  A remarkable hardening (~ 30 cm-1) of the normal mode of vibration associated
with the symmetric stretching of the oxygen octahedra for the Ba2FeReO6 and
Sr2CrReO6 double perovskites is observed below the corresponding magnetic
ordering temperatures. The very large magnitude of this effect and its absence
for the anti-symmetric 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 local Fe or Cr 3d and Re 5d occupations and spin magnitudes.
