Spin-density-wave effects on the vibrational anharmonicity of acoustic phonons in Cr-Ir alloy single crystals

Measurements are reported of the dependences on hydrostatic pressure (p) of the elastic constants (c(ij)) of Cr-Ir alloy single crystals containing 0.07 at.% and 0.20 at.% Ir. For the former concentration the alloy exhibits a transition from an incommensurate (I) spin-density-wave (SDW) to a paramagnetic state at the Neel temperature (T-N). dc(ij)/dp was studied as a function of temperature through T-N. The concentration of the Cr0.998Ir0.002 crystal is larger than the triple-point concentration on the Cr-lr magnetic phase diagram and for this crystal there appears a phase transition from an ISDW to a commensurate (C) SDW at T-IC. dc(ij)/dp was measured for this crystal as a function of temperature through T-IC. The acoustic mode Gruneisen parameters were calculated for the two crystals as a function of temperature through T-N (for Cr0.9993Ir0.0007) and through T-IC (for Cr0.998Ir0.002). Large negative Gruneisen parameters are observed in the ISDW and CSDW phases of Cr0.998Ir0.002. The results indicate stronger magnetoelastic interactions in the CSDW than in the ISDW phase of this crystal. It is shown that the strong magnetoelastic coupling in the ISDW and CSDW antiferromagnetic phases of the Cr-Ir system takes mainly place through volume effects.