Relationship of the elastic and nonlinear acoustic properties of the antiferromagnetic fccFe60Mn40single-crystal alloy to Invar behavior

The elastic and nonlinear acoustic properties of an antiferromagnetic fcc iron-manganese alloy single crystal with composition 60 at. % Fe have been studied with the ultrasonic pulse-echo-overlap technique. Velocity measurements of the three ultrasonic modes that can be propagated along the [110] direction have been made between 4.2 and 750 K to obtain all 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. At 293 K the elastic stiffnesses are ${\mathit{C}}_{11}$=170 GPa, ${\mathit{C}}_{12}$=98 GPa, and ${\mathit{C}}_{44}$=142 GPa; hence ${\mathit{C}}_{11}$ and ${\mathit{B}}^{\mathit{S}}$=123 GPa are small, conforming with recognized trends for 3d transition-metal alloys. The Néel temperature ${\mathit{T}}_{\mathit{N}}$, assessed from electrical-resistance measurements and the steplike decrease in the temperature dependence of the shear modulus (${\mathit{C}}_{11}$-${\mathit{C}}_{12}$)/2, is 467 K. The contributions of antiferromagnetic ordering to ${\mathit{C}}_{\mathit{L}}$[=(${\mathit{C}}_{11}$+${\mathit{C}}_{12}$+2${\mathit{C}}_{44}$)/2], (${\mathit{C}}_{11}$-${\mathit{C}}_{12}$)/2, and ${\mathit{B}}^{\mathit{S}}$ are negative for all temperatures, while ${\mathit{C}}_{44}$ is stiffened slightly. Measurements of the hydrostatic pressure dependences of the velocities of ultrasonic modes propagated along the [110] direction have been used to obtain the hydrostatic pressure derivatives (∂${\mathit{C}}_{\mathit{I}\mathit{J}}$/∂P${)}_{\mathit{P}=0\mathrm{}}$ of the elastic-stiffness-tensor components as a function of temperature in the antiferromagnetic Invar state.