Long-Wavelength Magnons in Heavy Rare Earths: Free or Frozen Lattice?

We show that in the long-wavelength region the magnetoelastic effect allows both the free-lattice and the frozen-lattice mode to exist in the heavy rare earths Tb and Dy. The former may be more easily detected by ferromagnetic resonance at low frequencies and low temperatures, and the latter predominates at high frequencies and high temperatures.