CRITICAL CHANGES IN VIBRATIONAL-MODES IN A MAGNETIC DEBYE CRYSTAL WITH A MAGNON-PHONON COUPLING

Using the Green function technique we calculate the mean square of the vibrational amplitude of an ion (or equivalently the Debye Waller factor W, or the Mössbauer recoil free fraction f{hook}) in a magnetic Debye crystal near the magnetic phase transition temperature Tc. We find that the magnon-phonon coupling which leads to the phonon spectrum broadening and frequency shift leads to sharp changes in W near Tc. The frequency shift leads to an increase in W, the broadening leads to a decrease in W. These results can explain the anisotropic critical changes in W near Tc observed recently in the magnetostrictive materials RFe2 and RCo2 (R = rare earth). In addition we calculate the expected changes in the second order Doppler shift near Tc. We find that when T approaches Tc from above a positive discontinuity and decrease in absolute value of slope of the thermal shift should occur. © 1979.