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In this study, monodisperse Fe 3 O 4 nanoparticles were synthesized successfully using a sonochemical method in the presence of 3-aminopropyltriethoxysilane (APTES). The morphology, microstructure and magnetic properties of the bare Fe 3 O 4 and APTES-coated Fe 3 O 4 were investigated in detail by TEM, XRD, FTIR and SQUID. It was found that APTEScoated Fe 3 O 4 showed relatively gooddispersion with anarrow size distribution of 8.4±2.1 nmdiameter. The functionalization of Fe 3 O 4 was proved to be covalent linking between Fe 3 O 4 and APTES. The field-dependent magnetization curve indicated superparamagnetic behavior of Fe 3 O 4 -APTES with a saturation magnetization (M s ) of 70.5 emu g −1 at room temperature. Amagnetorheological (MR) fluid was prepared using the obtained Fe 3 O 4 -APTES nanoparticles with 25 wt% particles, and its MR properties weretested usinga Physica MCR301 rheometer fitted with an MRmodule. The results showed that the as-prepared APTES-coated Fe 3 O 4 nanoparticle-based MR fluid exhibited typical MR effects, with increasing viscosity, shear stress and yield stress depending on the applied magnetic field strength.

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