Raman Spectral Probe on Size-Dependent Surface Optical Phonon Modes and Magnon Properties of NiO Nanoparticles

The particle size-dependent surface optical (SO) phonon and magnon modes of NiO nanoparticles were extensively studied using room-temperature confocal Raman spectroscopy. The required nanoparticles were synthesized using a simple co-precipitation method, and the suitable particle sizes were obtained by varying the annealing temperature from 300 to 900 °C. The SO phonon modes of the prepared NiO nanoparticles were blue shifted and broadened as the nanoparticle size was reduced due to the lattice contraction and boundary relaxation, respectively. The presence of SO phonon modes was studied using the dielectric continuum (DC) model for a nearly spherical-like particle, and a violation of DC model was observed in the experimental SO mode frequency due to the presence of first order magnon background over the phonon modes. The magnon background and SO mode frequency differences were increased with the nanoparticle size. The absence of magnon and second-order phonon modes in smaller NiO nanoparticles indicates the reduced spin correlation between the next nearest neighbor Ni2+ ions in the crystal lattice and the presence of uncompensated surface spins. Moreover, the size-dependent magnetic properties were investigated using the M–H curve, and the NiO nanoparticle shows the mixed faces of antiferromagnetic (AFM) and ferromagnetic (FM) properties in the smaller size particle and correlates the Raman scattering results.