Phase transition from the ferromagnetic to superparamagnetic with a loop shift in 5-nm nickel particles

Monodispersed 5-nm nickel particles have been successfully prepared by the microwave-assisted polyol method and confirmed by TEM and HRTEM analysis. Magnetic hysteresis loops, measured at different temperatures from 4.2 to 300 K after the sample was cooled from 300 K with a 20 kOe field, have found to occur at the temperature of a phase transition, T_pt = 100 K, i.e., the temperature at which the ferromagnetic to superparamagnetic transition was observed. It is unexpected for magnetic nanoparticles that the loop shift both zero-field cooling and field cooling occurs above T_pt and it is perfectly symmetrical below the temperature. The large shift observed may suggest the existence of the disfigurement and small cavities of nanoparticles, which cause surface spin disorder or canting above T_pt.