Elastic properties of single-crystal NiF2

Abstract

The elastic properties of Ni${\mathrm{F}}_{2}$ were studied at $65<T<300$ K using the ultrasonic pulse superposition method. At 300 K, the measured adiabatic elastic stiffness moduli, in units of ${10}^{11}$ dyn/${\mathrm{cm}}^{2}$, are ${C}_{11}=14.50$, ${C}_{12}=11.04$, ${C}_{13}=9.09$, ${C}_{33}=22.08$, ${C}_{44}=4.65$, and ${C}_{66}=9.94$. The measured moduli are compared with the theoretical values calculated from Pandey's model by using published Raman frequencies. Near 300 K, each $\frac{d{C}_{\mathrm{ij}}}{\mathrm{dT}}$ has a negative value. But $\frac{d{C}_{S}}{\mathrm{dT}}$, where ${C}_{S}=\frac{1}{2}({C}_{11}\ensuremath{-}{C}_{12})$, is positive between 300 K and the N\'eel temperature ${T}_{N}$. Other anomalous features are the following. There is a long precursor above ${T}_{N}$ in ${C}_{33}$ and ${C}_{44}$, ${C}_{33}$ exhibits critical exponents near ${T}_{N}$, whereas the values of ${C}_{11}$, ${C}_{66}$, ${C}_{S}$, etc. show sharp "cut-off" anomalies below ${T}_{N}$. These can be eliminated by a 4-kOe magnetic field along suitable directions. These properties of the elastic moduli are discussed in terms of a lattice instability or volume magnetostricitve coupling mechanism above ${T}_{N}$, linear magnetoelastic couplings, shear instability, and weak ferromagnetic domain effects, respectively. Possible existence of twin domains is proposed.