Exchange Length Tailored Magnetic Resonance for Broadband Absorption in FeCo‐Based Alloys

The broadband electromagnetic wave absorption (EWA) of soft magnetic alloys (SMAs) is severely constrained by the rapid decay of their magnetic loss at gigahertz frequencies. To address this issue, an intergranular coupling strategy guided by the magnetic exchange length (L_ex) is proposed in this study. Doping with Cu systematically increases L_ex in SMAs from 12.7 nm to 19.1 nm, thereby enhancing the collective precession of magnetic moments over a larger spatial range. This Cu doping concurrently reduces the effective magnetic anisotropy (K_eff) from 9.2 to 8.6 kJ m^-3 and increases the magnetic exchange strength by 25%. These coordinated changes effectively shift the resonance peak to 12.0 GHz and amplify the magnetic loss. As a result, the optimized alloy achieves an effective absorption bandwidth (EAB) of 8.1 GHz at a minimal thickness of 1.8 mm, representing a 47.1% improvement over its Cu-free counterpart. This work pioneers the use of L_ex-driven mechanisms for regulating magnetic loss and establishes a fundamental framework for enhancing the broadband EWA performance of SMAs through targeted resonance modulation.