Fe80.7Si4B13Cu2.3 nanocrystalline soft magnetic alloy powders with outstanding electromagnetic wave absorption properties in X-band (8–12 GHz) have been successfully synthesized by a ball-milling method. As milling time increases from 40 to 160 h, the mean particle size of the near spherical powders with a bcc-Fe nanocrystalline/amorphous composite structure decreases from 9 to 6 μm, while the average nanocrystalline grain size stabilizes at around 10 nm. The alloy powders possess high saturation magnetization of 178.2–183.4 emu/g. The alloy powders/paraffin (3:1 in mass ratio) composites exhibit real permeability and permittivity of 2.18–2.42 and 7.3–9.7 at 2 GHz, respectively, and imaginary permeability of 0.68–0.77 at resonance frequencies of around 12.5 GHz. The composites with thicknesses of 2.1–2.3 mm exhibit large reflection losses (RL) of −39.1 to −50.5 dB with effective absorption bandwidths (RL <  −10 db) of 4.5–5.0 ghz covering the x-band. the excellent absorption properties are benefited from the high permeability and matchable permittivity of the nanocrystalline alloy powders in ghz range.

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