Magnetoelasticity of dilute Cr-Si alloy single crystals

Abstract

Measurements are reported of the temperature dependence of the elastic constants and thermal expansion of dilute Cr-Si alloy single crystals containing 0.5, 1.2, 1.6 and 3.0 at.% Si. Well defined anomalies in all elastic constants, and in the thermal expansion, were observed at the Neel points (TN) and at the commensurate-incommensurate spin density wave transition temperatures (TIC) of the alloys. Elastic anomalies were also observed at the longitudinal-transverse incommensurate spin density wave transition temperature of the Cr+0.5 at.% Si crystal, but not in the thermal expansion thereof. The transition at the Neel point of the Cr+0.5 at.% Si crystal is continuous, that of the Cr+1.2 at.% Si and Cr+1.6 at.% Si crystals of first order, while the order of the transition in the Cr+3.0 at.% Si crystal could not be determined unambiguously. For the Cr+1.2 at.% Si and Cr+1.6 at.% Si crystals the discontinuous transitions at TN show hysteresis behaviour. In the 1.2 at.% Si crystal it is also accompanied by structure effects that were absent for 1.6 at.% Si. Hysteresis, accompanied by structure effects, was also observed at TIC for Cr+1.2 at.% Si. The structure effects are ascribed to the possibility of mixed incommensurate-commensurate spin density wave states in the dilute Cr-Si magnetic phase diagram. The magnetovolume and magnetic contributions to the bulk modulus have been found to fit the equation a+bT2+cT4, predicted by theory, rather well up to temperatures (T) close to the Neel points of Cr+0.5 at.% Si and Cr+1.6 at.% Si.