The Microstructure And Magnetic Properties Of Fe/SiO₂ Nanocomposites

Nanocomposites composed of ferromagnetic nanoparticles dispersed in oxide matrix have attracted much attentions because of their special magnetic properties and potential applications as microwave absorbers. In this work, the microstructure and magnetic properties of Fex(SiO2)1-x nanocomposties have been studied to understand the effect of interparticle dipolar interactions on the magnetic properties of system and the microwave absorption mechanism in ferromagnetic metal-oxide nanocomposites.The Fes(SiO2)1-x nanocomposites were synthesized by mechanochemical reduction reaction of Fe2O3 and Si powders. The Fex(SiO2)1-x nanocomposites of the single crystal Fe nanoparticles with equiaxed (or quasi-sphere) shape dispersed randomly in the amorphous SiO2 matrix were preprared. The Fe nanoparticles in the Fex(SiO2)1-x nanocomposites have a narrow particle size distribution; and the Fe average particle size of about 11 nm keeps almost unchanged for the Fe contents x between 22wt% and 51wt%.It has been shown that the interparticle dipolar interactions have an effect not only on the magnetic state of Fe particles but also on the moment arrangements of the Fe particles. With increasing x, the dipolar interaction strength among the Fe particles increases. The surperparamagnetic Fe particles in Fes(SiO2)1-x nanocomposites become blocked, which may be due to that the dipolar interaction increases the energy barrier for the magnetization reverse. The moment arrangements of the Fe particles change from the random orientation to the local "chain-like" arrangements and then to locally forming flux-closure arrangements, which leads to the first increase and then decrease of the coercivity and remanence ratio.The study of microwave absorption mechanism in the Fex(SiO2)1-x nanocomposites indicates that a broad resonance band in the 1-16 GHz range observed in these nanocompsites results from the overlap of the natural resonance and exchange resonance. The natural resonance and exchange resonance frequencies depend on the surface anisotropy of the Fe nanoparticles. The enhanced surface anisotropy resulting from the reduced Fe particle size shifts the natural resonance to high frequency. For exchange resonance, the surface anisotropy weakens the dependence of exchange resonance frequency on the square of the reciprocal particle size.The reflection loss (RL) values calculated for the Fex(SiO2)1-x nanocomposites-paraffin mixtures reveal that their microwave absorption performance is improved with increasing x when the thickness of the mixtures keeps unchanged. For the mixture with thickness of 1.4-2.0 mm and x=51wt%, the frequency band of the RL values smaller than-20 dB, corresponding to a microwave absorption of 99%, is from 9.6 to 17.1 GHz. A minimum RL of-33 dB is reached at 14.9 GHz for this mixture with the thickness of 1.5 mm.