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Fine flakes of Fe–Si–Al produced by long-duration microforging exhibit a peculiar dual-peak dispersion in their frequency characteristics of the imaginary part of the permeability. This dual dispersion has a high potential to be a electromagnetic noise suppressor, works effectively in the microwave band. In a previous report we clarified that one of the dispersions (D II), which appears in the lower frequency range, is correlated to a shape anisotropy of the flakes. In this article, the origin of another dispersion (D III) is studied by analyses of crystalline structure changes during forging (100–180 h) and annealing processes. X-ray diffraction and Mössbauer spectra strongly suggest that the annealed flakes have a composition gradient structure consisting of an Fe-rich mother phase and a Si/Al-rich surface layer. The fact that dispersion D III is enhanced with development of this phase separation leads to the conclusion that dispersion D III is caused by a magnetoelastic anisotropy near the flake surface.

The electromagnetic-noise suppression sheets made of Fe–Si–Al thin flakes embedded in polymer improve their noise suppressing performance appreciably in the magnetic field. We have already confirmed that the transmission characteristics of a microstrip line exhibit a high-frequency suppressing feature when a small piece of the sheet is placed on it [S. Yoshida et al., IEEE Trans. Magn. 37, 2401 (2001)]. In this article we found that this filtering performance is appreciably improved by a dc magnetic field. The effect becomes explicit in a field with intensity higher than a few hundred Oe and rise of the transmission loss becomes much steeper near the frequency of magnetic resonance. An example is that in the field of 1 kOe the 3 dB width of half to the peak of frequency profile of the transmission loss at resonance is reduced to less than a quarter of the one without magnetic field. This improvement brings about a new research subject to develop the high performance electromagnetic noise absorber that is expected to work under the optimized conditions in various digital electronic devices with operation frequencies up to the GHz band.

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