Most of microwave energy transforms into thermal energy, and chemical reactions are driven by thermal energy. In the past decade, however, many studies have been reported about "microwave effect", which is difficult to explain by thermodynamics, e.g., decrystallization of the ferromagnetic materials, creating the supersaturated solid solutions, etc. In particular, it is noteworthy of the antiferromagnetic-ferromagnetic transition of a spinel ferrite. Although it is known that the microwave magnetic field (H-field) plays an important role in this process, the details are still not clear. In previous work, FeAl2O4, which is a typical antiferromagnetic spinel, was synthesized using single-mode 2.45 GHz microwave H-field irradiation and the effect of the magnetic fields on magnetic characteristics of antiferromagnetic oxide was studied. However, the authors do not reach to reveal a basic mechanism of decreasing the crystal diameter (increasing the grain boundaries) by microwave irradiation. In the study, to examine the effect of the ratio of y-Fe2O3 in starting materials for the change of magnetic properties by microwave H-field irradiation, FeAl2O4 is synthesized from 90 wt% Fe2O3-10 wt% Al2O3 mixed powder.
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