Hexaferrite Films Grown on Single Crystal 6-H SiC With Graduated Interfacial MgO Buffer Layers

M-type barium hexaferrite films were processed by pulsed laser deposition on single-crystal 6-H silicon carbide substrates. MgO buffer and barrier layers were introduced to improve the film quality. Samples were characterized by X-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, X-ray diffraction, vibrating sample magnetometry, and ferromagnetic resonance (FMR). X-ray thetas-2thetas diffraction measurements indicated a strong (0, 0, 2n) crystallographic alignment. The magnetization of the BaM film is comparable to bulk values (4piM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> ~4320 G). A derivative power FMR linewidth of 500 Oe was measured at 55 GHz for the as-deposited films. This paper explores a potential next generation of microwave and millimeter-wave monolithic integrated circuit technology based upon a wide band-gap semiconducting material

Dielectric properties of M-type barium hexaferrite prepared by co-precipitation

Highly stoichiometric and phase pure barium hexaferrite has been synthesized by co-precipitation as well as a solid-state preparative method using high purity nitrates, oxides and carbonates of iron(III), barium(II) and ammonium hydroxide. The isochronal and isothermally measured complex permittivity and dielectric loss tangents over 1 MHz–1 GHz in frequency remained relatively stable until a sintering temperature between 1100 and 1300 °C. A high value of relative permittivity of 32 and low loss tangent of 0.0329 indicate suitable high frequency characteristics for barium hexaferrite. The measured apparent densities before and after each firing cycle showed a monotonic increase. The crystal structure determined from X-ray diffraction studies confirmed the presence of single phase belonging to the theoretical space group P63/ mmc with calculated cell parameters of a = b = 5.895 Å and c = 23.199 Å. In addition the DTA hexaferrite formation temperature was found to be 1050 °C.

Nanostructural M-type barium hexaferrite synthesized by spark plasma sintering method

This investigation shows that the spark plasma sintering (SPS) method may not only absolutely prevent the formation of intermediate phase α-Fe2O3 but also significantly lower the crystallization temperature of M-type barium hexaferrite. A kind of textured sample composed of major phase M-type barium hexaferrite and trace amount of BaFe0.24Fe0.76O2.88 has been successfully synthesized in one step by SPS process at 800°C for 10min. Scanning electron microscopy, energy dispersive spectroscopy, x-ray-diffraction, transimission electron microscopy, and vibrating-sample magnetometer techniques are used to study the microstructure, composition, phase formation, crystal orientation, and magnetism of the textured sample, respectively. It is confirmed that nanostructural M-type barium hexaferrites in the holes of the textured sample are in nanobelt and nanorod microstructures; these nanobelts and nanorods are nonstoichiometric and completely random and are characterized by barium or iron surplus, and that their composition may be described as BaFe12+xO19+1.5x (−4.49⩽x⩽6.60). The prolongation orientation of a nanobelt is parallel to the zone axis [101¯0]. The values of σs and Hc of the textured sample are 65.52emu∕g and 1412.17Oe, respectively. The Hc value decreases to one-fourth of that of randomly packed platelike M-type barium hexaferrite with same composition synthesized by sol-gel process at 900°C for 4h, while the σs value was independent of the synthesis conditions. This indicates that M-type barium hexaferrite magnets synthesized by SPS process may be applied to electromagnetic wave absorption and/or magnetic recording.

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.

Electromagnetic properties and microwave absorbing characteristics of doped barium hexaferrite

M-type barium hexaferrite BaFe 12− x (Mn 0.5Cu 0.5Ti) x /2O 19 ( x varying from 0 to 3 in steps of 1) have been synthesized by the usual ceramic sintering method. The ferrite powders possess hexagonal shape and are well separated from one another. The powder of these ferrites were mixed with polyvinylchloride plasticizer to be converted in to a microwave absorbing composite. X-ray diffraction (XRD), scanning electron microscope (SEM), ac susceptometer, vibrating sample magnetometer, and vector network analyzer were used to analyze its structure, electromagnetic and microwave absorption properties. The results showed that, the magnetoplumbite structures for all the samples have been formed. The sample having higher magnetic susceptibility and coercivity exhibits a larger microwave absorbing ability. Also, the present investigation demonstrates that microwave absorber using BaFe 12− x (Mn 0.5Cu 0.5Ti) x /2O 19 ( x = 2 and 3)/polyvinylchloride can be fabricated for the applications over 15 GHz, with reflection loss more than −25 dB for specific frequencies, by controlling the molar ratio of the substituted ions.

Properties of Cr-substituted M-type barium ferrites prepared by nitrate–citrate gel-autocombustion process

The Cr-substituted M-type barium hexaferrites, BaFe 12− x Cr x O 19, with x = 0.0 – 0.8 have been successfully prepared by nitrate–citrate auto-combustion process using citric acid as a fuel/reductant and nitrates as oxidants. The resulting precursors were calcined at 1100 °C for 1 h and followed by sintering at 1200 °C for 12 h in oxygen atmosphere. The ferrites were systematically investigated by using powder X-ray diffractometer (XRD), magnetic hysteresis recorder, Mössbauer spectrometer, and scanning electron microscope (SEM). The XRD data show the formation of pure magnetoplumbite phase without any other impurity phases. Both a and c lattice parameters calculated by the Rietveld method systematically decrease with increasing Cr content. The effects of Cr 3+ ions on the barium ferrites were reported and discussed in detail. The site preference of Cr 3+ and magnetic properties of the ferrites have been studied using Mössbauer spectra and hystereses. The results show that the magnetic properties are closely related to the distributions of Cr 3+ ions on the five crystallographic sites. The saturation magnetization systematically decreases, however, the coercivity increases with Cr concentration. The magnetization and Mössbauer results indicate that the Cr 3+ ions preferentially occupy the 2a, 12k, and 4f VI sites. The average size of hexagonal platelets obtained by SEM photographs tends to decrease with respect to Cr content.

Magnetic structure of Sn 2+Ru 4+-substituted barium hexaferrites prepared by mechanical alloying

M-type barium hexaferrite fine particles with composition BaFe 12−2 x Sn x Ru x O 19 (0.0≤ x≤0.3) were obtained by mechanical alloying. The effect of Sn 2+–Ru 4+ substitution on the magnetic properties and microstructure of the powders was investigated by Mössbauer spectroscopy, XRD, vibrating sample magnetometry (VSM) and thermomagnetic analysis. Mössbauer studies revealed that Sn 2+ ions replaced Fe 3+ ions on octahedral sites (4f 2 and 2a), while Ru 4+ preferred the tetrahedral 4f 1 and bipyramidal 2b sites. M s rise up to obtain a value of 64.2 A m 2/kg as x increased. The increase of M s owes to the site preference occupancy of Sn 2+–Ru 4+ mixture. The sharp decrease of the intrinsic coercivity (381.1–109 kA/m) has its origin on 4f 2 and 2b sites replacement.

Thick M-type barium hexaferrite films grown on garnet substrates

The deposition and characterization of thick M-type barium hexaferrite (BaFe12O19) films produced by a modified liquid phase epitaxy deposition technique are reported. The films are deposited on (111) oriented single crystal garnet (Gd3Ga5O12) substrates. The thickness of the films ranged between 45 and 80μm with growth rates of up to 40μm∕h. This growth rate is about five times greater than films grown on substrates of (111) magnesium oxide. Although the films are relatively thick, the ferrimagnetic resonance linewidth is remarkably narrow (∼50Oe) for external fields applied along the c axis.

Preparation of thick M-type barium hexaferrite films on double side of substrates by liquid-phase epitaxy deposition technique

Abstract Liquid-phase epitaxy deposition has been investigated to obtain thick and high quality of barium hexaferrite films on (1 1 1) gadolinium gallium garnet (GGG) and m-plane (1 1 0 0) aluminum oxide (Al 2 O 3 ) substrates. For films deposited on (1 1 1) GGG substrate the c -axis was aligned perpendicular to the film plane, but in the plane for films deposited on Al 2 O 3 . The films were deposited on both sides of the substrates, where the total thickness of the films ranged between 70 and 190 μm. The thickness and quality of these films meet the requirement for applications at high frequencies.

Single crystal hexaferrite phase shifter at Ka band

A phase shifter was fabricated from a scandium doped M-type (SCM) barium hexaferrite single crystal substrate, and tested over microwave frequencies from 20 to 40 GHz and bias magnetic fields from 0 to 2 kOe. At a microwave frequency of 40 GHz and bias magnetic field of 1.3 kOe, the differential phase-shift rate was 42°/kOe mm and the loss rate was 0.7 dB/mm. The phase shifter utilized the large uniaxial anisotropy field inherent in ScM single crystals, where the crystallographic c axis and the magnetic bias field were parallel to each other and in the substrate plane. The performance of the phase shifter was modeled by the conjugate method for transmission electron microscopy wave and compared with experimental results.

Microwave properties of ZnTi-substituted M-type barium hexaferrites

We have prepared magnetic anisotropic polycrystalline hexaferrites Ba(ZnTi)/sub x/Fe/sub 12-2x/O/sub 19/ with high orientation degrees f and low dielectric loss tangent tg/spl delta//sub /spl epsiv// by standard ceramic techniques with magnetic alignment of the grains' c axis during wet pressing and in flowing-oxygen sintering. When x increases, the saturation magnetization M/sub s/, the Curie temperature T/sub c/, and the anisotropy field H/sub a/ of the hexaferrites decrease. The results can be interpreted by assuming that Zn/sup 2+/ ions replace Fe/sup 3+/ ions of the tetrahedral 4f/sub 1/ sites and hexahedral 2d sites while Ti/sup 4+/ ions replace spin up Fe/sup 3+/ ions of octahedral sublattices 12k and 2a.

Mössbauer spectroscopic determination of magnetic moments of Fe 3+ and Co 2+ in substituted barium hexaferrite, Ba(Co,Ti) xFe (12−2 x) O 19

We report the distribution of magnetic moments of Fe 3+ and Co 2+ in Co 2+-, Ti 4+-substituted M-type barium hexaferrite, Ba(Co,Ti) x Fe (12−2 x) O 19, as a function of doping rate, x. The substitution, x, for iron has been varied with x=0, 0.25, 0.50, 0.70 and 0.85. The magnetic moments of Fe 3+ and Co 2+ were calculated from the combined results of Mössbauer measurements for Fe 3+ ions in the sublattices and neutron diffraction data for the total moments of Fe 3+ and Co 2+. A comparison of the signs of the magnetic moments of Fe 3+ and Co 2+ ions enabled us to attribute spin directions of the Co 2+ ions in the sublattices of the substituted ferrite samples. The spin directions of Co 2+ are opposite to those of Fe 3+ in the 4f 2 and 2b sublattices. They are reversed from the original directions in the 4f 1 and 12K sublattices when the value of x⩾0.70. A quantitative analysis shows that Co 2+ and Ti 4+ ions are preferably substituted into 4f 2 and 12K sublattices, respectively. In addition, while the hyperfine field of Fe 3+ in the 2b sublattice gives rise to the 2b–4f 2 interaction it is the partially substituted Co 2+ ions in the 4f 1 and 12K sublattices that contribute to the near neighbour 2a–4f 1 and 2b–12K types of interaction.

Cation distribution in bismuth-doped M-type barium hexaferrite

Mössbauer studies were conducted on Bi-doped barium ferrites BaFe 12–xBi xO 19 (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.6) with special attention paid to the behavior of the line widths in the Mössbauer sextets. Assuming that the broadening of the line widths in the various sextets is due to increases in the fluctuations in the hyperfine field (acting on a Fe ion in given site) caused by the introduction of the Bi ions into the neighboring Fe sites, the observed broadening of the lines belonging to the 4f vi and 4e patterns indicated that the impurity ions were entering into the 4e and 4f vi sites. The initial decrease in M s would arise if the Bi ions first substituted in the 4f vi and then into the 4e sites as more Bi is added.

Mossbauer and X-ray investigations of the kinetics and mechanism of M-type barium hexaferrite formation from hydroxide–carbonate precipitates

Mossbauer and X-ray investigations of the kinetics and mechanism of M-type barium hexaferrite formation from hydroxide–carbonate precipitates

Mechanism of the formation of nanoscale M-type barium hexaferrite in the citrate precursor method

Nanoscale particles of BaFe 12O 19 in the size range 10–20 nm have been prepared at a substantially low temperature of 425°C by a citrate precursor method. The low-temperature processes involved in the precursor synthesis are examined and compared with the processes involved in other chemical methods and conventional ceramic methods to show how ferrite formation becomes possible at relatively lower temperatures in the precursor method. The precursor decomposes at 425°C to form a metastable spinel-like structure which undergoes time/temperature-dependent changes to transform to the final hexagonal structure. Using XRD and IR data it is shown that the spinel-like structure is actually a ‘masked’ hexaferrite structure where the Ba 2+ ion in the hexagonal layer of the unit cell is engaged due to the presence of traces of carbon. When the Ba 2+ ions are freed by the removal of carbon, the hexaferrite structure is completed. The Mössbauer spectra illustrate how various sublattice sites gradually develop with increasing heat treatment temperature and particle size to complete the complex barium ferrite structure. The magnetic measurements show how the above changes in structure are reflected as changes in magnetization and as increases in T C. The increase in T C is attributed to possible changes in the number of effective magnetic interactions.

Mössbauer investigation of aluminium substituted barium hexaferrite in the paramagnetic state

57Fe-Mössbauer measurements have been performed on aluminium substituted M-type barium hexaferrites of composition BaFe 12− x Al x O 19 (0 ≤ x ≤ 9.6) above their Curie temperatures. The quadrupole splitting values for 57Fe nuclei in the five iron sublattices of the crystalline structure as a function of x have been determined up to the highest x value. For the non-substituted BaFe 12O 19 ferrite, the comparison of the obtained data with that measured below the Curie temperature, allowed a determination of the angle between the direction of the magnetization and the electric field gradient axis at the 57Fe nuclei in the octahedrally coordinated lattice sites. For x ≤ 6 a high rate of substitution of Fe 3+ by Al 3+ ions occurs in the k lattice sites responsible for the magnetic interactions between the spinel and the hexagonal structural blocks.