Ultrasonic study of the temperature and pressure dependences of the elastic properties of a Mn78Pt22-alloy single crystal.

Abstract

The elastic and nonlinear acoustic properties of monocrystalline ${\mathrm{Mn}}_{78}$${\mathrm{Pt}}_{22}$ alloy have been studied by the ultrasonic pulse-echo-overlap technique. To obtain the three independent elastic-stiffness tensor components ${\mathit{C}}_{\mathit{I}\mathit{J}}$ and the adiabatic bulk modulus ${\mathit{B}}^{\mathit{S}}$ as a function of temperature in the antiferromagnetic D phase, velocity measurements of the three ultrasonic modes, which can be propagated along the [110] direction, have been made between 10 K and room temperature; for the longitudinal mode measurements have been made up to 550 K. At 293 K the elastic stiffnesses are ${\mathit{C}}_{11}$=94.2 GPa, ${\mathit{C}}_{12}$=35.8 GPa, and ${\mathit{C}}_{44}$=84.7 GPa; hence ${\mathit{C}}_{11}$ and in turn ${\mathit{B}}^{\mathit{S}}$=55.2 GPa are comparatively small: the elastic moduli conform with trends with the e/a ratio previously recognized for 3d transition-metal antiferromagnetic Invar alloys. In the temperature range 10--225 K the C' mode exhibits shear-lattice softening that parallels mode softening found in the longitudinal second-order elastic-stiffness tensor component ${\mathit{C}}_{11}$. There is a sharp dip at the N\'eel temperature (515 K) in the velocity of the longitudinal ultrasonic wave propagated along the [110] direction.Measurements of the hydrostatic-pressure dependences of the velocities of ultrasonic modes with a [110] propagation vector have been used to obtain the hydrostatic pressure derivatives (\ensuremath{\partial}${\mathit{C}}_{\mathit{I}\mathit{J}}$/\ensuremath{\partial}P${)}_{\mathit{P}=0}$ of the elastic-stiffness tensor components. At 293 K (\ensuremath{\partial}${\mathit{C}}_{11}$/\ensuremath{\partial}P${)}_{\mathit{P}=0}$, (\ensuremath{\partial}${\mathit{C}}_{12}$/\ensuremath{\partial}P${)}_{\mathit{P}=0}$, (\ensuremath{\partial}${\mathit{C}}_{44}$/\ensuremath{\partial}P${)}_{\mathit{P}=0}$, and (\ensuremath{\partial}${\mathit{B}}^{\mathit{S}}$/\ensuremath{\partial}P${)}_{\mathit{P}=0}$ are 6.2\ifmmode\pm\else\textpm\fi{}0.1, 4.2\ifmmode\pm\else\textpm\fi{}0.3, 4.3\ifmmode\pm\else\textpm\fi{}0.2, and 4.8\ifmmode\pm\else\textpm\fi{}0.2, respectively. To establish the vibrational anharmonicity of the long-wavelength acoustic modes, the results obtained for ${\mathit{C}}_{\mathit{I}\mathit{J}}$ and (\ensuremath{\partial}${\mathit{C}}_{\mathit{I}\mathit{J}}$/\ensuremath{\partial}P${)}_{\mathit{P}=0}$ have been used to calculate the corresponding Gr\"uneisen parameters. In general the elastic and nonlinear acoustic properties show a number of features that resemble those of Invar materials.