Microwave absorption properties of Al- and Cr-substituted M-type barium hexaferrite

Aluminum- and chromium-substituted barium ferrite particles with single magnetic domain were prepared using self-propagating combustion method. The crystalline structure, size, coercivity and microwave absorption property of the particles were investigated by means of X-ray diffraction, transmission electron microscopy, vibrating sample magnetometry and vector network analyzer. The results show that the crystalline structure of BaFe_12−xAl_xO₁₉ is still hexagonal. But when the chromium substitution amount y exceeds 0.6, the extra chromium ions cannot enter the lattice of BaFe_12−yCr_yO₁₉. After Fe³⁺ is partly substituted with Al³⁺ and Cr³⁺, the microwave absorption properties of barium ferrite are improved. The maximum absorption reaches 34.76 dB. The ferromagnetic resonance is an important channel of barium ferrite to absorb microwaves with high frequency. Aluminum and chromium substitutions change the ferromagnetic resonant frequency of barium ferrite. The multipeak phenomenon of the ferromagnetic resonance increases the microwave absorption capability of barium ferrite.     

Determination of cation distribution in Ti4+ and Co2+ substituted barium ferrite by Mössbauer spectroscopy

In barium ferrite, BaFe₁₂O₁₉, the Fe³⁺ ions were gradually substituted by Ti⁴⁺ and Co²⁺. The cation distribution of the various lattice sites is determined as a function of the degree of replacement. The preferred site population is discussed by a substitution model.     

Magnetic properties of barium ferrite synthesized using a microemulsion mediated process

Ultrafine barium ferrite particles have been synthesized using a microemulsion mediated process. The aqueous cores (typically 10–25 nm in size) of water-in-oil microemulsions were used as constrained microreactors for the precipitation of precursor carbonates of Ba²⁺ and Fe³⁺. These precursors (5–15 nm in size) when heated at 950°C, transformed to the hexagonal ferrite BaFe₁₂O₁₉ as confirmed by X-ray diffraction. This barium ferrite powder had an intrinsic coercivity of 5089 Oe and a saturation magnetization of 60.1 emu/g.     

Suppression of the exaggerated growth of barium ferrite nanoparticles from solution using a partial substitution of Sc<Superscript>3+</Superscript> for Fe<Superscript>3+</Superscript>

The effect of the substitution of Sc³⁺ for Fe³⁺ in barium ferrite on the size of the resulting nanoparticles was studied. These nanoparticles, with the nominal compositions BaFe₁₂O₁₉ and BaFe_11.5Sc_0.5O₁₉, were synthesized hydrothermally at 90–240 °C or by coprecipitation under reflux at 140 °C. The precursors were obtained using (co)precipitation at room temperature. The sizes and morphologies of the precursors and nanoparticles were inspected with transmission electron microscopy, while their structures were confirmed with a combination of X-ray powder and electron diffraction. The samples’ compositions were analyzed with energy-dispersive X-ray spectroscopy. The evolution of the particle size and its distribution with the synthesis temperature and time were studied in pure and Sc-substituted barium ferrite and correlated with the evolution of the magnetic properties. The Sc substitution in the barium ferrite results in the formation of magnetic nanoparticles with applicable magnetic properties and in a significant reduction of the exaggerated particle growth. This was explained on the basis of the reaction kinetics.     

Mössbauer spectra of Fe3+ and Fe2+ ions in hexagonal ferrite with w-structure

The hexagonal ferrite Fe₂W = BaFe₂²⁺Fe³⁺₁₆O₂₇ exhibits a sharp ⁵⁷Fe Mössbauer spectrum at 300 and 78 K. All seven sublattices in this complicated crystal structure are detected. Fast electron exchange between Fe²⁺ and Fe³⁺ ions gives rise to sharp lines and makes them indistinguishable. At 5 K the exchange is slow and the Fe²⁺ ions are detected from the presence of a weak subspectrum with broadened lines separated from the main spectrum of the Fe³⁺ ions. Analysis shows that the Fe²⁺ ions reside exclusively on one of the seven sublattices, which is occupied statistically by Fe²⁺ and Fe³⁺ ions in the ratio of 2 : 1. For SrFe²⁺₂Fe³⁺₁₆O₂₇ the situation is the same.     

Characterisation of synthetic trioctahedral micas by Mössbauer spectroscopy

Trioctahedral potassium micas |K}[M₃]〈T₄〉O₁₀(OH)₂ have been synthesized by hydrothermal techniques with various cationic substitutions in the octahedral and the tetrahedral sheet. Taking annite |K}[Fe ₃ ²⁺ ]〈AlSi₃〉O₁₀(OH)₂ as the reference mineral, [Fe²] was replaced by [Mg²] and [Ni²], 〈Al³⁺〉 by 〈Fe³⁺〉 and finally [Fe²⁺] + 〈Si⁴⁺〉 by [Al³⁺] + 〈Al³⁺〉. Mössbauer spectra were evaluated in terms of quadrupole splitting distributions (QSDs) using three generalized sites for 〈Fe³⁺〉, [Fe³⁺] and [Fe²]. Annites, nominally free of 〈Fe³⁺〉, show a lower limit of [Fe³⁺]/Fe _tot of 0.10, which stabilizes the structure. The ferrous iron, [Fe²], QSD consists of two main components. In some of the solid solution series, there is strong experimental evidence for a third ferrous component, particularly at higher [Al³⁺] contents. This third component is centered at low quadrupole splittings and may be assigned to a defect [Fe²] site, forming 1:2 structures with two neighbouring trivalent octahedral cations. For charge compensation one OH^? is replaced by O^2? for each [M³⁺] cation. The ferrous QSDs vary systematically with chemical composition. Compared to those of annite, the QSD parameters (mean quadrupole splitting 〈QS〉 and quadrupole splitting with maximum probability, QS _peak ) are shifted towards higher values with increasing [Mg²] and [Ni²] contents, and decrease slightly with increasing content of trivalent cations. These trends can be interpreted in terms of changes in the local environment around the Fe probe nucleus, i.e., in terms of decreasing or increasing distortions from the ideal octahedral configurations.     

Co2+-Ti4+和Cu2+-Nb5+离子取代BaFe12O19单晶体中Fe3+离子的研究

本文对用Co²⁺-Ti⁴⁺和Cu²⁺-Nb⁵⁺离子取代BaFe₁₂O₁₉单晶体中Fe³⁺离子进行了研究,以Bi₂O₃作为助熔剂生长出了BaFe_12-2xCo_x²⁺Ti_x⁴⁺O₁₉(x=0;0.04;0.09;0.13;0.27和0.68)以及BaFe_12-x[Nb_1/3⁵⁺Cu_2/3²⁺]_xO₁₉(x=0;0.28;0.44和0.60)这两系列的单晶体,测定了100—300K温度范围内样品的磁化强度σ与单轴各向异性常数K₁,我们发现,对Co²⁺-Ti⁴⁺取代的样品,当x≤0.09时,其σ与K₁随x的增加而缓慢增加;当x>0.09时,其σ与K₁随x的增加而迅速降低,至x=1.1时,K₁变为零,对Nb⁵⁺-Cu²⁺取代的样品,其σ值在整个成份范围内基本保持不变,且有缓慢增加趋势;而K₁值则随x增加而单调下降,提出了取代离子在M型六角铁氧体中可能的分布模型来解释我们的结果。 关键词：     

Structural and magnetic behaviour of aluminium doped barium hexaferrite nanoparticles synthesized by solution combustion technique

Nanocrystalline M-type Al³⁺ substituted barium hexaferrite samples having generic formula BaFe_12−xAl_xO₁₉ (where x=0.00, 0.25, 0.50, 0.75, 1.00) were synthesized by the solution combustion technique. The precursors were prepared using stoichiometric amounts of Ba²⁺, Fe³⁺ and Al³⁺ nitrate solutions with citric acid as a chelating agent. The barium nitrate to citric acid ratio was taken as 1:2 and pH of the solution was kept at 8. The sintered samples were characterized by XRD, EDAX, SEM, TEM and VSM techniques. Pure barium hexaferrite shows only single phase hexagonal structure while samples at 0.25≤x≥1.00 show α-Fe₂O₃ peaks with M-phase of barium hexaferrite in the X-ray diffraction pattern. The lattice parameters (a and c) obtained from XRD data decreases with increase in aluminium content x. The particle size obtained from X-ray diffraction data is in the nanometer range. The magnetic behaviour of the samples was studied using vibrating sample magnetometer technique. The saturation magnetization (M_s) and magneton number (n_B) decrease from 38.567 to 21.732 emu/g and from 7.6752 to 4.2126μ_B, respectively, with increase in Al³⁺ substitution x from x=0.0 to 1.0.     

Mössbauer study of proton radiation effects in FeCl24H2O

Abstract

The proton radiation effects in ferrous chloride are studied by means of the Mössbauer spectroscopy. The irradiation with protons of energy of 0.68 to 1.5 MeV has been found to cause dehydration and chemical decomposition of ferrous chloride. FeCl_{2 ·} 2H₂O, and Fe₃O₄ in superparamagnetic and ferromagnetic states, as well as Fe_1?x O were formed. The formation of a superparamagnetic phase of Fe₃O₄ within the “spike” regions was verified by low temperature measurements. The effects observed were interpreted in terms of the “thermal spike” model. The calculated temperatures and radii of “spikes” formed by iron, chloride and oxygen ions are in good agreement with observation for superparamagnetic Fe₃O₄.     

Dipole contribution of Fe3+ ions in 12k positions to the anisotropy energy constant of the BaFe12O19 hexaferrite

The anisotropy of local fields on ⁵⁷Fe nuclei of Fe³⁺ ions located in the 12k positions of the BaFe₁₂O₁₉ ferrite is studied at low temperatures. The contributions of the anisotropy of the dipole and hyperfine fields to the anisotropy of local fields are separated. The contribution of Fe³⁺ ions in the 12k positions to the anisotropy energy constant K ₁ is calculated in the case of the interionic magnetic dipole-dipole interaction. This contribution comprises more than one-half the experimental K ₁ value.     

Mössbauer effect and electron spin resonance study of CdxMg1−xFe2O4 system

This work deals with the application of Mossbauer Effect (ME) in studying the crystal electric field and the cation distribution among tetrahedral (A) and the octahedral (B) sites of the spinal structure in the ferrite system Cd_x Mg_1–x Fe₂O₄ (x=0, 0.2,...1). The electron spin resonance technique (ESR) was also applied for studying this ferrite system. It was possible to characterize the ESR spectra of ferrite through the combination with the ME spectra.The ESR spectra of magnesium ferrite showed two resonance positions of Fe³⁺ ions and indicated that a strong exchange interaction is dominant in the pure Mg-ferrite. For high Cd concentration ferrites only single resonance line was observed. These results could be interpretted on the basis of the ME results where it indicated that Cd²⁺ ions prefer tetrahedral positions, forcing the Fe³⁺ ions from these positions to join those in the octahedral sites. The complete site occupation with different types of cations was successfully achieved from the ME spectra. The values of the quadrupole splitting indicated that for each ferrite in the system there exists an electric field gradient surrounding the Fe³⁺ ions in each of the octahedral and tetrahedral sites. The increase in the Mg-concentration increases the symmetry of the electric field at these sites.     

NMR spectra of57Fe in hexagonal ferrites with magnetoplumbite structure

The NMR spectra of⁵⁷Fe in domains of MFe₁₂O₁₉ (M=Ba, Sr, Pb) were measured by spin echo technique at 4.2 K. The change of the heavy ion causes frequency shifts of lines corresponding to Fe³⁺ ions in 2b and 4f₂ sites while leaving other lines essentially unchanged; the significant role of different Fe³⁺−M²⁺ bonding was found. The dipolar broadening of lines in BaFe₁₂O₁₉ caused by random and static displacement of bipyramidal Fe³⁺ ions from the mirror plane is calculated and the results are compared with the experiment.     

Mössbauer-effect study of Mg-Al ferrites having a rectangular hysteresis loop

A Mössbauer-effect study was made of Mg-Al ferrites having a rectangular hysteresis loop. The replacement of Fe³⁺ ions by A1³⁺ ions produces an additional component in the Mössbauer spectra, due to second-phase inclusions. The phase composition of the Mg-Al ferrite has a marked effect on the shape of the hysteresis loop. As the second-phase inclusions increase in relative importance, rectangularity coefficient D decreases. Local lattice distortion has a lesser effect on D. At high Al₂O₃ concentrations (above 15 mole%) in the Mg ferrite there is a sharp increase in the coercive force with increasing Al₂O₃ content. This increase is also due to second-phase inclusions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 2, pp. 105–109, February, 1971.In conclusion the authors thank I. I. Sil'vestrovich and B. A. Pavel'ev for furnishing the samples.     

A superior microwave absorption material: Ni2+-Zr4+ Co-Doped barium ferrite ceramics with large reflection loss and broad bandwidth

Large reflection loss and wide bandwidth are significant targets, determining the microwave absorption ability. However, it is still a challenge to simultaneously satisfy the two conditions. As a multifunctional material, BaFe₁₂O₁₉ possess excellent electromagnetic properties in the microwave frequency band. Due to the natural resonance phenomenon of the material, BaFe₁₂O₁₉ can produce a large magnetic loss which correlates with Fe³⁺ content, and the microwave absorption characteristics of barium ferrite can be modulated by ion doping. As a typical magnetic metal, Ni coupled with high-valence state Zr⁴⁺ doping helps to produce double resonance peaks. In this work, Ni²⁺-Zr⁴⁺ co-doping M-type barium ferrites (BaFe_12-2xNi_xZr_xO₁₉, BNZFO-x, x = 0–0.8) were prepared conveniently by solid-state reaction method. Several necessary measurements to characterize its microwave absorption property have been operated such as morphology, magnetic performance and electromagnetic parameters. The results show that reflection loss and bandwidth can be simply tuned by tailoring Ni²⁺-Zr⁴⁺ content. The reflection loss peak drifts from 18 GHz to 9.76 GHz, which involves a half of the studied frequency range. The maximum reflection loss achieves −60.6 dB and the corresponding bandwidth over −10 dB is 7.68 GHz for BNZFO-0.6 ceramic with only 2.1 mm thickness. Thus, the doping of Ni²⁺-Zr⁴⁺ ion pairs is beneficial to improve the absorbing properties of the material, and the superior microwave absorption property may originate from its inner double natural resonance in micro-scale. The excellent microwave absorption properties suggest that BNZFO-x is a promising candidate applied for designing electromagnetic shielding devices.     

Superparamagnetic behaviour of cement clinker and its ferrite phase doped with different impurities

Two oxide mixtures of clinker and its ferrite phase of compositions (66.5 wt.% CaO+24.5 wt.% SiO₂+6.0 wt.% Al₂O₃+3.0 wt.% Fe₂O₃) and (60.4 wt.% CaO+15.4 wt.% Al₂O₃+24.2 wt.% Fe₂O₃) respectively were divided into protions and were mixed individually with 0.5, 1, 1.5 or 3 wt.% of LiF, MgF₂, CaF₂, CaCl₂ or ZnO. Each portion of clinker was fired at 1450°C and each portion of ferrite was fired at 1350°C for 30 min. then quenched in air. Mössbauer effect and X-ray diffraction measurements were performed on each sample. The impurities doping produced small particle size. The LiF doping gave the smallest particle size and the highest blocking temperature. The ferrite with LiF exhibited two Fe³⁺ sites while the other used impurities gave one site only. The superparamagnetic relaxation appeared only in the spectra of ferrite with impurities, which means that the impurities in clinker have a tendency to combine with the calcium silicate phases not with C₄AF.     

Preparation of substituted barium ferrite BaFe12−x(Ti0.5Co0.5)xO19 by citrate precursor method and compositional dependence of their magnetic properties

As a possible candidate for the left-handed metamaterial with negative permeability, a series of Ti, Co-substituted M-type barium hexaferrite BaFe_12−x(Ti_0.5Co_0.5)_xO₁₉ (x=0, 1, 2, 3, 4 and 5) was prepared by citrate precursor method. The formation processes of the substituted barium hexaferrite compounds from the precursors were followed by the measurements of powder X-ray diffraction (XRD), Infrared absorption spectra (FT-IR), and thermogravimetry and differential thermal analysis (TG/DTA) coupled with mass spectroscopy (MS). In the case of the non-substituted sample, the formation of the barium hexaferrite is regulated by the thermal decomposition of BaCO₃ and the solid-state reactions of BaO and Fe₂O₃ in the temperature range from 800 to 1100 K. The formation temperature of the substituted BaFe_12−x(Ti_0.5Co_0.5)_xO₁₉ is higher than that for the non-substituted sample and increases with the value of x, due to the effects of carbonate ions incorporated by the partial substitution of Fe³⁺ by (Ti_0.5Co_0.5)³⁺. On heating up to ca. 1200 K, all the substituted samples transform into the magenetoplumbite phase as is the non-substituted sample. The compositional dependence of the magnetic properties of the substituted barium hexaferrite was investigated by the magnetization measurement. The decrease in the magnetic anisotropy was confirmed by the change in the magnetization curve and coercivity H_C with the composition x. A negative permeability spectrum was observed in the BaFe₉(Ti_0.5Co_0.5)₃O₁₉ in the frequency range from 2 to 4 GHz.     

Relaxation phenomena in Cr3+ doped Al2O3−Fe 2 57 O3 systems

Mössbauer absorption spectra have been measured for Cr³⁺ doped Al₂O₃?Fe ₂ ⁵⁷ O₃ systems for different values of the Cr³⁺ concentration at room temperature. The cross relaxation between Fe³⁺ and Cr³⁺ ions, which destroys the paramagnetic hyperfine structure of Fe⁵⁷ observed in undoped Al₂O₃?Fe ₂ ⁵⁷ O₃, is thoroughly studied. The experimental results suggest a new kind of cross-relaxation process involving three spins, i.e. two Fe³⁺.ions and one Cr³⁺. The process, though it is a higher-order one, is highly effective because it is energy-conserving.     

Microstructure characterization and magnetic behavior of NiAlxFe2−xO4 and Ni1−yMnyAl0.2Fe1.8O4 ferrites

NiAl_xFe_2−xO₄ and Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites were prepared by the conventional ceramic method and were characterized by X-ray diffraction, scanning electron microscopy, and magnetic measurements. The single spinel phase was confirmed for all prepared samples. A proper explanation of data is possible if the Al³⁺ ions are assumed to replace Fe³⁺ ions in the A and B sites simultaneously for NiAl_xFe_2−xO₄ ferrites, and if the Mn²⁺ ions are assumed to replace Ni²⁺ ions in the B sites for Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites. Microstructural factors play an important role in the magnetic behavior of Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites with large Mn²⁺ content.     

Effect of Fe3+-doped Ca12Al14O33 cement on optical and thermal properties

This study aims to investigate the effect Fe ions doped into Ca₁₂Al₁₄O₃₃ (C12A7, 12CaO-7Al₂O₃) cement compound on its thermal and optical properties. Polycrystalline samples of Ca₁₂Al_14?xFe_xO₃₃ (where x?=?0.0, 0.5, and 1.0) were prepared via a solid state reaction in an oxygen atmosphere. The lattice constant of Ca₁₂Al₁₄O₃₃ determined using an XRD technique was in excellent agreement with first-principles calculations. With increasing Fe concentrations, the lattice constants were found to have increased. Additionally, the optical gaps of Ca₁₂Al_14?xFe_xO₃₃, (x?=?0, 0.25, 0.5, and 1.0) were 3.9?eV, 3.77?eV, 3.75?eV and 3.63?eV, respectively. It was clearly seen that the optical gap decreased with increasing Fe concentrations. As revealing by first-principles calculations, the optical gap was directly related to the electronic transition from the occupied electronic state of extra-framework O^2? ions (as free O^2? ions inside nano-cage) to the conduction band. Moreover, we also found that the thermal conductivity Ca₁₂Al_14?xFe_xO₃₃ was reduced when the larger atomic mass and atomic radii Fe was substituted into Al sites. Hence, this indicated that Fe³⁺-substitution into Al³⁺ sites of Ca₁₂Al₁₄O₃₃ cement directly affected both its optical gap and thermal conductivity.     

Improvement of the magnetic properties of barium hexaferrite nanopowders using modified co-precipitation method

Barium hexaferrite BaFe₁₂O₁₉ powders have been synthesized using the modified co-precipitation method. Modification was performed via the ultrasonication of the precipitated precursors at room temperature for 1 h and the additions of the 2% KNO₃, surface active agents and oxalic acid. The results revealed that single phase magnetic barium hexaferrite was formed at a low annealing temperature of 800 °C for 2 h with the Fe³⁺/Ba²⁺ molar ratio 8. The microstructure of the powders appeared as a homogeneous hexagonal platelet-like structure using 2% KNO₃ as the crystal modifier. A saturation magnetization (60.4 emu/g) was achieved for the BaFe₁₂O₁₉ phase formed at 1000 °C for 2 h with Fe³⁺/Ba²⁺ molar ratio 8 using 5 M NaOH solution at pH 10 in the presence of 2% KNO₃. Moreover, the saturation magnetization was 52.2 emu/g for the precipitated precursor at Fe³⁺/Ba²⁺ molar ratio 12 in was achieved for the precipitated precursor ultrasonicated for 1 h and then annealed at 1200 °C for 2 h. Coercivities from 956.9 to 4558 Oe were obtained at different synthesis conditions.