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.
enough grain size which exceed the critical grain size for transition from single-domain to multi-domain grains. As for magnetic properties, the H c values were found to increase as the sintering temperature increased from 900 to 1200 °C and subsequently reduced at 1350 °C sintering temperature. The increased value of coercivity for lower sintering was due to shape and magnetocrystalline anisotropy for small enough grains. An integrated analysis of phase, microstructural and hysteresis data pointed to existence of three distinct shape-differentiated groups of B-H hysteresis loops which belong to samples with moderate and strong magnetism.
The causes of electromagnetic energy loss in very low loss yttrium iron garnet (YIG) are not fully understood. Thus, we have studied and now report on new findings concerning this problem at MHz and low GHz frequencies. We chose to observe if there would be new revealing data if a polycrystalline YIG sample was subjected to an isochronal recovery behaviour process, a procedure normally employed to study electrical resistivity and shear stress in metals by metallurgist. Thus, isochronal recovery behaviour of YIG samples' complex permeability components, l 0 and l 00 as well as their electrical resistivity (q), was studied in this work. The YIG samples were prepared via mechanical alloying of pure Y 2 O 3 and Fe 2 O 3 powders followed by appropriate sintering, measurement of properties, quenching, re-measurement of properties, isochronal annealing and a final repeat of property measurement. Results obtained from the experiments showed that isochronal recovery behaviour could be clearly exhibited by both the complex permeability components and the electrical resistivity. The measured property values gave their isochronal recovery behaviour in which the values, though suppressed after the quenching were recovered even higher after undergoing the annealing. We believe the mechanisms which produced the changes should involve atomic scale and submicron defects in the form of vacancies, interstitials, microcracks, dislocations etc. created in the quenched samples. It seems plausible that changes in the atomic and submicron defects concentration gave rise to changes in the values of the complex permeability components and electrical resistivity. Generally, the connection between such defects and isochronal recovery behaviour of material properties has been indicated in the literature.
In an attempt to obtain the best possible properties of barium hexaferrite (BaFe 12 O 19 ), 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 BaFe 12 O 19 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 BaFe 12 O 19 ceramics obtained at a lower sintering temperature of 1,150°C which is lower than the normally reported sintering temperature of C1,200°C. The thermal treatment can markedly affect the grains in compact samples.
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