Hollow glass microspheres coated with thin Ni films of various thicknesses (about 50–250nm) were obtained by electroless plating technique. The resultant magnetic microspheres had very low densities (∼0.39–1.28gcm−3) and low coercivities (<40Oe). As-plated films comprised of nanocrystallites embedded in amorphous matrix and postannealing treatment led to a submicrocrystalline structure and an increased saturation magnetization. The high frequency properties of the composites consisting of Ni-plated microspheres and polymer were investigated. The permittivity and permeability increased with increasing microsphere content in the composite and increasing Ni film thickness. These composites had ferromagnetic resonance peak in the range of 5–12GHz. Postannealing also had an effect on the microwave properties. The reflection loss of the composite was found dependent on the absorber material thickness, polymer:microsphere ratios, the Ni film thickness, as well as the heat treatment of the microspheres.

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Double-layered Ni-Co1−xFex hollow microspheres with a mean size of ∼1 µm 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 <20 dB is obtained for the CoFe coated hollow Ni microspheres with an absorber thickness of 1.5–3.0 mm.