The magnetic-order induced effects in nanocrystalline NiO are investigated through the phonons and magnons observed in the Raman spectra. The key observations are (i) an anisotropy of the first-order transverse and longitudinal optical phonons, with a splitting on the order of 5 meV and (ii) a marked size and excitation wavelength variation of the two-magnon peak, which varies linearly with a redshift of $\sim 50\mathrm{c}{\mathrm{m}}^{-1}$ with a size reduction from 105 to 30 nm. The magnon-related peaks, in contrast to the phonons, are suppressed for near-resonance laser excitations. The experimental results are interpreted in terms of the exchange interactions and strong electron-electron correlations. The magnetization measurements shows a crossover to ferromagnetism with large coercivities and magnetization with decreasing size, which is shown to be due to the thermoinduced contribution.

• Received 19 May 2020
• Revised 19 June 2020
• Accepted 2 July 2020

DOI:https://doi.org/10.1103/PhysRevB.102.024423