ACOUSTIC-MODE VIBRATIONAL ANHARMONICITY IN CR-SI ALLOY SINGLE-CRYSTALS

Measurements are reported of the hydrostatic-pressure dependences of t he elastic constants of Cr + 0.5 at. % Si and Cr + 1.6 at. % Si alloy single crystals through the Neel transition. In the former crystal the Neel transition is a second-order incommensurate (I) spin-density-wav e (SDW)-to-paramagnetic (P) transition, while it is a first-order comm ensurate (C) SDW-to-P transition in the latter crystal. The pressure d erivatives of the elastic constants at different constant temperatures are used to calculate the acoustic-mode Gruneisen parameters, which q uantify the vibrational anharmonicity, as a function of temperature th rough the Neel transition of each crystal. For the Cr + 0.5 at. % Si c rystal the mean long-wavelength acoustic mode Gruneisen parameter, <(g amma)over bar>(el)approximate to-5, jumps by Delta<(gamma)over bar>(el )=30, from <(gamma)over bar>(el)approximate to-5 to <(gamma)over bar>( el)=+25, in going through the second-order ISDW-P Neel transition, and then decreases to a value of <(gamma)over bar>(el)approximate to+5 at temperatures well into the paramagnetic phase. This jump is much larg er, Delta<(gamma)over bar>(el) = 170, from <(gamma)over bar>(el) = -50 to <(gamma)over bar>(el) = + 150, in going through the first-order CS DW-P Neel transition of Cr + 1.6 at. % Si, after which <(gamma)over ba r>(el) decreases to <(gamma)over bar>(el)approximate to 0 at temperatu res deep into the paramagnetic phase. The results show very strong cou pling of the SDW to the long-wavelength longitudinal-acoustic phonons in both crystals. Coupling to the long-wavelength shear acoustic phono ns is relatively weak. In both crystals there is also strong coupling of the longitudinal phonons with the spin fluctuations above T-N. The present measurements are compared with anharmonic effects observed thr ough the second-order CSDW-P Neel transitions of Cr + 0.3 at. % Ru and Cr + 3.5 at. % Al alloy single crystals. [S0163-1829(97)00342-1].