A remarkable hardening ($\sim 30{\mathrm{cm}}^{-1}$) of the normal mode of vibration associated with the symmetric stretching of the oxygen octahedra for the ${\mathrm{Ba}}_2{\mathrm{FeReO}}_6$ and ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$ double perovskites is observed below the corresponding magnetic ordering temperatures. The very large magnitude of this effect and its absence for the antisymmetric stretching mode provide evidence against a conventional spin-phonon coupling mechanism. Our observations are consistent with a collective excitation formed by the combination of the vibrational mode with oscillations of Fe or Cr $3d$ and Re $5d$ occupations and spin magnitudes.

• Received 21 December 2011

DOI:https://doi.org/10.1103/PhysRevLett.108.177202