ULTRASONIC STUDY OF THE ELASTIC AND NONLINEAR ACOUSTIC PROPERTIES OF THE PARAMAGNETIC CR-5 AT-PERCENT-V SINGLE-CRYSTAL ALLOY

To assist understanding the effects of pressure on the magnetoelastic interactions in antiferromagnetic Cr alloys, the elastic and nonlinear acoustic properties of single-crystal Cr-5 at.% V alloy, which is par amagnetic at all temperatures, have been studied by the ultrasonic pul se-echo-overlap technique. Measurements of the hydrostatic-pressure de pendences of the velocities of ultrasonic modes, propagated along the [100] and [110] directions, have been used to obtain the hydrostatic-p ressure derivatives (partial derivative C-IJ/partial derivative P)(P=0 ) of the elastic-stiffness tensor components C-IJ. At 296 K, (partial derivative C-11/partial derivative P)(P=0), (partial derivative C-L/pa rtial derivative P)(P=0), (partial derivative C-44/partial derivative P)(P=0), (partial derivative C'/partial derivative P)(P=0) and (partia l derivative B-S/partial derivative P)(P=0) are 9.1 +/- 0.1, 8.7 +/- 0 .1, 1.0 +/- 0.02, 1.74 +/- 0.02 and 6.8 +/- 0.1 respectively. To estab lish the vibrational anharmonicity of the long-wavelength acoustic mod es, the results obtained for C-IJ and (partial derivative C-IJ/partial derivative P)(P=0) have been used to calculate the corresponding Grun eisen parameters. The mean long-wavelength acoustic Gruneisen paramete r gamma(el)(= 1.43) is similar to the thermal Gruneisen parameter gamm a(th)(= 1.45). Comparison of the results obtained for C-IJ, (partial d erivative C-IJ/partial derivative P)(P=0) and gamma(el) for Cr-5 at.% V with data available for antiferromagnetic Cr-0.3 at.% Ru and Cr-3.5 at.% Al alloys have established that pressure effects strongly the mag netoelastic coupling between the long-wavelength longitudinal acoustic phonons and the spin-density wave in the antiferromagnetic alloys; sh ear mode interactions are very small. The magnetic contributions to th e longitudinal elastic stiffnesses C-11 and C-L and bulk modulus B-S o f these alloys increase sharply upon application of pressure, while th ose to the shear elastic stiffnesses C-44 and C' are influenced only s lightly.