Epsilon-negative response induced by magnetic/dielectric dual percolation in Ag/Bi25FeO40 metacomposites

Metacomposites exhibiting ε′-negative (ε′ < 0) responses have introduced composite design concepts and processing methods into the field of advanced electromagnetic (EM) devices. However, the percolation theory and its underlying microscopic mechanisms remain insufficiently understood. In this work, a co-continuous phase structure was fabricated by in situ constructing a three-dimensional silver (Ag) network within a magnetic Bi25FeO40 matrix, thereby realizing magnetic/dielectric dual percolation behavior. Magnetic percolation was confirmed by abrupt changes in saturation magnetic induction, coercivity, and remanence with increasing Ag content. Concurrently, during the evolution of the percolation structure, Ag/Bi25FeO40 metacomposites exhibited three distinct dielectric response mechanisms—interface polarization, dielectric resonance, and plasmonic oscillation—reflected in the enhancement of positive permittivity (ε′ > 0), a resonance peak near 400 MHz, and an ε′-negative response spanning the 100 MHz–1 GHz range. Using first-principles calculations, we analyzed the differential charge density, density of states, and Fermi level of Ag@Bi25FeO40 heterostructures to elucidate the transition of electric dipole states from static alignment to rotation and finally to collective oscillation. Owing to the effective blocking effect of Bi25FeO40 cuboid building blocks on the Ag network, a broadband weak ε′-negative response (300 MHz–1 GHz, 0 < |ε′| < 1000) was achieved. Its inductive characteristics and potential for EM shielding were further clarified through equivalent circuit analysis and EM simulation.