Ultrasonic study of the temperature and pressure dependences of the elastic propertiesof fcc Co-Mn alloy single crystals

To search for ramifications of magnetovolume effects in the magnetoelasticity of Co-Mn alloys, the elastic and nonlinear acoustic properties of single crystals with compositions ${\mathrm{Co}}_{75}$${\mathrm{Mn}}_{25}$, ${\mathrm{Co}}_{68}$${\mathrm{Mn}}_{32}$, ${\mathrm{Co}}_{52}$${\mathrm{Mn}}_{48}$, and ${\mathrm{Co}}_{46}$${\mathrm{Mn}}_{54}$, which have different magnetic configurations, have been determined ultrasonically. Pulse-echo-overlap measurements of the velocities of ultrasonic modes have been used to determine the three independent elastic-stiffness tensor components ${\mathrm{C}}_{\mathrm{IJ}}$ and the adiabatic bulk modulus ${\mathrm{B}}^{\mathrm{S}}$, as a function of temperature in the range from 4.2 up to about 800 K, and the pressure derivatives (\ensuremath{\partial}${\mathrm{C}}_{\mathrm{IJ}}$/\ensuremath{\partial}P${)}_{\mathrm{P}\mathrm{=}0}$ and (\ensuremath{\partial}${\mathrm{B}}^{\mathrm{S}}$/\ensuremath{\partial}P${)}_{\mathrm{P}\mathrm{=}0}$ at room temperature. These measurements of the elastic and nonlinear acoustic behavior of fcc Co-Mn alloys assist understanding of the structural and magnetic phase stability in this alloy system. Two of the alloys studied (${\mathrm{Co}}_{52}$${\mathrm{Mn}}_{48}$ and ${\mathrm{Co}}_{46}$${\mathrm{Mn}}_{54}$) are antiferromagnetic at room temperature; their elastic-stiffness tensor components and bulk moduli are affected by the onset of antiferromagnetic ordering and strong magnetoelastic coupling in the antiferromagnetic phase. The ${\mathrm{Co}}_{68}$${\mathrm{Mn}}_{32}$ alloy, with composition centered between the superantiferromagnetic and superparamagnetic regions in the phase diagram, does not undergo any transition. The temperature dependences of the elastic stiffnesses associated with the three independent ultrasonic modes propagated in the austenite and martensite phases of the ferromagnetic ${\mathrm{Co}}_{75}$${\mathrm{Mn}}_{25}$ alloy show that the \ensuremath{\epsilon}\ensuremath{\rightarrow}\ensuremath{\gamma} transition is strongly first order; the absence of long-wavelength acoustic-mode softening shows that its driving mechanism is not elastic in origin. Application of pressure does not induce acoustic mode softening for any of these alloys: the pressure derivatives (\ensuremath{\partial}${\mathrm{C}}_{\mathrm{IJ}}$/\ensuremath{\partial}P${)}_{\mathrm{P}\mathrm{=}0}$ and (\ensuremath{\partial}${\mathrm{B}}^{\mathrm{S}}$/\ensuremath{\partial}P${)}_{\mathrm{P}\mathrm{=}0}$ are positive. The long-wavelength longitudinal-acoustic-mode Gr\"uneisen parameters of antiferromagnetic ${\mathrm{Co}}_{46}$${\mathrm{Mn}}_{54}$ show the unusual feature of being smaller than those for the shear waves: magnetovolume contributions to vibrational anharmonicity are stronger for longitudinal than for shear waves. The longitudinal-mode gamma ${\ensuremath{\gamma}}_{\mathrm{L}[110]}$ of this ${\mathrm{Co}}_{46}$${\mathrm{Mn}}_{54}$ alloy passes through a maximum value of 7.0 at 388 K: the vibrational anharmonicity of this mode is markedly enhanced in the vicinity of the N\'eel temperature.