A series of ferrite nanoparticles, such as Fe₃O₄, Zn_xFe_3-xO₄，Mn_1-xZn_xFe₂O₄, and Dy_xFe_3-xO₄, were prepared by adjusting ions doping via chemical coprecipitation. The ferrite particles were coated by surfactant and separately dispersed for synthesizing various perfluoropolyether oil (PFPE-oil) based ferrofluids. The micro-structures, magnetization-temperature of nanoparticles and ferrofluids were characterized by VSM, Mössbauer Spectrum, XRD, FTIR and TEM, respectively. The results show that with increasing ion doping, the saturation magnetization of various ferrite nanoparticles increase firstly and then reduces at room temperature. The thermomagnetic coefficient k_T (k_T=ΔM/ΔT) of various nanoparticles are also great different. The ferrite nanoparticles with the maximum saturation magnetization could be obtained, and the magnetization-temperature of ferrofluids could be significantly improved by preferably matching ferrite particles,  and the ferrofluids could be used in under various tempertuare environments, such as high temperature, low temperature and variable temperature. Our results are useful for basic research and for promoting the sealing application under various temperatures.
 

Ferrite nanoparticles, Magnetism-temperature, Ferrofluids, Ions doping