Third-order elastic constants, vibrational anharmonicity, and the Invar behavior of the Fe72Pt28 alloy.

Abstract

In the ferromagnetic state the alloy ${\mathrm{Fe}}_{72}$${\mathrm{Pt}}_{28}$ shows strong Invar effects governed by magnetoelastic interactions, including a negative hydrostatic pressure dependence of the bulk modulus: it becomes easier to squeeze under pressure. This remarkable behavior has prompted study of its nonlinear acoustic properties by determining its third-order elastic stiffness tensor components (TOEC) from measurements of the influence of uniaxial stress on the velocities of ultrasonic waves between 230 and 370 K, covering the range in which ${\mathrm{Fe}}_{72}$${\mathrm{Pt}}_{28}$ has a negative thermal expansion, a property central to the Invar behavior. To obtain complete sets of the TOEC from 230 K up to the Curie point (367 K), the results have been combined with data for the hydrostatic pressure dependences of ultrasonic wave velocities. The TOEC quantify the first-order anharmonic terms in the interatomic potential and hence the long-wavelength acoustic phonon anharmonicities which are central to the Invar properties of this alloy. The tensor components ${\mathit{C}}_{111}$, ${\mathit{C}}_{112}$, and ${\mathit{C}}_{123}$ are anomalously positive, in accord with the negative values previously determined for the hydrostatic pressure derivatives (\ensuremath{\partial}${\mathit{C}}_{11}$/\ensuremath{\partial}P${)}_{\mathit{T},}$${\mathit{P}}_{=0}$ and (\ensuremath{\partial}${\mathit{B}}^{\mathit{S}}$/\ensuremath{\partial}P${)}_{\mathit{T},}$${\mathit{P}}_{=0}$. Thus, the large stress-induced longitudinal-acoustic-mode softening in the ferromagnetic phase is confirmed, reinforcing the suggestion that the vibrational anharmonicities of the longitudinal acoustic modes, which have large negative mode Gr\"uneisen parameters, play an important part in causing the thermal expansion of ${\mathrm{Fe}}_{72}$${\mathrm{Pt}}_{28}$ to be negative.