Synthesis, structure and dynamic magnetic properties of double-layered Ni-Fe1-xCox hollow microspheres

Abstract

Double-layered Ni-Co1-xFex hollow microspheres with a mean size of ∼1 νm were fabricated by electroless plating of Co or Co-rich CoFe films on Ni hollow spheres (HS); the latter was synthesized by an autocatalytic reaction process. The Ni HS have a microstructure consisting of both fcc and hcp phases and have a very low coercivity. Coating a layer of Co leads to a high coercivity due to the formation of the hcp Co phase with high magnetocrystal anisotropy. Addition of 10% Fe into the Co layer decreases the coercivity from 432 to 140 Oe by transforming the hcp Co phase to the fcc CoFe phase; the latter has a low anisotropy field. Addition of 20% Fe results in a further decrease in coercivity to 69 Oe due to the formation of the bcc CoFe phase. The microwave properties of the composites consisting of HS and a polymer were investigated. Ni HS + a polymer composite have high permittivity but relatively low permeability. Co or CoFe coating decreases the permittivity but slightly increases the permeability of the composite. A reflection loss of