Carbon Confinement as a Design Principle for Multiphase Magnetic Nanocomposites With Broadband Functionality

ABSTRACT While broadband electromagnetic (EM) loss mechanisms have critical implications for both electromagnetic absorbers and magnetic hyperthermia, integrating diverse loss channels into single material architecture remains a key challenge for next‐generation multifunctional composites. Herein, we introduce carbon confinement of Cobalt Ferrite nanoparticles (CFO@C) as a design principle to simultaneously address the performance‐processability trade‐off for broadband functionality. Mesoporous activated carbon acts as a reactive template that constrains CFO nanoparticle growth (), mitigates agglomeration, and provides conductive pathways for complementary dielectric response (ε). Static magnetometry reveals complex magnetic behavior driven by coexisting hard and soft phases, which is quantitatively resolved using Voigt‐profile deconvolution of the wasp‐waisted hysteresis loops, enabling phase‐resolved analysis of reversal processes. Ferromagnetic Resonance (18–30 GHz) reveals a stable g‐factor and large damping (α = 0.14) indicative of efficient GHz‐frequency energy dissipation governed by spin–lattice relaxation. Low‐frequency magnetic hyperthermia validates linear‐response relaxation as the dominant loss channel under physiological field conditions (310 kHz, 400–800 Oe). These results establish CFO@C as a multifunctional nanocomposite that unifies broadband EM dissipation with efficient low‐frequency heating, establishing the pathway for μ − ε co‐design in frequency‐adaptive materials relevant to printed electronics, EMI mitigation, and magnetically driven functional devices.