Microwave synthesis and magnetic properties of amorphous ferrites

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Various amorphous ferrites were synthesized by microwave irradiation on the powder mixture of component oxides using 28 GHz frequency. Microwave absorbing behavior of component oxides was analyzed by measuring the temperature-time profiles under microwave irradiation. Strong microwave absorption was observed in powder mixture of oxides with spinel-and magnetoplumbite-type compositions. After irradiation of high power microwaves, the specimens turned to amorphous phases showing ferromagnetism. These amorphous phases did not crystallize by high-temperature annealing at 1200℃, but easily crystallized by applying shear stress. It was found that the tendency of amorphisation strongly correlate with the magnetostriction constant of the crystalline ferrites. The obtained amorphous ferrites shows ferromagnetic behavior similar to those of crystalline phases with the same compositions, i.e., the Curie temperature and saturation magnetization are nearly the same for both amorphous and crystalline phases, but the coercive force (Hc) of the amorphous phase is much smaller than that of crystalline phase. In the case of ZnFe_2O_4, the amorphous phase shows ferromagnetism although the crystalline ZnFe_2O_4 (normal spinel-type) is antiferromagnetic. The possible presence of tetrahedral Fe^<3+> ion confirmed by XPS spectra. The mechanism of amorphisation is discussed in terms of material-microwave interactions, especially, structure fluctuation induced by coupling with microwave energy.