Microwave synthesis and magnetic properties of amorphous ferrites

采用28ghz频率微波辐照复合氧化物粉末，合成了多种非晶态铁氧体。通过测量微波辐照下的温度-时间曲线，分析了各组分氧化物的微波吸收行为。尖晶石型和磁铅矿型氧化物的粉末混合物具有较强的微波吸收。经高功率微波辐照后，试样转变为非晶相，表现出铁磁性。这些非晶态相在1200℃高温退火时不结晶，但在施加剪切应力时容易结晶。结果表明，非晶化倾向与晶体铁氧体的磁致伸缩常数密切相关。所得的非晶铁氧体表现出与晶相相似的铁磁性行为，即非晶相和晶相的居里温度和饱和磁化强度基本相同，但非晶相的矫顽力（Hc）远小于晶相。对于ZnFe_2O_4，非晶相表现出铁磁性，而晶体ZnFe_2O_4（正常尖晶石型）是反铁磁性的。XPS光谱证实了四面体Fe^<3+>离子的存在。从材料与微波相互作用的角度讨论了非晶化的机理，特别是与微波能量耦合引起的结构波动。

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.