Ultrasonic study of the temperature and pressure dependences of the elastic properties of a single-crystal Heusler structure Cu41Mn20Al39 alloy

Pulse-echo-overlap measurements of ultrasonic wave velocity have been used to determine the elastic-stiffness tensor components C-u and the adiabatic bulk modulus, BS, of a ferromagnetic Heusler structure Cu41Mn20Al39 at % al loy single crystal as functions of temperature in the range 14-300 K and hy drostatic pressure up to 0.2 GPa at room temperature. At 295 K the elastic stiffnesses are: C-11 = 133 GPa, C-44 = 92 GPa, C' (= (C-11 - C-12)/2) = 17 GPa, C-12 = 99 GPa, C-L(= C-11 + C-44 - C') = 205 GPa, and B-S (= C-11 - 4 C'/3) = 106 GPa. Cu41Mn20Al39 is a comparatively soft material elastically because its elastic properties are influenced strongly by magnetoelastic ef fects. The results of measurements of the effects of hydrostatic pressure o n the ultrasonic wave velocity have been used to obtain the hydrostatic-pre ssure derivatives of the elastic-stiffness tenser components. At 295 K (par tial derivative C-11/partial derivative P)(P = 0), (partial derivative C-44 /partial derivative P)(P=0), (partial derivative C'/partial derivative P)(P =0), (partial derivative C-12/partial derivative P)(P=0), (partial derivati ve C-L/partial derivative P)(P=0), and (partial derivative B-S/partial deri vative P)(P=0) are 5.0 +/- 0.1, 3.0 +/- 0.1, 1.0 +/- 0.2, 3.0 +/- 0.3, 7.7 +/- 0.4 and 3.7 +/- 0.4, respectively. Application of hydrostatic pressure does not induce acoustic-mode softening: the pressure derivatives (partial derivative C-IJ/partial derivative P)(P=0) and (partial derivative B-S/part ial derivative P)(p=0) and the acoustic-mode Gruneisen parameters are posit ive. An interesting feature of the non-linear acoustic behaviour of this al loy is that the value obtained for (partial derivative C'/partial derivativ e P)(P=0), associated with the softer shear mode propagated a long the [110 ] direction and polarized along the [110] direction, is small in comparison with those of the other shear and longitudinal modes. The Gruneisen parame ter of this mode, and hence its vibrational anharmonicity, is much larger t han those of the other long-wavelength acoustic phonon modes. (C) 1998 Kluw er Academic Publishers.