In this paper, nickel ferrite nanostructures were fabricated via a simple and green hydrothermal method using lemon juice. For the synthesis of NiFe2O4 nanoparticles, the materials were placed in the autoclave reactor for 2 and 12 hours. The morphology of particles was studied by scanning electron microscope (SEM). Although the NiFe2O4 nanoparticles after 2 h heating are agglomerated increasing the heating time to 12 h leads to the formation of sphere shape nanosize particles. The crystal structure of nickel ferrite nanoparticles was investigated via an X-ray diffraction pattern and the crystallite size of particles was calculated using the Debye Scherrer formula. The magnetic hysteresis loop of the synthesized ferrite was studied by a vibration sample magnetometer (VSM). The magnetic parameters of remanence Mr, saturation magnetization Ms, and coercivity Hc, increase as the synthesis time increases. The optical properties of NiFe2O4 were investigated by ultraviolet-visible (UV-vis) spectra. The results show that the particle band gaps decrease with increasing the synthesis time
