Evolution of Magnetic Properties in Ferrites: Trends of Single- Sample and Multi-Sample Sintering

Microstructure of magnetic materials greatly influences the performance of magnetic properties, and sintering has been used as an agent to tailor the microstructure of these magnetic materials especially ferrites. Nanostructured ferrites prepared by high-energy milling method are often inherently unstable owing to their small constituent sizes, non-equilibrium cation distribution, disordered spin configuration, and high chemical activity. Therefore, sintering of the milled ferrites recrystallizes the nanostructure and causes its transition from an excited metastable (activated) state into the low-energy crystalline state. A better understanding of the response of nanoscale ferrites with changes in temperature is crucial not only for basic science (the development of an atomistic and microscopic theory of the mechanochemical processes) but also because of the technological high-temperature applications in catalysis, ferrofluids and information storage. This chapter discusses on two different sintering schemes, which are a commonly applied multi-sample sintering and a rarely adopted single-sample sintering. Experimental results of single-sample and multi-sample sintering of NiZn ferrites and yttrium iron garnet (YIG) were highlighted, and their microstructural consequences on the magnetic properties were also discussed

Dependence of developing magnetic hysteresis characteristics on stages of evolving microstructure in polycrystalline yttrium iron garnet

The microstructure evolution in several polycrystalline yttrium iron garnet samples as a result of a sintering scheme was studied in detail, in parallel with the changes in their magnetic properties. Samples with nanometer sized starting powder were synthesized by employing the High-Energy Ball Milling technique and then sintering toroidal compacts of the milled powder. Nine sintered samples were obtained, each corresponding to a particular sintering from 600 °C to 1400 °C. The samples were characterized for their evolution in crystalline phases, microstructure and magnetic hysteresis-loops parameters. The results showed an increasing tendency of the saturation magnetization and saturation induction with grain size, which is attributed to crystallinity increase and to reduction of demagnetizing fields in the grains. The variation in coercivity could be related to anisotropy field changes within the samples due to grain size changes. In particular, the starting appearance of room temperature ferromagnetic order suggested by the sigmoid-shaped B–H loops seems to be dependent on a sufficient number of large enough magnetic domain-containing grains having been formed in the microstructure. Viewed simultaneously, the hysteresis loops appear to belong to three groups with different magnetism-type dominance, respectively dependent on phase purity and three different groups of grain size distributions

Recent developments of smart electromagnetic absorbers based polymer-composites at gigahertz frequencies

The rapid increase in electromagnetic interference has received a serious attention from researchers who responded by producing a variety of radar absorbing materials especially at high gigahertz frequencies. Ongoing investigation is being carried out in order to find the best absorbing materials which can fulfill the requirements for smart absorbing materials which are lightweight, broad bandwidth absorption, stronger absorption etc. Thus, to improve the absorbing capability, several important parameters need to be taken into consideration such as filler type, loading level, type of polymer matrix, physical thickness, grain sizes, layers and bandwidth. Therefore, this article introduces the electromagnetic wave absorption mechanisms and then reveals and reviews those parameters that enhance the absorption performance

Magnetite nanoparticles in wastewater treatment

Clean water is very important for health and well-being of humans and ecosystem. However, over the year, a billion tons of industrial waste, fertilizers and chemical waste were dumped untreated into water bodies, such as rivers, lake and oceans contributing towards water pollution, then threatening human health and ecosystem. Hence, the need for clean water has urged scientists to research and find solutions for improving water quality. Application of nanoparticles in wastewater treatment improves the environmental quality by elimination of harmful pollutants in wastewater. Magnetite is one of the nanoparticles used in wastewater treatment because of its specific large surface area, high reactivity in adsorption and recoverable from treated water via magnetic separation technology. Preparation method of magnetite nanoparticles is the important key to its adsorption efficiency

Sintering temperature dependence of optimized microstructure formation of BaFe12O19 using sol–gel method

In an attempt to obtain the best possible properties of barium hexaferrite (BaFe12O19), the sol–gel synthesis method was chosen and, the optimum sintering conditions were established. The effects of the sintering temperature on the structural, morphological and magnetic properties of hexaferrite were studied. X-ray analysis indicates that the sintered samples (1,000–1,150 °C) remained in the hexagonal structure. From this analysis, no secondary phases are identified. The effect of sintering temperature on the grain growth of BaFeBaFe12O19 is confirmed by the microstructure using HR-SEM and is in good agreement with the XRD analysis based on the peak intensity of the (107) plane. The samples sintered at 1,150 °C showed the densities as ~93 % of theoretical density. Sintering temperature affected the grains in compact samples. The results show that homogeneous and dense BaFeBaFe12O19 ceramics obtained at a lower sintering temperature of 1,150 °C which is lower than the normally reported sintering temperature of ≥1,200 °C. The thermal treatment can markedly affect the grains in compact samples

Compositional and frequency dependent-magnetic and microwave characteristics of indium substituted yttrium iron garnet

Effect of In3+ ion substitutions on yttrium iron garnet samples’ morphology, magnetic properties and dc electrical resistivity was examined closely and reported in this study. A series of polycrystalline garnet ferrites with composition of Y3Fe5−xInxO12 (0 ≤ x ≤ 0.5) were prepared by using the mechanical alloying technique. The morphological properties of the samples was analysed by using a TEM, XRD, FESEM and EDX. The electrical dc resistivity of the samples was investigated as a function of temperature and composition by using a Picoammeter, the complex permeability was analysed by using an Impedance Materials Analyzer meanwhile microwave properties was measured by using VNA. The lattice constant increases as In3+ content increases which can be understood by the difference in ionic radii of In3+ ions replacing the smaller Fe3+ ions. The grain size also increased with In3+ content, indicating that the In3+ ion acts as a grain growth promoter. Both complex permeability components, µ′ and µ″ reaching about 92.75 and 85.03 respectively at x = 0.3; later decreased with further In substitution. This result is synchronized with FMR linewidth which manifests magnetic loss of the samples. By correlating the phase analysis, morphology, electrical resistivity and complex permeability results, it is believed that there was an increase in number of crystalline-growth regions with increasing In3+ content, which together increased a total mass of ferromagnetic grains with the latter starting to dominate the samples. The results also showed that In3+ ions increase the dc resistivity of the system

Magnetic and microwave properties of polycrystalline gadolinium iron garnet

The microwave loss in nanosized GdIG particles synthesized using mechanical alloying technique was investigated. There were very few of research on the microwave properties of nanosized particle GdIG and there is no attempt investigating on the material at C-band frequency range and its correlation with the microstructure. Gadolinium (III) iron oxide and iron (III) oxide, α-Fe2O3 were used as the starting materials. The mixed powder was then milled in a high-energy ball mixer/mill SPEX8000D for 3 hours. The samples were sintered at temperature 1200°C for 10 hours in an ambient air environment. The phase formation of the sintered samples was analyzed using a Philips X’Pert Diffractometer with Cu-Kα radiation. Complex permeability constitutes of real permeability and magnetic loss factor were measured using an Agilent HP4291A Impedance Material Analyzer in the frequency range from 10 MHz to 1 GHz. A PNA-N5227 Vector Network Analyzer (VNA) was used to obtain the information on ferromagnetic linewidth broadening, ΔH that represents the microwave loss in the samples in in frequency range of 4 to 8 GHz (C-band). The ΔH value was calculated from the transmission (S21) data acquired from VNA. The single phase GdIG showed low initial permeability and low magnetic loss when applied with low-frequency range energy. From these data, it is validated that GdIG is a suitable material for microwave devices for the high-frequency range

Influence of microstructural evolution on the magnetically group dominance in polycrystalline Y3Fe5O12 multi-samples

In present work, the effect of changing microstructure on magnetic properties which evolves in parallel, in particular from amorphous-to-crystalline development, in yttrium iron garnet was investigated. 9 toroidal samples of polycrystalline yttrium iron garnets were prepared by using the mechanical alloying technique and sintered at low to high sintering temperature for microstructure-dependent-magnetic evolutions. A brief, yet revealing characterization of the samples were carried out by using an X-ray Diffraction, Field Emission Scanning Electron Microscopy, Impedance Material Analyzer, LCR-meter and, Picoammeter. It is believed that microstructural features such as amorphous phase, grain boundary, secondary phase and intergranular pores contribute significant additional magnetic anisotropy and demagnetizing fields, thus affecting the initial permeability accordingly. A scrutinizing observation of the permeability component results show that they tend to fall into three groups of magnetic permeability according to degree of magnetic behaviour dominance. The Curie temperature remained relatively stable and unaffected by the evolution, thus confirming its intrinsic character of being dependent only on the crystal structure and compositional stoichiometry. The increased electrical resistivity while the microstructure was evolving is believed to strongly indicate improved phase purity and compositional stoichiometry

Electromagnetic wave absorbing characteristics of C/Co-Mn and C/Co-Zn doped barium hexaferrite sandwiched nanocomposites

The development of high reflection loss and broad frequency bandwidth for microwave absorbing materials (MAMs) has been intensified in recent years. Carbon nanocomposites have stimulated attention as the EM-wave absorbing materials due to their high conductivity with high dissipating capability of electrostatic charges and EM-wave radiation shielding. In this study, we report on the analysis of microwave absorption through absorption spectrum of carbon/barium hexaferrite nanocomposites with Co-Mn and Co-Zn as dopants. The nanoparticles of Co-Mn-doped and Co-Zn-doped barium hexaferrites were synthesised using the mechanical alloying technique. The resultant particles were then mixed thoroughly with epoxy resin with ratio of 8:92 wt.% prior to sandwich with carbon black layer at different thicknesses of 1, 2 and 3 mm to form nanocomposites. The samples were characterised on their morphological, magnetic and microwave properties using field emission scanning electron microscope (FESEM), vibrating sample magnetometer (VSM) and vector network analyser (VNA) respectively; there follow rational discussions on the effect of dopant substitutions and microstructure on the absorption/reflection loss and bandwidth broadening of MAM. The results show that the saturation magnetisation of BaCoZnFe16O27 is higher as compared to that of BaCoMnFe16O27 and pure BaFe12O19 in which Zn behaves as an ion modifier that compensates the magnetic moment in hexaferrite sublattices, thus translated into an increase of net magnetic moment per unit volume of the sample. By using carbon black as a matching layer and BaCoMnFe16O27 as an absorbing layer with total sandwich thickness of 2 mm, the highest reflection loss of 39.83 dB was obtained. The capability to tune the absorption and bandwidth of all the samples at various frequencies indicates that these materials would be a superior MAM. Through scrutinisation of relationship between absorption/reflection loss with different dopants and thickness variation, further discussion on mechanism of adsorption/reflection characteristics for research optimisation would be provided

A comparative study of different sintering routes effects on evolving microstructure and B–H magnetic hysteresis in mechanically-alloyed Ni–Zn ferrite, Ni0.3Zn0.7Fe2O4

A comparative study between two sintering routes has been carried out to reveal the parallel evolutions of microstructure and B–H hysteresis in a mechanically-alloyed Ni–Zn ferrite. The starting powders were mixed and crushed via the mechanical alloying technique. Subsequently two portions of the resulting powder were subjected to two respective sintering routes: multi-sample and single-sample. In the multi-sample sintering, the samples were sintered from 600 to 1400 °C with any one sample being subjected to only one sintering temperature. For the single-sample sintering, only a single sample was subjected to repeat sintering from 600 to 1400 °C. In B–H hysteresis measurement, the same trends but with different values were observed for both sintering routes. Saturation induction, Bs, values range from 23.9 to 1076.0 G for multi-sample sintering and from 23.4 to 930.7 G for single-sample sintering. Three distinct behaviour groups could be distinguished which correspond to a particular range of grain sizes and domain state of the samples. The activation energies of grain growth for multi-sample and single-sample sintering show three different ranges of values for each route which are 6.80, 99.31 and 143.39 kJ/mol and 14.60, 29.42 and 162.83 kJ/mol respectively. These different ranges of values characterized the different diffusion mechanisms