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.     

Effect of aluminum substitution on the structural and magnetic properties of cobalt ferrite synthesized by sol-gel auto combustion process

Aluminum substituted cobalt ferrite powders (CoFe_2−xAl_xO₄) with varying composition from 0.0 to 1.0 in the step of 0.2 have been obtained by sol-gel auto combustion technique using citric acid as a fuel. The metal nitrate to fuel ratio was maintained 1:4 throughout the synthesis of CoFe_2−xAl_xO₄. The thermal analysis of as prepared samples is done by TGA technique. The compositional stoichiometry of the prepared samples is confirmed by Energy dispersive X-ray analysis technique. Single phase cubic spinel structure and nano phase structure of the synthesized powders were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The crystallite size of 16-26 nm was obtained using Scherrer formula. SEM analysis shows the formation of uniform grain growth. The grain size obtained from SEM results is of the order of 30 nm. Maximum specific surface area was observed to be of the order of 52 m²/gm. The highest value of saturation magnetization and coercivity was observed for pure cobalt ferrite sample and it decreases as the aluminum content x increases. A strong co-relation between the saturation magnetization and aluminum content was observed. The decrease in magnetic properties is due to the substitution of aluminum ions in place of Fe³⁺.     

Structural and magnetic study of the Ti-doped barium hexaferrite ceramic samples: Theoretical and experimental results

We present a theoretical and experimental study of the structural and magnetic properties of M-type Ti⁴⁺-doped barium hexaferrite BaFe_{(12−(4/3)x)}Ti_xO₁₉ with x=0, 0.1, 0.2, 0.3, 0.5, 0.7 and 1.3. The XRD patterns and magnetic measurements show appreciable variations in the values of the saturation magnetization and the magnetic anisotropy field, H_an, with increasing Ti⁴⁺ content. We did not observe significant changes in the Lotgering factor along the (0 0 l) direction and in the texture coefficient, C_ex, which was estimated from the torque curves. The magnetic properties of these materials are explained by the combined effect of the coherent rotation of the magnetic domains and the replacements of Fe³⁺ by Ti⁴⁺ ions in the octahedral and tetrahedral sites. The influence of the Ti⁴⁺ content on the samples was studied theoretically by using a statistical phenomenological model. The main purpose of the model is to make preliminary predictions of the distribution of any dopant cation in the Fe³⁺ sites. As a result, we are able to analyze both structural and magnetic features of M-type barium hexaferrite.     

Magnetic and Microwave Absorbing Properties of Electrospun Ba(1−x)LaxFe12O19 Nanofibers

Ba_(1−x)La_xFe₁₂O₁₉ (0.00≤x≤0.10) nanofibers were fabricated via the electrospinning technique followed by heat treatment at different temperatures for 2 h. Various characterization methods including scanning electron microscopy (SEM), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and microwave vector network analyzer were employed to investigate the morphologies, crystalline phases, magnetic properties, and complex electromagnetic parameters of nanofibers. The SEM images indicate that samples with various values of x are of a continuous fiber-like morphology with an average diameter of 110±20 nm. The XRD patterns show that the main phase is M-type barium hexaferrite without other impurity phases when calcined at 1100 °C. The VSM results show that coercive force (H_c) decreases first and then increases, while saturation magnetization (M_s) reveals an increase at first and then decreases with La³⁺ ions content increase. Both the magnetic and dielectric losses are significantly enhanced by partial substitution of La³⁺ for Ba²⁺ in the M-type barium hexaferrites. The microwave absorption performance of Ba_0.95La_0.05Fe₁₂O₁₉ nanofibers gets significant improvement: The bandwidth below −10 dB expands from 0 GHz to 12.6 GHz, and the peak value of reflection loss decreases from −9.65 dB to −23.02 dB with the layer thickness of 2.0 mm.     

Physical and magnetic properties of highly aluminum doped strontium ferrite nanoparticles prepared by auto-combustion route

Highly Al³⁺ ion doped nanocrystalline SrFe_12−xAl_xO₁₉ (0≤x≤12), were prepared by the auto-combustion method and heat treated in air at 1100 °C for 12 h. The phase identification of the powders performed using x-ray diffraction show presence of high-purity hexaferrite phase and absence of any secondary phases. With Al³⁺ doping, the lattice parameters decrease due to smaller Al³⁺ ion replacing Fe³⁺ ions. Morphological analysis performed using transmission electron microscope show growth of needle shaped ferrites with high aspect ratio at Al³⁺ ion content exceeding x≥2. Al³⁺ substitution modifies saturation magnetization (M_S) and coercivity (H_C). The room temperature M_S values continuously reduced while H_C value increased to a maximum value of 18,100 Oe at x=4, which is an unprecedented increase (∼321%) in the coercivity as compared to pure Sr-Ferrite. However, at higher Al³⁺ content x>4, a decline in magnetization and coercivity has been observed. The magnetic results indicate that the best results for applications of this ferrite will be obtained with an iron deficiency in the stoichiometric formulation.     

Composition and magnetic studies of ultrafine Al-substituted Sr hexaferrite particles prepared by citrate sol-gel method

Ultrafine aluminum-substituted strontium hexaferrite particles have been prepared via citrate sol-gel route. Gels were synthesized with molar ratios [Al³⁺]:[Fe³⁺] of 0.4:11.6, 1:11, 1.5:10.5 and 2:10 and the ferrite particles were obtained by annealing the gels at 950 °C for 2 and 24 h. Electron energy dispersive X-ray spectroscopy (EDX) verified the presence of Al in the substituted samples. X-ray diffraction (XRD) confirmed the formation of the M-type hexaferrite phase in the samples with some indication of α-Fe₂O₃. Scanning electron microscope showed that the hexaferrite powder consists hexagonal crystals with average diameter D_av (80-186 nm) that decreases with increasing Al content and increases with increasing annealing time. Magnetic properties were determined using a pulsed-field magnetometer and a vibrating sample magnetometer (VSM). The saturation magnetization at room temperature and the Curie temperature were found to decrease while the coercivity increases with increasing the Al content. The highest coercivity of 10.1 kOe was achieved for the sample with the molar ratio [Al³⁺]:[Fe³⁺]=2:10 annealed for 24 h. The influences of the particle size, composition and impurity on the magnetic properties were discussed.     

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.     

BaCO3 mediated modifications in structural and magnetic properties of natural nanoferrites

Preparing M-type barium hexaferrite and improving the magnetic response of natural ferrites by incorporating barium carbonate (BaCO₃) is ever-demanding. Series of barium carbonate doped ferrites with composition (100−x)Fe₃O₄·xBaCO₃ (x=0, 10, 20, 30 wt%) are prepared through solid state reaction method and sintered gradually at temperatures of 800 and 1000 °C. Nanoparticles of natural ferrite and commercial BaCO₃ are used as raw materials. Impacts of BaCO3 on structural and magnetic properties of these synthesized ferrites are inspected. The obtained ferrites are characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) at room temperature. Uniform barium hexaferrite particles in terms of both morphology and size are not achieved. The average crystallite size of BaFe₁₂O₁₉ is observed to be within 30–600 nm. The sintering process results phase transformation from Fe₃O₄ (magnetite) to α-Fe₂O₃ (hematite) and the formation of hexagonal barium ferrite crystals. The occurrence of barium crystal is found to enhance with the increase of BaCO₃ concentrations up to 20 wt% and suddenly drop at 30 wt%. Saturation and remanent magnetization of the doped ferrites are significantly augmented up to 16.37 and 8.92 emu g⁻¹, respectively compared to their pure counterpart. Furthermore, the coercivity field is slightly decreased as BaCO₃ concentrations are increased. BaCO₃ mediated improvements in the magnetic response of natural ferrites are demonstrated.     

Effect of substitution of K ions on the structural and electrical properties of nanocrystalline MgAl2O4 spinel oxide

Potassium substituted nanosized magnesium aluminates having a nominal composition Mg_1−xK_xAl₂O₄ where x=0.0, 0.25, 0.5, 0.75, 1.0 have been synthesized by the chemical co-precipitation method. The samples have been characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM), and dc electrical resistivity measurements. The XRD results reveal that the samples are spinel single phase cubic close packed crystalline materials. The calculated crystallite size ranges between 6 and 8 nm. The behaviour of the lattice constant seems to deviate from the Vegard's law. While X-ray density clearly increases, the bulk density and consequently, the percentage porosity do not exhibit a significant change on increasing the K⁺ content. The SEM micrographs suggest homogeneous distribution of the nanocrystallites in the samples. The dc electrical resistivity exhibits a typical semiconducting behaviour. Substitution of a Mg²⁺ ion by a K⁺ ion provides an extra hole to the system, which forms small polaron. Thermally activated hopping of these small polarons is believed to be the conduction mechanism in the Mg_1−xK_xAl₂O₄. The activation energy of hopping of small polarons has been calculated and found K⁺ ions content dependent.     

Preparation and investigation of magnetic properties of MnNiTi-substituted strontium hexaferrite nanoparticles

A series of M-type strontium hexaferrite powders with substitution of Mn²⁺, Ni²⁺ and Ti⁴⁺ ions for Fe³⁺ ions according to the formula SrFe₉(Mn_0.5−xNi_xTi_0.5)₃O₁₉, where x ranges from 0 to 0.5 with a step of 0.1, has been prepared via the conventional ceramic method. In order to get nanoparticles, the obtained powders were milled in a high energy SPEX mill for 1 h. XRD investigations of the unmilled and milled powders show that the prepared samples are all single phase hexaferrite. Lattice parameters and mean crystallite sizes of the powders were determined from the XRD data and Scherrer’s formula. Transmission electron microscope (TEM) was used to analyze their structures. Room temperature magnetizations and coercivities of the samples in a magnetic field of 15 kOe have been determined from the hysteresis loops. It was found that magnetizations of the milled samples were smaller than the magnetization of the unmilled samples. This decrease, based on core-shell model, has been attributed to the presence of a magnetically dead layer on the particles’ surface of the milled powders. In addition, the magnetizations of the milled samples decrease with the increase in x value. This decrease has been discussed according to site occupation of the substituted cations on the sublattices. The discussion also supports the increase of lattice parameters and the decrease of Curie temperature as x increases.     

Effect of boric acid on structure,morphology and luminescent properties of divalent europium doped calcium aluminate phosphors

Blue phosphors Ca_1 − xAl₂O₄: xEu²⁺ were prepared by high temperature solid-state method. Their structure, morphology and luminescent properties were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and fluorescence spectroscopy. The effect of different amounts of fluxing agent H₃BO₃ on structure, morphology and luminescent properties of blue phosphors Ca_1 − xAl₂O₄: xEu²⁺ luminous intensity caused by different amount of H₃BO₃ was also investigated. The amount of H₃BO₃ doped Ca_1 − xAl₂O₄: xEu²⁺ in optimal luminous intensity had been determined. The results showed that both the excitation and emission spectra of samples were all broad bands, and that the peak of emission spectra was near 442 nm, which was corresponding to the 4f⁶5d → 4f⁷ transition of Eu²⁺ illuminating blue light. Ca_1 − xAl₂O₄: xEu²⁺ (x = 3.5 mol%) could be gained with good morphology and the best luminous intensity when H₃BO₃ mass ratio was 0.5 wt%.     

Effect of Pb on structural and magnetic properties of Ba-hexaferrite

Lead was doped in barium hexaferrite by co-precipitation method and the Pb-doped Ba-hexaferrite with compositions of Ba_1−xPb_xFe₁₂O₁₉ was investigated for the first time at x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0. The molar ratio (Fe³⁺/Ba²⁺) of the solutions was kept 12 while pH was maintained at 13 by using NaOH (M=5) as precipitating agent. Prepared samples were sintered at 965±5 °C for three hours. Structural and morphological studies were done by X-ray diffractometer (XRD) and the scanning electron microscope (SEM). SEM micrographs confirmed the formation of hexagonal plate like structures and particle size was observed to be increased with the increase in Pb concentration. The hysteresis loops obtained from the magnetometer showed that with the increase in Pb concentration, the coercivity decreased while magnetic induction and remanence increased, which in turn increased the maximum energy product (BH)_max. Lower coercivity and the moderate increase in saturation magnetization obtained from Pb doping makes the material useful for magnetic recording media and other frequency based applications.     

Formation of submicron-sized SrFe12−xAlxO19 with very high coercivity

Submicron-sized SrFe_12−xAl_xO₁₉ (x=1.3) was formed in glass-ceramic matrix using controlled thermocrystallization of the SrO–Fe₂O₃–Al₂O₃–B₂O₃ glass and the hexaferrite powder was obtained by removing the matrix phases. The samples were characterized by X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray (EDX) analysis and magnetization measurements. The glass-ceramic material exhibits very high coercivity value up to 10.18 kOe which approaches a theoretically estimated maximum value for the compound. The hexaferrite powder consists of well faceted single crystals, which adopt the shape of a truncated hexagonal bipyramid. The powder saturation magnetization value is close to the theoretically estimated one for bulk material. Crystal structure of the powder was refined by Rietveld method and distribution of Al atoms on Fe sites was determined. Al atoms occupy 41% of 2a sites, 14% of 12k sites and 5% of 4e(1/2) sites, while 4f sites are not affected.     

The structure and luminescence properties of a novel orange emitting phosphor Y3MnxAl5−2xSixO12

A series of phosphors with the composition Y₃Mn_xAl_5−2xSi_xO₁₂ (x=0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.6) was prepared through solid state reactions. X-ray powder diffraction analysis of samples shows that when co-doping content does not exceed 16% of Al³⁺, equimolar co-doping of Mn²⁺ and Si⁴⁺ does not change the garnet structure of phosphors, but makes the interplanar distance to decrease a certain extent. However, if the co-doping content exceeds 16%, new phases will form in the samples. The excitation and emission spectra of samples show that Mn²⁺ in Y₃Mn_xAl_5−2xSi_xO₁₂ emits broadband orange light (peak wavelength varies from 586 to 593 nm). With an increment in co-doping content, the emission intensity of the phosphors increases when the value of x is lower than 0.1 while it decreases when it is higher than 0.1 and the emission peak moves to a longer wavelength.     

Physical, electrical and dielectric properties of Ca-substituted strontium hexaferrite (SrFe12O19) nanoparticles synthesized by co-precipitation method

Calcium substituted strontium hexaferrite Ca_xSr_1−xFe₁₂O₁₉ (x=0.0−0.6) nanoparticles are synthesized by chemical co-precipitation method. The synthesized samples are characterized by Fourier Transform Infrared (FTIR), X-ray diffraction (XRD), Scanning Electron Microscopy, Transmission Electron Microscopy, DC electrical resistivity and dielectric measurements. FTIR data of uncalcined sample shows that nitrate ions are present which disappeared on calcination at 920 °C. The XRD data shows that a single hexagonal magnetoplumbite phase is formed in samples in which the calcium content, x, is ≤0.20. However, a nonmagnetic phase (α-Fe₂O₃) in addition to the hexagonal phase is also present in samples with x>0.20. The average crystallite size is found between 17 and 29 nm. The DC electrical resistivity increases with increase of calcium content up to level of x=0.2 but decreased on further addition of calcium. The enhanced resistivity of the calcium doped material has potential applications in microwave devices. The variations of dielectric constant and dielectric loss angle are explained on the basis of Maxwell-Wagner and Koops models.     

Mn-doped (Ba,Y)Fe12O19 hexaferrites: Crystal structure and oxidation states of Mn and Fe

We have fabricated Ba_0.95Y_0.05Fe_12-xMn_xO₁₉ samples with large Mn-doping amounts of x = 4 and 6, using the mechanical milling and heat treatment. X-ray diffraction analysis indicated the samples crystallized in the M-type hexaferrite structure. The Mn doping caused the modification, shift and broadening of some characteristic phonon-vibration modes, which were recorded by Raman spectroscopy. This is due to an incorporation of Mn ions into the M-type structure that disorders the periodic lattice and changes symmetry. Basing on X-ray absorption spectroscopy, we have found Fe in all samples stable with an oxidation state 3+ (Fe³⁺). Though Mn²⁺ and Mn³⁺ ions coexist, the concentration of Mn²⁺ in x = 4 is larger than that in x = 6. The analysis of Fourier-transform spectra have demonstrated the replacement of Mn^2+,3+ ions for Fe³⁺ in the M-type structure. The sites of Mn^2+,3+ ions in this structure have been discussed.     

Cr3+ doped Al2O3 nanoparticles: Effect of Cr3+ content in intensifying red emission

Cr³⁺-doped α-Al₂O₃ nanoparticles (Al_2−xCr_xO₃, 0.005 ≤ x ≤ 0.05) were synthesized by co-precipitation method. X-ray diffraction (XRD) patterns of Cr³⁺:Al₂O₃ nanoparticles revealed the crystallite size of ∼53 nm and electron microscopy (SEM & TEM) confirmed the spherical nanoparticle formation. Diffuse reflectance spectra (DRS) displayed peaks at 406 and 558 nm corresponding to the Cr³⁺ transitions which became prominent with the increase in Cr³⁺ concentration which was also evidenced by the gradually increasing pink coloration of the samples. Photoluminescence (PL) studies showed the sharp red emission at 694 nm (ruby line) which was observed for all samples. The Dq/B value for all samples was found to be greater than 2.3 confirming the presence of Cr³⁺ ions in the octahedral sites. Chromaticity diagrams displayed the maximum red appearance for the sample with x = 0.01 and a lifetime of 4 ms. The synthesized Cr³⁺:Al₂O₃ nanoparticles with smaller crystallite sizes and narrow near monochromatic emission can be used in various applications including sensing, lasing, and bioimaging applications.     

Mössbauer Studies and the Microwave Properties of Al<Superscript>3+</Superscript>- and In<Superscript>3+</Superscript>-Substituted Barium Hexaferrites

The correlation of the chemical composition, the structure, and the microwave characteristic of solid solutions of the BaFe_{12 – x}D_xO₁₉ (0.1 ≤ x ≤ 1.2) barium hexaferrite substituted with diamagnetic Al³⁺ and In³⁺ ions has been studied. The precise data on the crystal structure have been obtained by powder neutron diffraction using a high-resolution Fourier diffractometer (Dubna, JINR). The data on the distribution of the diamagnetic substituting ions in the hexaferrite structure have been obtained by Mössbauer spectroscopy. The microwave properties (the transmittance and the reflectance) have been studied in the frequency range 20–65 GHz and in external magnetic fields to 8 kOe. It is found that the transmission spectra are characterized by a peak that corresponds to the resonant frequency of the electromagnetic energy absorption, which is due to the ferromagnetic resonance phenomenon. The correlation of the chemical composition, the features of the ion distribution in the structure, and the electromagnetic properties has been revealed. It is shown that external magnetic fields shift the absorption peak of electromagnetic radiation to higher frequencies due to an increase in the magnetocrystal anisotropy. The results enable the conclusion that the features of the intrasublattice interactions and the electromagnetic properties should be explained using the phenomenological Goodenough–Kanamori model.     

Effect of substitution of Co ions on the structural and electrical properties of nanosized magnesium aluminate

Cobalt substituted nanosized magnesium aluminates having a nominal composition MgAl_2−xCo_xO₄ where x = 0.0, 0.5, 1.0, 1.5, 2.0 were synthesized by the chemical co-precipitation method. Aluminium (Al³⁺) ions were completely and successfully substituted by Co²⁺ ions, which yielded an electron rich terminal compound MgCo₂O₄. All the samples were characterized by means of X-ray diffraction (XRD), thermogravimetry and differential thermal analysis (TG/DTA) and dc electrical resistivity measurements. The investigated samples were found to be spinel single phase cubic close packed crystalline materials as demonstrated by XRD data. Using the Debye Scherer formula, the calculated crystallite size was found Co²⁺ concentration dependent and varied between 7 and 19 nm. The lattice constant, X-ray density and bulk density were found to increase while percentage porosity decreases on increasing the Co²⁺ concentration. The dc electrical resistivity was found to decrease as a function of temperature as expected for a typical semiconductor. The doped Co²⁺ ions are believed to form small polarons and hopping of these small polarons between the adjacent sites seems to be partially responsible for conduction in the system. The activation energy of hopping of small polarons was also calculated.     

Luminescent properties of stabled hexagonal phase Sr1−xBaxAl2O4:Eu (x=0.37-0.70)

Stabled hexagonal phase Sr_1−xBa_xAl₂O₄:Eu²⁺ (x=0.37-0.70) was prepared by solid-state method. Result revealed that the structure behavior of the SrAl₂O₄:Eu²⁺ calcined at 1350 °C in a reducing atmosphere for 5 h strongly depended on the Ba²⁺ concentration. With increasing Ba²⁺ concentration, a characteristic hexagonal phase can be observed. When 37-70% of the strontium is replaced by barium, the structure of the prepared sample is pure hexagonal. Photoluminescence and excitation spectra of the samples with different x and doped with 2% Eu²⁺ were investigated. Changes in the emission spectra were observed in the two different phases. The green emission at 505 nm from Eu²⁺ was found to be quite strong in the hexagonal phase. The intensity and peak position of the green luminescence from Eu²⁺ changed with increasing content of Ba²⁺. The strongest green emission was obtained from Sr_0.61Ba_0.37Al₂O₄:Eu²⁺. The decay characteristics of Sr_1−xBa_xAl₂O₄:Eu²⁺ (x=0.37-0.70) showed that the life times also varied with the value of x. Furthermore, the emission colors and decay times varying with x could be ascribed to the variation of crystal lattice.