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

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 t he ultrasonic waves propagated along the [100] and [110] directions have be en measured as a function of hydrostatic pressure up to 0.2 GPa at fixed te mperatures in the range from room temperature up to 400 K, which covers the Neel temperature (T-N) The results provide complete sets of the elastic-st iffness tensor components and their pressure derivatives as a function of t emperature. The longitudinal acoustic-mode Gruneisen parameters are positiv e at room temperature, but they become negative in the range from about 300 K to T-N indicating longitudinal-mode softening due to a strong magnetoela stic interaction between the SDW and long-wavelength longitudinal acoustic phonons: the mode softening is much enhanced just below the Neel point. The Gruneisen gamma gamma(S){N[110]U[1 (1) over bar 0]} corresponding to the s hear C' mode increases markedly with increasing temperature up towards T-N The mode gamma gamma(S){N[100]U(001)} of the shear C-44, mode has a small p ositive value and is essentially temperature independent throughout the AFM and PM phases. The vibrational anharmonicities of the longitudinal C-11 an d C-L and the shear C' modes are greatly influenced by the magnetoelastic i nteraction between the SDW and long-wavelength acoustic phonons. The longit udinal acoustic-mode Gruneisen parameters for Cr in the PM phase above the Neel point have large positive values, which decrease substantially with in creasing temperature; those for both shear modes are also positive and have values appropriate for a paramagnetic material in marked contrast to the s oft mode behaviour in the antiferromagnetic spin-density wave state.