Spin–phonon coupling in NiO nanoparticle

Nickel oxide (NiO) has a cubic rock salt crystal structure at room temperature. Raman scattering of the transverse optical (TO) and longitudinal optical (LO) phonon in NiO is Raman inactive. Thus, it is difficult to employ the Raman scattering technique to study the lattice vibration dynamics and spin–phonon coupling in NiO. In this work, crystalline stoichiometry of NiO nanoparticles with different nanocrystalline sizes was tuned to make the Raman scattering selection rules dramatically relaxed. Well-defined Raman scattering peaks of the two zone-boundary folded modes TO(Δ) and LO(Δ) were observed. These two modes are situated at the midpoint along the Γ–Δ–X direction in the Brillouin zone. The Raman scattering of these two modes are induced by magnetostriction and nonstoichiometric Ni–O stretching, respectively. The well-defined Raman peaks of TO(Δ) and LO(Δ) allow us to study the spin–phonon coupling effect in NiO. It is found that spin–phonon coupling is responsible for the Raman scattering anomalies, namely, the relatively large Raman shift hardening and peak width narrowing below the Néel temperature for LO and its overtone 2LO phonons.