High-pressure ultrasonic properties of spin-density-wave Cr-Re alloy single crystals

The pressure dependence of the elastic constants (c(ij)) of Cr-Re alloy single crystals, containing 0.3 and 0.5 at.% Re, are reported. For 0.3 at.% Re the concentration (c) is below the triple point concentration (c) on the (c-T) magnetic phase diagram. This crystal therefore remains in the incommensurate (I) spin-density-wave (SDW) antiferromagnetic phase at all temperatures below the ISDW-paramagnetic (P) Neel transition temperature (T-N). The pressure derivatives of the elastic constants, dc(ij)/dp, were measured for the Cr+0.3 at.% Re crystal as a function of temperature through T, The Cr+0.5 at.% Re crystal has c > c(t) and exhibits an incommensurate-commensurate (I-C) SDW phase transition at a temperature T-IC < T-N on heating. In the case of this crystal dc(ij)/dp was studied at temperatures close to and above T-IC. The acoustic mode Gruneisen parameters (gamma(n)), which quantify the lattice vibrational anharmonicity, were calculated as a function of temperature from the dc(ij)/dp data for each crystal. These calculations indicate much larger coupling of the SDW to the long-wavelength longitudinal phonons than to the shear modes for both Cr-Re crystals. Longitudinal mode softening under applied pressure, due to strong magnetoelastic interactions between the SDW and the longitudinal acoustic phonons, in Cr+0.3 at.% Re at T < T-N gives nearly discontinuously way to extremely large mode stiffening as the crystal is heated through T-N. gamma(n) in the CSDW and pressure-induced ISDW phases of the Cr+0.5 at.% Re crystal shows unusual behaviour with temperature. This is particularly so for the shear mode Gruneisen parameters which are relatively large and negative just above T-IC, implying significant shear mode softening under applied pressure. gamma(n) (shear) then increases on further increasing the temperature of this crystal. In comparison, gamma(n) (shear) for both the CSDW and pressure-induced ISDW phases of other Cr alloys, with c > c(i), for instance Cr-Ru and Cr-Ir alloys, is near zero, slightly positive, and remains constant with temperature. The measurements on the Cr+0.5 at.% Re crystal suggest the existence of a new phase line, separating the ISDW phase present at atmospheric pressure from that induced from the CSDW phase by applying high pressure, on the pressure-temperature magnetic phase diagrams of dilute Cr alloys with c > c(i). More experimentation, particularly high pressure neutron diffraction studies, are needed to verify this point. (C) 2002 Elsevier Science B.V. All rights reserved.