AbstractWe synthesized Fe and Co‐based (Fe48Co41Si3B8) alloy nanopowders by a mechanochemical technique and investigated their structure, magnetic and electromagnetic properties. The nanopowders have a bcc (A2) phase and demonstrate high magnetization with low coercivity. The analysis reveals that their magnetization process may be described by a two‐phase model. The microwave measurements performed by the coaxial method over a broad band up to 18 GHz show that the nanocomposites comprising the nanopowders possess high, broadband magnetic permeability, promising applications in electromagnetic shielding and microwave absorption. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)