Facile synthesis of FeCo–MnO2 core–shell nanoparticles as high-frequency microwave absorbers using a two-step method

•FeCo–MnO2 core–shell nanoparticles were fabricated using a two-step method.•Porous structure of MnO2 layer caused electromagnetic wave scattering inside pores.•Thus, the impedance matching improved.•Lorentz-type resonance induced by displacement current lag at core–shell interface.•1.9-mm-thick FeCo–MnO2 (71 nm) nanoparticles had a high RL of − 36 dB.•The nanoparticles had a wide bandwidth of 4.9 GHz.