Phase transition from the ferromagnetic to superparamagnetic with loop shift about 5 nm nickel particles

Adopting the microwave-assisted (MW) polyol method, we have successfully prepared monodispersed nickel particles with size ranging from 5 nm to 10 nm. It is confirmed by HRTEM that the prepared nickel particles with plane gap of 0.2033 nm have F C C crystal structure and are in good agreement with the reported value of 0.2034 nm for the reflecting plane (100) of the FCC structure [JCPDS: 4-850]. From magnetic hysteresis loops measured from 4.2 to 300 K, we have found the loop shift in both ZFC and FC curves at different temperatures. The temperature of the phase transition from the ferromagnetic to the superparamagnetic T_pt = 100 K. The large shift observed may be suggested to be the existence of the disfigurement and small cavities of nanoparticles, which cause surface spin disorder or canting above T_pt.