We investigate for the first time the magnetorheological (MR) properties of bimetallic alloy nanocomposites based on cross-linked polydimethylsiloxane elastomer and ferromagnetic FeCo 3 nanoparticles. The nanoparticles (∼30 nm), with a saturation magnetization value of 166 emu/g, are synthesized by hydrazine reduction of Fe 2+ and Co 2+ metal ions. Isotropic and anisotropic nanocomposite films are prepared by a solution casting technique with 5, 10, and 20 wt % FeCo 3 in the absence and presence of 0.2 T magnetic field, respectively. The structural, morphological, and magnetic properties of nanoparticles and their composites are characterized by X-ray diffraction, transmission electron microscopy, field emission scanning electron microscopy, confocal and optical microscopy, and vibrating sample magnetometer analysis. Steady-state and dynamic mechanical properties of the nanocomposite under a magnetic field are evaluated by rotary shear, strain amplitude sweep, angular frequency sweep, and magnetic flux density sweep tests using a parallel plate rheometer. The effects of particle concentration, particle alignment on the magnetic properties and anisotropic coefficient of the nanocomposites are determined by measuring the hysteresis property parallel and perpendicular to the particle chain alignment. The anisotropic nanocomposites show saturation magnetization higher than that of the isotropic nanocomposites, except for the particle concentration at 20 wt %. Magnetorheological study reveals that the isotropic nanocomposites have higher absolute and relative MR effect than that of their anisotropic counterpart. Under 1.098 T magnetic field, the highest absolute MR effect of ∼21 600 Pa is found for 5 wt %, whereas the highest relative MR effect of ∼8.4% is obtained with 20 wt % isotropic composites.

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The combined effect of both hollow, mesoporous structure of carbon nanofiber and Fe3O4 nanoparticles on the microwave absorption properties of polydimethylsiloxane nanocomposites have been investigated. Nanofibers with above characteristics were prepared via coelectrospinning the solutions of polyacrylonitrile/FeCl3 and poly­(methyl methacrylate) followed by stabilization and carbonization at elevated temperatures. Carbonized nanofibers contained Fe3O4 nanoparticles with average crystallite size of ∼12.3–14.6 nm and exhibited a surface area of 126.4–377.7 m2/g. Catalytic graphitization surrounding Fe3O4 nanoparticles was seen in high-resolution transmission electron microscopy and also supported by a decrease in intensity ratio of D to G bands in Raman spectra. Microwave absorption properties of nanocomposites were investigated in a vector network analyzer using a coaxial waveguide in the frequency range of 2–18 GHz and found to be dependent on thickness, filler loading, and Fe3O4 content of the nanofibers. At an absorber thickness of 7.5 mm with 25 wt % carbon nanofibers (consisting 5 wt % Fe3O4), the absorption bandwidth was found to be a maximum of 4.33 GHz with reflection loss of −25 dB. However, corresponding bandwidth was increased to 4.51 GHz with reflection loss of −44 dB for nanocomposite with 25 wt % carbon nanofibers (but containing 7.5 wt % Fe3O4) at only 5.5 mm absorber thickness.

Cobalt ferrite nanofiber filled polydimethylsiloxane nanocomposites show 100–400% improvement in magnetorheological properties compared to that with nanoparticles.