Structural and magnetic properties of cadmium substituted Ni-Al ferrites

Nickel Cadmium Aluminum Ferrites with the general formula Ni_1−xCd_xAl_0.6Fe_1.4O₄ where x=0, 0.25, 0.50 and 0.75 were prepared through standard double sintering reaction method. The crystallography, surface morphology and magnetic properties were studied by X-ray diffractometer (XRD), Scanning Electron Microscope (SEM) and Vibrating Sample Magnetometer (VSM), respectively. The expected single phase spinel structure was confirmed by XRD analysis. Lattice parameter and X-ray density were increased monotonically by increasing Cd concentration due to the larger ionic radii of the cadmium ion. Surface topography of the samples consists of fine cubical shape microstructures. The average grain size increased with increase in cadmium concentration. The saturation magnetization was found to be increased with increase in cadmium content up to x=0.50 and then decreased with further increasing cadmium concentration for x=0.75.     

The role of Mg substitution on the microstructure and magnetic properties of Ba Co Zn W-type hexagonal ferrites

A series of W-type hexagonal ferrites with the composition BaCoZn_1−xMg_xFe₁₆O₂₇ (0?x?0.6) were prepared by the conventional ceramic method to study their structural and magnetic properties as a function of temperature and composition. The characterization using X-ray diffraction indicated that a hexagonal W-type single-phase structure and the effect of composition on the unit cell parameters, density and porosity was studied. The variation of the magnetic susceptibility (χ_M) with temperature for all the investigated samples in the temperature range (300–800 K) shows three regions of behavior that was explained on the basis of the distribution of Zn²⁺ and Mg²⁺ ions in the lattice and leads to the anomalous behavior of the effective magnetic moment μ_eff. The Curie temperature indicated that the critical concentration is at x=0.5. Paramagnetic nature of the samples above the Curie temperature is observed. The Curie Weiss constant θ calculated from the plot of 1/χ_M vs. T (K) is in agreement with the expected value. The effective magnetic moment μ_eff decreases with increasing the intensity of magnetic field. The possible mechanisms contributing to these properties are discussed in the text.     

Synthesis,crystal structure and magnetic characterization of Pr3+ and Zn2+ ions co-doped hexagonal ferrites via the ceramic process

M-type hexaferrites Ca_0.2Sr_0.8-xPr_xFe_12-yZn_yO₁₉ (0.00?≤?x?≤?0.40, 0.00?≤?y?≤?0.30) were synthesized by the ceramic process. The X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and a vibrating sample magnetometer (VSM) were used to investigate microstructure and magnetic properties of the M-type hexaferrites. The single-phase with hexagonal structure was obtained in all Pr–Zn substituted M-type hexaferrites, and with increasing Pr–Zn content, the 2θ values of (107) and (114) peaks shifted towards higher angles. With increasing Pr–Zn content, the lattice constant a basically kept unchanged, while the lattice constant c decreased. FESEM images of the hexaferrites showed that the hexagonal platelets had formed in the hexaferrites and the average grain size increased with increasing Pr–Zn content. The saturation magnetization (M_s), remanent magnetization (M_r) and M_r/M_s ratio first increased with increasing Pr–Zn content (0.00?≤?x?≤?0.24, 0.00?≤?y?≤?0.18), and then decreased with further increasing Pr–Zn content. The coercivity (H_c), magnetic anisotropy field (H_a) and effective magnetic anisotropy constant (K_eff) increased with increasing Pr–Zn content (0.00?≤?x?≤?0.16, 0.00?≤?y?≤?0.12), and then decreased with further increasing Pr–Zn content.     

Microstructural and magnetic properties of Pr–BiCo co-substituted M-type Sr–Ca hexaferrites

We have synthesized the Pr–BiCo substituted hexaferrites with compositions of Sr_0.8-xCa_0.2Pr_xFe_12.0-x(Bi_0.5Co_0.5)_xO₁₉ (0.0?≤?x?≤?0.5) by the standard ceramic method. Results of X-ray diffraction analysis exhibits that the synthesized hexaferrites with x from 0.0 to 0.3 are in single magetoplumbite structure, and impurity phases are observed when x?≥?0.4. The surface morphology of magnets shows that hexaferrite grains have a hexagonal platelet shape with clear grain boundaries. The remanence first increases with x from 0.0 to 0.1, and then decreases when x?≥?0.1. The intrinsic coercivity decreases with x from 0.0 to 0.1, and then increases when x?≥?0.1. With x from 0.0 to 0.4, the changing trend of magnetic induction coercivity is in agreement with that of H_cj, while at x?≥?0.4, H_cb decreases. The maximum energy product initially increases with x from 0.0 to 0.2, and then decreases when x?≥?0.2.     

Synthesis and characterization of a hexagonal mesoporous silica with enhanced thermal and hydrothermal stabilities

A synthetic route was developed for a novel hexagonal mesoporous silica that has remarkably wide channel diameters and thick walls. The procedure involved the acid-catalyzed hydrolysis of tetraethylorthosilicate in a water/ethanol/isopropoanol solvent mixture while employing 1-hexadecylamine as a templating agent and mesitylene as an auxiliary agent. After removal of the template by either extraction with ethanolic hydrochloric acid or by calcination at 550 °C, the resulting mesoporous materials had surface areas of 1283 and 1211 m²/g. The channel diameters were found to be 47.2-51.1 Å, while the wall thicknesses were 20.9-21.1 Å. X-ray powder diffraction demonstrated that the novel mesoporous silica belonged to the MCM-41 structural family. Notably, they displayed higher thermal and hydrothermal stabilities, and have higher surface areas than conventionally prepared MCM-41 silica. The thickest channel walls (21.1 Å) can withstand calcination to nearly 850 °C with minimal structural damage. The calcined sample was more resistant to hydrothermal treatment in boiling water than was the solvent-extracted product but both materials showed minimal change after 25 h of hydrothermal treatment.     

The magnetic properties of aligned M hexa-ferrite fibres

Aligned and random fibres of strontium hexaferrite (SrM, SrFe₁₂O₁₉) and barium hexaferrite (BaM, BaFe₁₂O₁₉) were manufactured by blow spinning from an aqueous inorganic sol–gel precursor, which was then fired to give the hexagonal ferrite fibre. Their magnetic properties were studied by VSM, investigating the evolution of these properties with firing and measurement temperature, and in particular the effects of fibre alignment. It has been predicted that aligned ferrite fibres will demonstrate an enhanced magnetisation along the axis of alignment with respect to perpendicular to the axis, and this has been demonstrated here for the first time. The optimum firing temperature was 1000 °C, at which point they still had submicron grains. In BaM random fibres M_s=63.8 emu g⁻¹ and H_c=428.1 kA m⁻¹, and in SrM random fibres M_s=63.3 emu g⁻¹ and H_c=452.8 kA m⁻¹, high values for polycrystalline materials. Fibres aligned parallel to the applied field had saturation magnetisation (M_s) values equal to those of the random fibres, whilst fibres aligned perpendicular to the field had M_s values 62% and 75% lower, for BaM and SrM, respectively. There was no change in coercivity (H_c) between random or aligned fibres of any orientation, and fibres aligned 45° and parallel to H appeared identical. Therefore, properties along the axis of alignment were superior when compared to measurements perpendicular to the axis of alignment, giving a directionality to the magnetisation in an otherwise randomly oriented ferrite material.     

Effect of cation distribution on the structural and magnetic properties of nickel substituted nanosized Mn–Zn ferrites prepared by co-precipitation method

Nano particles of Mn_(0.5–x)Ni_xZn_0.5Fe₂O₄ (x=0.0, 0.1, 0.2, 0.3) have been synthesized by chemical co-precipitation method. The lattice constant and distribution of cation in the tetrahedral and octahedral sites have been deduced through X-ray diffraction (XRD) data analysis. The lattice constant (Å) for all Mn/Ni concentration is found to be less than that for the corresponding bulk values. X-ray intensity calculations indicate that there is deviation in the normal cation distribution. Magnetization decreases with increasing Ni concentration except for x=0.3, where it shows increasing trend. This is due to migration of Fe³⁺ ions from B-site to A-site, which reduces the B–B coupling and there by the spin canting in the B sublattice. The Curie temperature was found to decrease with increase in nickel concentration except for x=0.3, where it shows a rise. Coercivity is very low and is found to be inversely proportional to the grain size.     

MFM studies of magnetic domain patterns in bulk barium ferrite (BaFe12O19) single crystals

Magnetic domain patterns in bulk barium ferrite (BaFe₁₂O₁₉; BaM) single crystals on the basal plane and the prism plane were measured and studied by magnetic force microscopy (MFM). The surface domain pattern is in the form of flowers or star on the basal plane and long elongated spikes or stripe domains on the prism plane. The change in domain structure with applied field (H_app) and the thickness (T) dependence on domain width (δ) was observed. The domain width decreased from 32 to 9 μm for the crystals of 800-100 μm thicknesses, respectively.     

Structural, magnetic and electrical properties of NiCuZn ferrites prepared by microwave sintering method suitable for MLCI applications

Polycrystalline NiCuZn soft ferrites with stoichiometric iron were prepared by a novel microwave sintering method. The powders were calcined, compacted and sintered at 950 °C for 30 min in a microwave sintering furnace. X-ray diffraction patterns confirm the formation of single phase cubic spinel structure. The grain size was estimated using SEM micrographs. The lattice constant is found to increase with increase in zinc concentration. The sintered ferrites have been investigated for their physical, magnetic and electrical properties such as bulk density, X-ray density, porosity, anisotropy constant, initial permeability, saturation magnetization, DC resistivity, dielectric constant and dielectric loss as a function of zinc concentration. Permeability, saturation magnetization, dielectric constant and dielectric loss were found to increase while DC resistivity was found to decrease with the replacement of Zn with Ni. The present series of ferrites are found to posses properties that are suitable for the core materials in multilayer chip inductors.     

Influence of rare earth Ce on structural, electrical and magnetic properties of Sr based W-type hexagonal ferrites

A series of single phase W-type Sr_3−xCe_xFe₁₆O₂₇ (x=0, 0.02, 0.04, 0.06, 0.08, 0.10) hexagonal ferrites prepared by the Sol-Gel method was sintered at 1050 °C for 5 h. The X-ray diffraction analysis reveals that all the samples belong to the family of W-type hexagonal ferrites. The c/a ratio falls in the range of W-type hexagonal ferrites. The grain size was measured by SEM varies from 0.7684 to 0.4366 μm which shows that the Ce³⁺ substituted samples have smaller grain size than pure ferrite Sr₃Fe₁₆O₂₇ which results from the difference in ionic radii of Ce³⁺ (1.034 Å) and Sr²⁺ (1.12 Å). The room temperature resistivity of the present samples varies from 6.5×10⁸ to 272×10⁸ Ω-cm. The coercivity increases from 1370 to 1993 Oe which is consistent with the decrease in grain size. The coercivity values indicate that the present samples fall in the range of hard ferrites. The large value of H_c may be due to domain wall pinning at the grain boundaries.     

Synthesis and characterization of Fe3O4@C@Ag nanocomposites and their antibacterial performance

We synthesized Fe₃O₄@C@Ag nanocomposites through a combination of solvothermal, hydrothermal, and chemical redox reactions. Characterization of the resulting samples by X-ray diffraction, Fourier-transform infrared spectroscopy, field-emission scanning and transmission electron microscopy, and magnetic measurement is reported. Compared to Fe₃O₄@Ag nanocomposites, the Fe₃O₄@C@Ag nanocomposites showed enhanced antibacterial activity. The Fe₃O₄@C@Ag nanocomposites were able to almost entirely prevent growth of Escherichia coli when the concentration of Ag nanoparticles was 10 μg/mL. Antibacterial activity of the Fe₃O₄@C@Ag nanocomposites was maintained for more than 40 h at 37 °C. The intermediate carbon layer not only protects magnetic core, but also improves the dispersion and antibacterial activity of the silver nanoparticles. The magnetic core can be used to control the specific location of the antibacterial agent (via external magnetic field) and to recycle the residual silver nanoparticles. The Fe₃O₄@C@Ag nanocomposites will have potential uses in many fields as catalysts, absorbents, and bifunctional magnetic-optical materials.     

Densification and magnetic properties of low-fire NiCuZn ferrites

The mixing of (Ni_0.38Cu_0.12Zn_0.50)Fe₂O₄ powders with Bi₂O₃ was performed using the solid-state mixing as well as wet chemical coating processes such as ammonia precipitation coating, urea precipitation coating, and solution coating. Ferrites prepared from the wet chemical coating processes could be densified at a lower sintering temperature without significant impact on the microstructural evolution compared with that prepared by solid-state mixing. In addition, samples prepared from the wet chemical coating process have a higher B_r and B_s and a lower H_c compared with that from solid-state mixing. Considering both the effects of sintering temperature and sintered density (>95% T.D.), ferrites with 1.5 wt% Bi₂O₃ addition by ammonia precipitation coating sintered at 900°C can provide the best permeability and quality factor (191 and 68.2, respectively) among all the cases studied.     

Indium-substitution and indium-less case effects on structural and magnetic properties of yttrium-iron garnet

The effect of indium (In) substitution in the dynamics of structure and ferrimagnetism of yttrium iron garnet (YIG) employing sintering temperature as a temporary agent of composition and structural changes was reported in this study. The nanoparticles of YIG powder samples with various In content (x=0.0–0.4) were prepared via the mechanical alloying (MA) technique. A brief, yet revealing characterization of the samples was carried out via transmission electron microscope, X-ray diffraction, Raman spectroscopy, B–H Hysteresisgraph, and LCR-metre. The X-ray diffraction analysis of the samples prepared via the MA indicates the formation of single phase YIG structure at much lower sintering temperature than that in the conventional ceramic technique. The lattice constant increases as In content increases which obeys Vegard's Law due to the larger In³⁺ ions replacing the smaller Fe³⁺ ions. The saturation induction increased reaching about 699.1 G for x=0.3 and decreased with further In substitution. Three stages of ordered magnetism formation were identified which attributed to development of crystallinity and larger grains for magnetic domain accommodation. The Curie temperature shows a decrement in their values with In content due to weakening of superexchange interactions. Raman shifts from 268.1 to 272.2 cm⁻¹ with increasing In content were observed due to stress developed in the YIG crystal structure.     

Electrical,structural, magnetic and transport properties of Zn2BaFe16O27 doped with Cu

Series of polycrystalline samples of Zn_2−xCu_xBaFe₁₆O₂₇ were prepared by usual ceramic methods, where x=0.0, 0.4, 0.6, 0.8, 1.0, 1.4. X-ray analysis done at room temperature using CoK_α with λ=1.790 Å confirms the presence of W-type hexaferrite phase structure. Saturation magnetization and hysteresis loops curves measurements at room temperature were studied as a function of Cu²⁺ substitution. It can be seen that the Cu²⁺ content slightly decreases the saturation magnetization from 25 to 20 emu g⁻¹; all hysteresis loops are closed, which indicates low anisotropy field and low saturation magnetization field. The dc conductivity and thermoelectric power were measured in a range from room temperature up to T=750 K for all samples. The thermoelectric power decreases on increasing Cu²⁺ content, and the conductivity increases with temperature. The value of the charge-carrier concentration increases by increasing the temperature and Cu²⁺ content.     

Influence of hydrogen on structural and magnetic properties of the hexagonal Laves phase HoMn2

The HoMn₂ compound crystallizes in the cubic C15 or hexagonal C14 Laves phases depending on preparation. The effect of hydrogen absorption on structural and magnetic properties of HoMn₂H_x hydrides for the C14 phase has been investigated by XRD and AC/DC magnetometry in the temperature ranges of 75-380 K and 4-390 K, respectively. In addition to general features revealed by RMn₂H_x compounds (R=rare earth or Yttrium), unusual behavior of these hydrides was found. In particular, a transformation from the hexagonal to the monoclinic structure was detected, the same as that observed for cubic HoMn₂H_x compounds. The structural transformations are correlated to the magnetic behavior. The presented results are compared mainly with the properties of the cubic HoMn₂H_x hydrides as well as with those of other RMn₂H_x hydrides. Tentative magnetic and structural phase diagrams are proposed.     

Effect of Ce and Cu co-doping on the structural,morphological, and optical properties of ZnO nanocrystals and first principle investigation of their stability and magnetic properties

Ce, Cu co-doped ZnO (Zn_1−2xCe_xCu_xO: x=0.00, 0.01, 0.02, 0.03, 0.04 and 0.05) nanocrystals were synthesized by a microwave combustion method. These nanocrystals were investigated by using X-ray diffraction (XRD), UV–visible diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). The stability and magnetic properties of Ce and Cu co-doped ZnO were probed by first principle calculations. XRD results revealed that all the compositions are single crystalline. hexagonal wurtzite structure. The optical band gap of pure ZnO was found to be 3.22 eV, and it decreased from 3.15 to 3.10 eV with an increase in the concentration of Cu and Ce content. The morphologies of Ce and Cu co-doped ZnO samples confirmed the formation of nanocrystals with an average grain size ranging from 70 to 150 nm. The magnetization measurement results affirmed the antiferro and ferromagnetic state for Ce and Cu co-doped ZnO samples and this is in agreement with the first principles theoretical calculations.     

Effect of Fe substitution on multiferroic hexagonal YMnO3

Polycrystalline YMn_1−xFe_xO₃ (x=0.02-0.20) powders were synthesized by means of modified citrate method. Powder X-ray diffraction gives evidence that all the samples are single phase and exhibit hexagonal structure with P6₃cm space group as observed for YMnO₃. The solubility limit of Fe was determined as about 6 wt.%. Cell parameter values were found to increase with Fe content, since Fe³⁺ and Mn³⁺ have the same ionic radii. This can be attributed to the increase of the tilting of MnO₅ bipyramid and the buckling of Y atoms. In addition, ⁵⁷Fe Mössbauer spectrometry provides evidence of two Fe³⁺ sites attributed to two different nearest atomic neighbours. Magnetic properties reveal a paramagnetic-to-antiferromagnetic transition, a possible increase of the magnetic anisotropy, and a competition between ferromagnetic and antiferromagnetic interactions.     

Study of dielectric behaviour of Mn-Zn nano ferrites

Dielectric properties of Mn_0.4Zn_0.6Fe₂O₄ ferrites synthesised by co-precipitation method have been investigated as a function of frequency (up to 30 MHz) at different temperatures. Dispersion in dielectric constant has been observed between temperatures 450-500 K. DC resistivity was found to increase up to 100 times greater than those for the samples prepared by the conventional ceramic methods. Resistivity variation with temperature is also reported in the present work. The particle size is calculated using Scherrer equation for Lorentzian peak, which comes out between 9 and 19 nm. Possible mechanisms contributing to these processes have been discussed.