Magnetic-order induced effects in nanocrystalline NiO probed by Raman spectroscopy

Abstract

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 $\ensuremath{\sim}50\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ 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.