Elastic properties of MnF2

The elastic properties of MnF2 have been investigated in the temperature range 4.2<T<330 K using a cw transmission technique. The measured values at T=300 K of the six adiabatic elastic constants in units of 1011 dyn/cm2 are c11=10.24, c33=16.55, c44=3.185, c66=7.208, c12=7.95, and c13=7.07. The elastic Debye temperature calculated from the low-temperature elastic-constant data is D(E)=261.6 K. Using the force-constant model of Matossi for the rutile structure and the published frequencies of the Raman-active vibrational modes, theoretical values of the elastic constants are calculated and found to be in fair agreement with experiment. Each elastic constant [except c=12(c11-c12)] shows the expected linear decrease with increasing temperature in the region TD(E). On the other hand, c has not yet reached this limiting high-temperature behavior at T=330 K. The effects on the elastic properties of volume magnetostrictive coupling to the spin fluctuations near the Néel temperature (TTN) and of linear magneto-elastic coupling to the magnetic modes in the antiferromagnetic state (T<TN) are discussed. The elastic constant c44 is found to have a component at low temperatures (TTN) decreasing approximately as T-1. This striking phenomenon is discussed on the basis of two possible processes, neither of which provides a completely satisfactory explanation. © 1970 The American Physical Society.