The nanocomposite BaFe12O19/α-Fe and nanocrystalline α-Fe microfibers with diameters of 1-5 μm, high aspect ratios and large specific areas are prepared by the citrate gel transformation and reduction process. The nanocomposite BaFe12O19/α-Fe microfibers show some exchange-coupling interactions largely arising from the magnetization hard （BaFe12019） and soft （a-Fe） nanoparticles. For the microwave absorptions, the double-layer structures consisting of the nanocomposite BaFe12O19/α-Fe and α-Fe microfibers each exhibit a wide band and strong absorption behavior. When the nanocomposite BaFel2O19/α-Fe microfibers are used as a matching layer of 2.3 mm in thickness and a-Fe microfibers as an absorbing layer of 1.2 mm in thickness, the optimal reflection loss （RL） achieves -47 dB at 15.6 GHz, the absorption bandwidth is about 12.7 GHz ranging from 5.3 to 18 GHz, exceeding -20 dB, which covers 72.5% C-band （4.2-8.2 GHz） and whole X-band （8.2-12.4 GHz） and Ku-band （12.4-18 GHz）. The enhanced absorption properties of these double-layer absorbers are mainly ascribed to the improvement in impedance matching ability and microwave multi-reflection largely resulting from the dipolar polarization, interfacial polarization, exchange-coupling interaction, and small size effect.
Nanocomposite BaFe12019/a-Fe microfibers with diameters of about 1-5 μm are prepared by the organic gel- thermal selective reduction process. The binary phase of BaFe12019 and a-Fe is formed after reduction of the precursor BaFel2019/a-Fe203 microfibers at 350 ℃ for 1 h. These nanocomposite microfibers are fabricated from a-Fe （16-22 nm in diameter） and BaFe12019 particles （36--42 nm in diameter） and basically exhibit a single-phase-like magnetization be- havior, with a high saturation magnetization and coercive force arising from the exchange--coupling interactions of soft a-Fe and hard BaFe12019. The microwave absorption characteristics in a 2-18 GHz frequency range of the nanocomposite BaFe12O19/a-Fe microfibers are mainly influenced by their mass ratio of a-Fe/BaFe12019 and specimen thickness. It is found that the nanocomposite BaFelzO19/a-Fe microfibers with a mass ratio of 1：6 and specimen thickness of 2.5 mm show an optimal reflection loss （RL） of -29.7 dB at 13.5 GHz and the bandwidth with RL exceeding -10 dB covers the whole Ku-band （12.4-18.0 GHz）. This enhancement of microwave absorption can be attributed to the heterostruc- ture of soft, nano, conducting a-Fe particles embedded in hard, nano, semiconducting barium ferrite, which improves the dipolar polarization, interfacial polarization, exchange--coupling interaction, and anisotropic energy in the nanocomposite BaFe12O19/a-Fe microfibers.
