  The physics of mutual interaction of phonon quasiparticles with electronic
spin degrees of freedom, leading to unusual transport phenomena of spin and
heat, has been a subject of continuing interests for decades. Despite its
pivotal role in transport processes, the effect of spin-phonon coupling on the
phonon system, especially acoustic phonon properties, has so far been elusive.
By means of inelastic neutron scattering and first-principles calculations,
anomalous scattering spectral intensity from acoustic phonons was identified in
the exemplary collinear antiferromagnetic nickel (II) oxide, unveiling strong
spin-lattice correlations that renormalize the polarization of acoustic phonon.
In particular, a clear magnetic scattering signature of the measured neutron
scattering intensity from acoustic phonons is demonstrated by its momentum
transfer and temperature dependences. The anomalous scattering intensity is
successfully modeled with a modified magneto-vibrational scattering cross
section, suggesting the presence of spin precession driven by phonon. The
renormalization of phonon eigenvector is indicated by the observed
"geometry-forbidden" neutron scattering intensity from transverse acoustic
phonon. Importantly, the eigenvector renormalization cannot be explained by
magnetostriction but instead, it could result from the coupling between phonon
and local magnetization of ions.
