Structural and magnetic studies on mechanosynthesized BaFe12−xMnxO19

Barium hexaferrite powders with manganese substitution were prepared by mechanosynthesis. The structural and magnetic properties were characterized by X-ray diffractometer and vibration sample magnetometer, respectively. XRD patterns were refined by Rietveld method. Preferential site occupation of manganese ion was investigated by room temperature (RT) Mössbauer measurements. XRD results showed a single-phase barium hexaferrite with some residual hematite. Crystallite size was observed to decrease with substitution amount. Lower saturation magnetization and increased coercivity is observed in substituted samples. RT Mössbauer measurements showed that manganese ions preferentially occupy 12k, 4f₂, and 2a sites.     

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.     

Structure and hard magnetic properties of barium hexaferrite with and without La2O3 prepared by ball milling

The barium hexaferrites have been prepared by ball milling of a BaO₂ and Fe₂O₃ mixture followed by thermal heat treatments. The structure and magnetic properties were investigated using X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer techniques. The effect of grain refiner was also studied and it was found that the hard magnetic properties were improved significantly. The sintered product of barium hexaferrite powders prepared from ball milling has higher coercive force than that of other barium hexaferrite made from oxide/carbonate.     

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.     

Effects of powder flowability on the alignment degree and magnetic properties for NdFeB sintermagnets

The magnetic powders for sintered NdFeB magnets have been prepared by using the strip casting (SC), hydrogen decrepitation (HD) and jet milling (JM) techniques. The effects of powder flowability and addition of a lubricant on the alignment degree and the hard magnetic properties of sintered magnets have been studied. The results show that the main factor affecting powder flowability is the aggregation of magnetic particles for powders in a loose state, but it is the friction between the powder particles for powders that are in a compact state. The addition of a lubricant with suitable dose can slightly prevent the congregating of powders, obviously decrease the friction between the powder particles, improve the powder flowability, and increase the alignment degree, remanence and energy product density of sintered magnets. Mixing a suitable dose of lubricant and adopting rubber isostatic pressing (RIP) with a pulse magnetic field, we have succeeded in producing the sintered NdFeB magnet with high hard magnetic properties of B_r=14.57 KG, _jH_c=14.43 KOe, (BH)_max=51.3 MGOe.     

Simple recipe to synthesize single-domain BaFe12O19 with high saturation magnetization

We report a new synthesis route for preparation of single-domain barium hexaferrite (BaFe₁₂O₁₉) particles with high saturation magnetization. Nitric acid, known as a good oxidizer, is used as a mixing medium during the synthesis. It is shown that formation of BaFe₁₂O₁₉ phase starts at 800 °C, which is considerably lower than the typical ceramic process and develops with increasing temperature. Both magnetization measurements and scanning electron microscope micrographs reveal that the particles are single domain up to 1000 °C at which the highest coercive field of 3.6 kOe was obtained. The best saturation magnetization of ≈60 emu/g at 1.5 T was achieved by sintering for 2 h at 1200 °C. Annealing at temperatures higher than 1000 °C increased the saturation magnetization, on the other hand, decreased the coercive field which was due to the formation of multi-domain particles with larger grain sizes. It is shown that the best sintering to obtain fine particles of BaFe₁₂O₁₉ occurs at temperatures 900-1000 °C. Finally, magnetic interactions between the hard BaFe₁₂O₁₉ phase and impurity phases were investigated using the Stoner-Wohlfarth model.     

Structural and magnetic properties of SrFe12O19 hexaferrite synthesized by a modified chemical co-precipitation method

M-type strontium hexaferrite (SrFe₁₂O₁₉) particles had been prepared by a modified chemical co-precipitation route. Structural and magnetic properties were systematically investigated. Rietveld refinement of X-ray powder diffraction results showed that the sample was single-phase with the space group of P6₃/mmc and cell parameter values of a=5.8751 Å and c=23.0395 Å. The results of field-emission scanning electronic microscopy showed that the grains were regular hexagonal platelets with sizes from 2 to 4 μm. The composition determined by energy dispersive spectroscopy is the stoichiometry of SrFe₁₂O₁₉. The ferrimagnetic to paramagnetic transition was sharp with Curie temperature T_C=737 K, which further confirmed that the samples were single phase. However, it was found that the coercivity, saturation magnetization and the squareness ratio of the synthesized SrFe₁₂O₁₉ samples were lower than the theoretical values, which could be explained by the multi-domain structure and the increase of the demagnetizing factor.     

Investigation of magneto-anisotropy field for system of particles with near superparamagnetic volume

We obtained the temperature dependence for low-field boundary of the anisotropy field distribution in a system of barium hexaferrite nanocrystals in the temperature range from 300 to 700 K. We treated the experimental data taking into account the influence of thermal fluctuations on the anisotropy field and the transition of particles into the paramagnetic state, stimulated by external magnetic field. We showed that the dependence under consideration is formed by particles of different volume, which increased from 3.5×10⁻¹⁸ to 40×10⁻¹⁸ cm³ while the particles lost their magnetic stability with the temperature growth.     

Preparation of the SrFe12O19-based magnetic composites via boron oxide glass devitrification

Magnetic composites were obtained in the system SrO–Fe₂O₃–B₂O₃ by oxide glass heat treatment at 600–950 °C. Samples of the composites were investigated using XRD analysis, magnetic measurements, electron microcopy, and thermal analysis. It was shown that chemical composition of the precursor oxide glass and thermal treatment conditions influenced on the SrFe₁₂O₁₉ particles morphology and magnetic properties. The composites and powders were obtained containing hexaferrite as single domain platelet crystals or polycrystalline aggregates with a coercive force up to 6300 Oe in the former case and 4200 Oe in the latter case.     

Fabrication of porous Ni–YSZ anodes by PSH-PIM

This paper has investigated the fabrication process of porous Ni–YSZ anodes by the powder injection molding method, in which a powder space holder (PSH) is used. Polymethyl methacrylate (PMMA) has been used as a PSH for mixing with NiO–YSZ powders. For this study, five kinds of feedstock containing 0%, 10%, 20%, 30%, and 40% PMMA by volume were prepared. The thermoplastic binder used for the process had a fixed 35 vol.%, and the powder loads formed the remaining 65, 55, 45, 35, and 25 vol.% of the feedstock. After molding and debinding, the parts were sintered at 1,500 °C. The obtained results showed that increasing the PMMA portion of the feedstock and reducing its powder load causes the viscosity of the feedstock to decrease. The amount of shrinkage of the samples containing 0–30% PMMA showed an almost linear increase with the increase of the PMMA content, and for the samples with 40% PMMA, this increase of shrinkage was higher. The amount of porosity in the samples having 0–30% PMMA increased with the rise in the PMMA content, but in the samples containing 40% PMMA, the amount of porosity decreased, such that it was less than that of the samples with 30% PMMA. The electrical conductivity and flexural strength of all the samples were also studied in this work.     

Improving magnetic properties of barium hexaferrites by La or Pr substitution

La or Pr substituted barium hexaferrites, Ba_1−x(La or Pr)_xFe₁₂O₁₉, x=0.00-0.20, were successfully prepared by a citrate combustion process. The sintered bodies were structurally and magnetically studied by powder X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). All XRD patterns show the single phase of the magnetoplumbite barium ferrite without other intermediate phases. Magnetization (M_S) and coercive field (H_C) could be improved by substitutions of La or Pr ions on Ba ion basis sites. The M_S reveal magnetic behavior with respect to La or Pr ions content, showing an increase at first and then a decrease. The H_C increases remarkably with increasing La or Pr ions content.     

Directional behavior of nuclear quadrupole resonance in YBa2Cu3O6

Powder samples of YBa₂Cu₃O₆ were magnetically aligned and the anisotropies in the systems were studied by means of Cu(1) nuclear quadrupole resonance (NQR) in the absence of external magnetic fields. Our room temperature measurements of the NQR lineshapes and the spin–lattice and spin–spin relaxation times as a function of the aligning magnetic field indicate that full microscopic alignment can be achieved by using a magnetic field of about 4.7 T, for which doublet line patterns arising from a hyperfine splitting were observed.     

Structure and magnetic properties of nanocrystalline Fe75Si25 powders prepared by mechanical alloying

Nanocrystalline Fe₇₅Si₂₅ powders were prepared by mechanical alloying in a planetary ball mill. The evolution of the microstructure and magnetic properties during the milling process were studied by X-ray diffraction, scanning electron microscope and vibrating sample magnetometer measurements. The evolution of non-equilibrium solid solution Fe (Si) during milling was accompanied by refinement of crystallite size down to 10 nm and the introduction of high density of dislocations of the order of 10¹⁷ m⁻². During the milling process, Fe sites get substituted by Si. This structural change and the resulting disorder are reflected in the lattice parameters and average magnetic moment of the powders milled for various time periods. A progressive increase of coercivity was also observed with increasing milling time. The increase of coercivity could be attributed to the introduction of dislocations and reduction of powder particle size as a function of milling time.     

Induced magnetic anisotropy in Co-doped Finemet-type nanocrystalline materials

Transverse magnetic anisotropy has been induced in the Fe_14.7Co_58.8Cu₁Nb₃Si_13.5B₉ and Fe_13.8Co₆₅Cu_0.6Nb_2.6Si₉B₉ amorphous ribbons by annealing under an external magnetic field. This anisotropy plays a predominant role, compared to magneto-crystalline and magneto-elastic anisotropies, in forming the magnetic properties and shaping the hysteresis loop. The effect of temperature and time of annealing on the induced magnetic anisotropy and magnetic properties (magnetic permeability, coercivity and power losses) in both alloys was investigated. Under this work, measurements of frequency dependence of the real and imaginary parts of magnetic permeability were made within a frequency range up to 110 MHz. It was found that as a result of magnetic field annealing the Snoek limit increases in both alloys compared to the Co-free Finemet.     

Magneto-optic Faraday effect in maghemite nanoparticles/silica matrix nanocomposites prepared by the Sol–Gel method

Bulk monolithic samples of γ-Fe₂O₃/SiO₂ composites with different iron oxide/silica ratios have been prepared by the sol–gel technique. Iron oxide nanoparticles are obtained in-situ during heat treatment of samples and silica matrix consolidation. Preparation method was previously optimized to minimize the percentage of antiferromagnetic α-Fe₂O₃ and parallelepipeds of roughly 2×5×12 mm³, with good mechanical stability, are obtained. RT magnetization curves show a non-hysteretic behavior. Thus, magnetization measurements have been well fitted to an expression that combines the Langevin equation with an additional linear term, indicating that some of the nanoparticles are still superparamagnetic as confirmed by X-ray diffraction and electron microscopy measurements. Zero field cooled /field cooled experiments show curves with slightly different shapes, depending on the size and shape distribution of nanoparticles for a given composition. Magneto-optical Faraday effect measurements show that the Faraday rotation is proportional to magnetization of the samples, as expected. As a demonstration of their sensing possibilities, the relative intensity of polarized light, measured at 5° from the extinction angle, was plotted versus applied magnetic field.     

Magnetic ordering in La0.75Sr0.25CrO3: A neutron diffraction study

Low-temperature neutron diffraction measurements were carried out on a powder sample of the compound La_0.75Sr_0.25CrO₃ in order to elucidate its magnetic structure. Rietveld analysis of the neutron diffraction data, as a function of temperature, showed that it possesses a G-type antiferromagnetic alignment of Cr spins at all temperatures below 300 K. Down to the lowest achievable temperature, viz. 17 K, the Cr site moments were found to be the weighted average of the 75% Cr³⁺ and 25% Cr⁴⁺ spin-only ionic moments. At 17 K, the Cr site moment was 2.71(5) μ_B/Cr ion. There is no observable change in the Cr–O bond lengths as a function of temperature. The tilt angles of the CrO₆ octahedra marginally increase with decreasing temperature.     

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.     

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.     

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.     

Synthesis of magnetoresistive La0.7Sr0.3MnO3 nanoparticles by an improved chemical coprecipitation method

La_0.7Sr_0.3MnO₃ nanoparticles were prepared by a simple chemical coprecipitation route. Structural, magnetoresistance (MR), and magnetic properties were investigated. Rietveld refinement of X-ray powder diffraction result shows that the sample is single-phase with the space group of R3¯C. The result of field-emission scanning electronic microscopy shows that most of the grain sizes are distributed from 50 to 200 nm. The composition determined by energy-dispersive spectroscopy is the stoichiometry of La_0.7Sr_0.3MnO₃. The ferromagnetic to paramagnetic transition is sharp with Curie temperature T_C=367 K, which further confirms that the sample is single-phase. The steep change in MR at low fields is attributed to the alignment of the magnetization, while the high-field MR is due to the grain boundary effect.