Defect induced ferromagnetic interaction in nanostructured nickel oxide with core–shell magnetic structure: the role of Ni2+ and O2− vacancies

Abstract

Nanostructured nickel oxide samples with crystallite sizes in the range 32–45 nm are synthesized through a facile chemical route using nickel chloride and ethanol amine as the starting materials. The analysis of the antioxidant activity and DC conductivity of the NiO samples confirmed the presence of both Ni²⁺ and O²⁻ vacancies. The temperature dependent magnetization studies of the samples are done using a Vibrating Sample Magnetometer in the range 20–300 K. The core–shell magnetic structure of the NiO nanoparticles with an antiferromagnetic core and a spin-glass shell is revealed from the zero field cooled and field cooled magnetization studies of the samples. The dependence of uncompensated moments on total spins contradicts Neel's models and is found to vary directly with O²⁻ vacancy concentration. The ferromagnetic response of NiO samples due to the interaction between the antiferromagnetic core and the ferromagnetic shell is evident from the magnetic hysteresis studies in the temperature range 20–300 K. The ferromagnetic response is traced to the concentration of O²⁻ vacancies, which act as donor impurities and mediate the alignment of magnetic moments associated with Ni²⁺ vacancies. The decrease of ferromagnetic contribution upon annealing is explained by the decrease in the concentration of O²⁻ vacancies which caused a reduction in the number of magnetic polarons and hence the effective magnetization.