CHANGES IN VIBRATIONAL-MODES NEAR THE MAGNETIC PHASE-TRANSITION IN MAGNETOSTRICTIVE MATERIALS

Mössbauer studies of 57Fe in RFe2 (R = Y, Tb, Dy, Ho) at temperatures 300 to 800 K and in R(Co0.98Fe0.02)2 (R = Tb, Dy) at temperatures 85 to 300 K have been performed. The spectra yield the temperature dependence of the easy axis of magnetization, the magnetic hyperfine fields, electric field gradients and isomer shift. From the total Mössbauer absorption area the recoil free fraction was determined. The relative intensities of the various Mössbauer absorption lines yield the anisotropy in the recoil free fraction. In all compounds, except in YFe2, all measured quantities show large variations near the magnetic phase transition temperature (Tc). The variation in the recoil free fraction and in its anisotropy and the variation in the electric field gradient are all consistent, quantitatively, with anisotropic softening in the vibration modes of the Fe nucleus near Tc In RFe2 the modes in which the iron vibrates along the local C3h symmetry axes are softened less than the perpendicular modes. In RCo2 it is the other way around. It seems, that these critical softening phenomena are due to the strong magnon-phonon coupling present in these materials and absent in YFe2. This conclusion is confirmed by a theoretical analysis given in an adjacent paper. In it we use the Green function method to calculate the average of the square of vibrational amplitudes in a Debye solid with a strong magnon-phonon coupling at temperatures close to the magnetic phase transition. We show that these amplitudes may increase or decrease near Tc depending on the details of the coupled system. © 1979.