Effects of temperature and hydrostatic pressure on the magnetoelasticity of a commensurate spin-density-wave antiferromagnetic Cr-3.5 at. % Al alloy single crystal.

Abstract

Measurements are reported of the temperature and hydrostatic-pressure dependences of the elastic constants and thermal expansion of a Cr--3.5 at. % Al alloy single crystal through the N\'eel transition. The magnetic contributions to the volume strain and bulk modulus have been found to fit the equation a+${\mathit{bT}}^{2}$+${\mathit{cT}}^{4}$, predicted by theory, rather well up to temperatures close to the N\'eel temperature (${\mathit{T}}_{\mathit{N}}$). The pressure derivatives of the elastic constants at different constant temperatures are used to calculate the acoustic-mode Gr\"uneisen parameters as a function of temperature through the N\'eel point. The mean long-wavelength acoustic Gr\"uneisen parameter, \ensuremath{\gamma}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}^{\mathit{e}\mathit{l}}$\ensuremath{\approxeq}-5, is negative in the commensurate spin-density-wave phase (T${\mathit{T}}_{\mathit{N}}$), it increases sharply to a positive value of \ensuremath{\gamma}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}^{\mathit{e}\mathit{l}}$\ensuremath{\approxeq}+35 just above ${\mathit{T}}_{\mathit{N}}$ and then decreases to a value \ensuremath{\gamma}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}^{\mathit{e}\mathit{l}}$\ensuremath{\approxeq}+2 at temperatures well into the paramagnetic phase. The spin density wave couples mainly with the longitudinal-mode acoustic phonons in the long-wavelength limit. Shear-mode anharmonicity is only weakly affected by the N\'eel transition.