Investigation of Fe₃O₄Aqueous Ferrofluids Before and After Freezing

The magnetization and magnetoviscous properties of Fe3O4 aqueous ferrofluids before and after zero-field freezing are investigated. The results show that the magnetization of ferrofluids is less than that of Fe3O4 nanoparticles. The saturation magnetization of ferrofluids after freezing is greater than that of ferrofluids before freezing due to the breakage of aggregations in ferrofluids. Our experimental data agree well with the model of gas-like compression. The viscosity-magnetic field hysteretic curves of condensed ferrofluids after freezing are initially studied. The results demonstrate a special curve with increasing and decreasing fields. The viscosity changes under magnetic fields are related to the magnetically induced structures.