Fig 1

Preparation and basic characterization of FeCo multiphase core-shell heterostructures with equiatomic ratios under gradient oxidation. (a) Schematic diagram of the synthetic route for FeCo@(Fe,Co)xO4 heterostructures with continuously tunable oxidation degree. (b) X-ray diffraction patterns of FeCo samples treated at oxidation temperatures from room temperature (RT) to 800 ℃. (c) Raman spectra of FeCo-RT to FeCo-800. (d) Room temperature hysteresis loops of FeCo-RT to FeCo-800."

Fig 2

Morphological evolution and surface chemical states of gradient-oxidized FeCo heterostructures. (a) SEM images of FeCo-RT, (b) FeCo-300, (c) FeCo-600, and (d) FeCo-800. (e–h) SEM images of FeCo-450 at different magnifications. (i–l) SEM images of FeCo-450 and corresponding EDS elemental distribution maps. High-resolution XPS spectra of FeCo-450: (m) C 1s, (n) O 1s, (o) Fe 2p, and (p) Co 2p."

Fig 3

Electromagnetic parameters and microwave absorption properties of gradient oxidized FeCo samples. (a) Real part (ε′) and (b) Imaginary part (ε′′) of the complex permittivity, and (c) Dielectric loss tangent (tanδε) as frequency for all samples. (d) Real part (μ′) and (e) Imaginary part (μ″) of the complex permeability, and (f) Magnetic loss tangent (tanδμ) as frequency. Two-dimensional contour plots of reflection loss and their corresponding three-dimensional surface plots for (g) FeCo-RT, (h) FeCo-450, and (i) FeCo-800 at different thicknesses."

Fig 4

Impedance matching, dielectric relaxation, and dissipation mechanism analysis of gradient-oxidized FeCo samples. (a) RL curves and (b) Normalized input impedance matching ratio(|Zin/Z0|) of FeCo-450 at different thicknesses as a function of frequency. (c) Smith chart of normalized input impedance for FeCo-450. (d) Cole-Cole curves for FeCo-RT, (e) FeCo-450, and (f) FeCo-800. (g) Conductivity loss and (h) Polarization loss as a function of frequency for all gradient-oxidized samples. (i) Quantitative contribution of conductivity loss and polarization loss to total dielectric dissipation of FeCo-450 at a specific frequency. (j) Atomic structure models of the FeCo and CoFe2O4 phases. (k) Spin density isosurface of the FeCo/CoFe2O4 heterojunction, showing the spin polarization charge density distribution and local magnetic moment configuration at the interface. (l) Three-dimensional differential charge density difference isosurface of the FeCo/CoFe2O4 heterojunction and Bader charge analysis plot. The yellow and cyan regions represent electron accumulation and depletion regions, respectively. (m) Planar average charge density difference (Δρ) along the z-direction of the FeCo/CoFe2O4 heterojunction."

Fig 5

Electronic structure, attenuation behavior, magnetic loss analysis, and electromagnetic field simulation of gradient-oxidized FeCo samples. (a) Planar average electrostatic potential distribution of FeCo, CoFe2O4, and FeCo@CoFe2O4, showing the vacuum level (Evac), Fermi level (EF), and work function (Φ) of each system. (b) Spin-polarized total density of states and partial density of states of the FeCo/CoFe2O4 heterojunction. The vertical dashed line represents the Fermi level, with positive and negative values representing spin-up and spin-down channels, respectively. (c) Attenuation constant (α) and (d) eddy current loss coefficient (C0) of all samples as a function of frequency. (e) Comprehensive comparison of FeCo-450 with recently reported magnetic-based electromagnetic absorbers in terms of minimum reflection loss and effective absorption bandwidth. (f) Volumetric power loss density and (g) 3D simulation distribution of internal electric field multi-section vector diagrams of all samples at their respective optimal matching frequencies."

Fig 6

Radar scattering simulation and microwave attenuation mechanism of gradient oxidized FeCo heterojunction. (a) Three-dimensional radar scattering plots of bare PEC, FeCo-RT, FeCo-450, and FeCo-800. (b) Radar cross section (RCS) curves of PEC and all coated samples as a function of incident angle. (c) Bistatic RCS polar plots of PEC and FeCo-450. (d) Bar chart of RCS reduction for all gradient oxidized FeCo samples at typical scattering angles. (e) Schematic diagram of microwave attenuation mechanism of FeCo-450, including impedance matching, interface polarization induced by built-in electric field, defect dipole polarization, natural/exchange resonance, eddy current loss, and interface spin pinning."