Study of NiO nanoparticles, structural and magnetic characteristics

NiO nanoparticles with different sizes were synthesized at different temperatures from 300 to $700{}^{\circ }$C to the study of behavior related to size. The nanoparticles show dimensions in the range 5.07–68.29 nm, which were determined by transmission electron microscopy images. The nanoparticles present an irregular morphology from 300 to $600{}^{\circ }$C, while the observed structure is a truncated octahedron with FCC structure only for samples at $700{}^{\circ }$C. X-ray diffraction measurements and Rietveld analysis verify this crystal structure, the crystal size, and the lattice parameters. Raman spectroscopy of the nanoparticles shows the normal modes of proposed truncated octahedrons related to longitudinal optical, transverse optical phonons, and a combination of both. Optical properties were measured by UV–visible spectroscopy to analyze the variation of the band gap in function of the size. In addition, magnetic measurements, magnetization versus temperature, and magnetization versus magnetic field present ferromagnetic behavior. M–H hysteresis curves show the coercive field with anisotropic characteristics that we related to the competition between two magnetic orders coexisting in the samples.