Ni-Fe oxide composite magnetic nanoparticles prepared by chemical co-precipitation method are introduced and ionic ferrofluids without surface-active agent is made by Massart method. The microstructure and grain size are analyzed through XRD, EDX, XPS and TEM, and the magnetization vs. magnetic field is measured by vibrating sample magnetometer (VSM, HH-15). The magnetizing curves have been fitted by using Langevin theory and gas-like compression model, respectively. The results indicate that there is an obvious deviation between Langevin theory and experimental data while the gas-like compression model can fit well, and that the compression parameter γ increases with the increasing ferrofluids volume fraction. The magnetization behaviors of Ni₂O₃/γ-Fe₂O₃ ferrofluids can be well illustrated by the field induced aggregation effect.

ferrofluids  /  magnetization  /  aggregations