MAGNETOELASTICITY OF DILUTE CR-IR ALLOY SINGLE-CRYSTALS

Measurements are reported of the temperature dependence of the elastic constants, ultrasonic attenuation and thermal expansion of dilute Cr- Ir alloy single crystals containing 0.07, 0.17, 0.20 and 0.25 at.% Ir. Well defined magnetic anomalies were observed in the elastic constant s and in the thermal expansion at the Neel points (T-N) and at the inc ommensurate-commensurate (I-C) spin-density-wave (SDW) transition temp eratures (T-IC). The transition at T-IC, observed in Cr + 0.20 at.% Ir and Cr + 0.25 at.% Ir, is hysteretic, of hysteresis width about 20 K. Magnetic anomalies were also observed in the elastic constants at the spin-flip transition temperature (T-sf), The attenuation coefficient (gamma) shows a large spike-like peak at T-IC (on heating) and at T-CI (on cooling) for Cr + 0.20 at.% Ir. These peaks are however smeared o ut by inhomogeneities in the crystal containing 0.25 at.% Ir. The atte nuation coefficient peaks at T-sf and T-N in Cr + 0.07 at.% Ir and in Cr + 0.17 at.% Ir. Due to very large attenuation and the high value of T-N, gamma could not be measured successfully around the Neel point f or the other crystals. The magnetovolume (Delta omega) and magnetic co ntributions (Delta B) to the bulk modulus (B) have been found to fit t he equation a + bT(2) + cT(4), predicted by theory, rather well up to temperatures (T) close to T-N. The volume derivative of T-N calculated from Delta omega and Delta B for Cr + 0.07 at.% Ir, compares fairly w ell with the value obtained from high-pressure measurements. High-pres sure measurements are not available for the other crystals for a simil ar comparison. The temperature dependence of gamma for Cr + 0.17 at.% Ir fits the power law (T/T-N - 1)(-5/4), predicted by theory for T jus t above T-N, rather well. The theory however fails in the case of Cr 0.07 at.% Ir and could not be tested for the other two crystals.