| In this paper, 10-20nm Mn-Zn ferrite nanoparticles were synthesized by hydrothermal method. The effects of pH value, reaction temperature, reaction time and additive on the formation of nanosized Mn-Zn ferrite crystallites were discussed. The products and magnetic properties were characterized by X-ray Diffraction , Transmission Electron Microscopy and Vibrating Sample Magnetometer etc. The changing rule of Mn-Zn ferrite's curie temperature and magnetization intensity related to composition were measured and analyzed. Water-based Mn-Zn ferrite magnetic fluid was prepared by choosing appropriate surfactant and the influence of the crafts of the preparation on the magnetic fluid stability were discussed. The particle distribution of the Mn-Zn ferrite magnetic fluid was detected. Saturation magnetization of the Mn-Zn ferrite magnetic fluid was measured and the magnetic moments of the nanoparticles were calculated. The principle of capillary tube viscometer was analyzed and the measuring apparatus was built to measure the viscosity of the Mn-Zn ferrite magnetic fluid. The effects of the volume fraction of the suspended magnetic particles, mass concentration of surfactants, temperature and the external magnetic field strength as well as the orientation on the viscosity of the magnetic fluid were analyzed.
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