Ultrasonic Propagation in RbMnF3. I. Elastic Properties

The elastic properties of RbMn${\mathrm{F}}_3$ have been investigated over the temperature range 4.2-300°K with a continuous-wave transmission technique. The measured values of the three adiabatic elastic constants ${\mathrm{C}}_{11}$, $C_{44}$, and $C^{\prime }=\frac{1}{2}(C_{11}-C_{12})$ at 300°K in units of ${10}^{12}$ erg/${\mathrm{cm}}^3$ are, respectively, 1.174±0.002, 0.3193±0.0008, and 0.3763±0.0008. The value of the Debye temperature calculated from the low-temperature elastic-constant data is ${\Theta }_D\left(\mathrm{elastic}\right)=386\mathrm{}\pm 1.5$ °K. For $T>\mathrm{}\frac{1}{2}{\Theta }_D$, the three elastic constants decrease linearly with temperature. Attempts at applying the present results to a three-force-constant theory of the cubic perovskite structure failed because the secular determinant for the principal oscillation frequencies results in an unstable solution. It is found that only the longitudinal elastic modes are affected by the onset of long-range order at the Néel temperature $T_N$. This is consistent with Pytte and Bennett's recent theory if the dominant spin-phonon interaction is taken to be the volume magnetostriction. In the antiferromagnetic state, the elastic constants are found to be sharply dependent on applied magnetic fields. A model is proposed which explains the magnetic-field dependence and temperature dependence of the elastic constants in this region. The basis of the model is that the coupling of the ultrasonic waves to the antiferromagnetic resonance modes in the low-frequency limit is determined by the sublattice magnetization orientation, which in turn is magnetic-field-dependent. This model will be described in more detail in a second paper.