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Mn 1- x Zn x Fe 2 O 4 ferrofluids were produced from natural sand for magnetic sensors and radar absorbing materials. The X-ray diffraction data showed that the Zn partially substituted the Mn and Fe ions to construct a spinel structure. The increasing Zn composition decreased the lattice parameters of the structure. The transmission electron microscopy images showed that the filler Mn 1- x Zn x Fe 2 O 4 nanoparticles tended to agglomerate in three dimensions. Lognormal and mass fractal models were used to fit the small-angle X-ray scattering data of the ferrofluids demonstrated that the ferrofluids formed chain-like structures with a fractal dimension of 1.12–1.67 that was constructed from primary particles with sizes of 3.6–4.1 nm. The filler, surfactant, and carrier liquid of the ferrofluids were confirmed by the functional groups of the metal oxides, tetramethylammonium hydroxide, and H 2 O, respectively. The secondary particles contributed to the saturation magnetization of the Mn 1- x Zn x Fe 2 O 4 ferrofluids. The Mn 1- x Zn x Fe 2 O 4 ferrofluids demonstrated excellent performance as magnetic sensors with high stability, especially compared with MnFe 2 O 4 ferrofluids. Furthermore, the ferrofluids exhibited excellent radar absorbing materials. The Mn 1- x Zn x Fe 2 O 4 ferrofluids prepared in this work may serve as a future platform for advancing magnetic sensors and radar absorbing materials.

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The preparation of Co-doped magnetite ferrofluids from natural sand was developed using a double-layer technique. The Co-doped magnetite nanoparticles formed a spinel phase with lattice parameters in the range of 8.355–8.422 Å and tended to agglomerate with the particle sizes of 7–12 nm. The presence of the first and second layers from oleic acid and DMSO was detected by the infrared spectrum as well as the olive oil used as a carrier liquid. The saturation magnetization of the superparamagnetic samples decreased from 24.4 to 4.8 emu/g with decreasing Co 2+ composition. The particle size and electrostatic forces between the magnetic particles and the microbes played an essential role in inhibiting microbial growth. Interestingly, the increasing Co 2+ composition enhanced the superior performance of the ferrofluids against E. coli, S. aureus, B. subtilis, and C. albicans . With additional extensive investigation, we believe that the prepared Co-doped magnetite double-layered ferrofluids from natural sand with superior antimicrobial performance can be new significant antimicrobial agents.