High-pressure ultrasonic study of monocrystalline Cr in the antiferromagnetic and paramagnetic phases

Abstract

Abstract An experimental ultrasonic study has been made of the elastic and nonlinear acoustic behaviour of single-crystal Cr in both the antiferromagnetic (AFM) spin-density wave (SDW) and paramagnetic (PM) phases. The velocities of the ultrasonic waves propagated along the [100] and [110] directions have been measured as a function of hydrostatic pressure up to 0.2 GPa at fixed temperatures in the range from room temperature up to 400 K, which covers the Néel temperature (TN). The results provide complete sets of the elastic-stiffness tensor components and their pressure derivatives as a function of temperature. The longitudinal acoustic-mode Griineisen parameters are positive at room temperature, but they become negative in the range from about 300 K to TN indicating longitudinal-mode softening due to a strong magnetoelastic interaction between the SDW and long-wavelength longitudinal acoustic phonons: the mode softening is much enhanced just below the Nkel point. The Grüneisen gamma γs{N[110]U[110]} corresponding to the shear C′ mode increases markedly with increasing temperature up towards T N. The mode gamma γs{N[100]U(001)} of the shear C44 mode has a small positive value and is essentially temperature independent throughout the AFM and PM phases. The vibrational anharmonicities of the longitudinal C 11 and CL and the shear C′ modes are greatly influenced by the magnetoelastic interaction between the SDW and long-wavelength acoustic phonons. The longitudinal acoustic-mode Grüneisen parameters for Cr in the PM phase above the Néel point have large positive values, which decrease substantially with increasing temperature; those for both shear modes are also positive and have values appropriate for a paramagnetic material in marked contrast to the soft mode behaviour in the antiferromagnetic spin-density wave state.