Surfactant-free synthesis and electromagnetic properties of Co-Ni-B composite particles

► The Co-Ni-B composite particles were surfactant-free synthesized in aqueous solution. ► The morphologies contain spheres, spheres in-pair, dentrites particles. ► The μr″ of amorphous Co-Ni-B shows negative values at reverse resonant frequency. ► The optimal RL of −32.4 dB is obtained with thickness of 1.2 mm. ► The bandwidth of reflection loss less than −10 dB covers Ku-band for a 1.1 mm layer.