• Co/Co3O4@NC nanosheets with gradient magnetic heterointerfaces are fabricated via high-temperature carbonization and low-temperature oxidation, enabling precise phase evolution control.
• Gradient magnetic heterointerfaces optimize impedance matching and enhance interfacial polarization, magnetic coupling, and long-range magnetic diffraction, leading to superior EM wave absorption.
• The optimized Co/Co3O4@NC absorbents achieve a strong reflection loss of −53.5 dB at 3.0 mm thickness and an effective absorption bandwidth of 5.36 GHz, outperforming single magnetic domain counterparts.
• This work provides a novel design strategy for advanced EM wave absorbing materials by engineering gradient magnetic heterointerfaces through controlled oxidation.