Role of electronic correlations on the phonon modes of MnO and NiO

The possibility of magnetic-order induced phonon anisotropy in single crystals of MnO and NiO is investigated using inelastic neutron scattering. Below ${T}_{N}$ both compounds exhibit a splitting in their transverse-optical phonon spectra of approximately 10%. This behavior illustrates that, contrary to general assumption, the dynamic properties of MnO and NiO are substantially noncubic.