The aim of this work was to investigate the size-related magnetism for the single-domain Co Fe_2O_4 nano-particles synthesized using the hydrothermal method. The effects of the reaction temperature and the reaction time on the lattice constants, particle morphologies, and the room-temperature magnetic properties were studied from the X-ray diffraction, the transmission electron microscope, and the vibrating-sample magnetometer. The experimental results show that the samples are composed of Co Fe_2O_4 nano-particles with an average crystallite size(D) smaller than 40 nm, and the magnetic properties of the samples can be manipulated in a wide range: the MS values vary from smaller than 50 emu/g to close to 80 emu/g, and the HC values are between about 200 Oe and 2000 Oe. Additionally, the relationship between HC and 1/D^(3/2) satisfi es linearship, showing the characteristic of single-domain structure. These results indicate that the single-domain Co Fe_2O_4nano-particles with size controlled between the superparamagnetic critical size and single-domain critical size can be easily prepared using this hydrothermal method.
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