Magnetoelastic coupling in Fe_(100-x)Ge_x single crystals with 4 < x < 18

摘要

In this paper we examine the elastic (c' and c_(44)) and magnetostrictive (λ_(100) and λ_(111)) behaviors of Fe_(100-x)Ge_x for 4 < x < 18, quantities used further to find the fundamental magnetoelastic coupling constants b_1 and b_2 at room temperature. The x dependence of b_1 and b_2 for Fe_(100-x)Ge_x is contrasted to those of Fe_(100-x)Ga_x and Fe_(100-x)A1_x. While the rhombohedral shear elastic constant c_(44) is almost insensitive to the type and amount of solute, the tetragonal shear constant c' shows a pronounced and rapid softening with increasing x for all three alloys but with different decreasing slopes. Similarly, while the rhombohedral magnetostriction λ_(111) behavior is analogous for all three alloy systems, showing a sign change from negative to positive at the onset of chemical order, the tetragonal magnetostriction λ_(100) behavior differs. For the Ga and Al alloys, λ_(100) maintains positive values over the entire x range, both curves showing large peak values, whereas λ_(100) of Fe_(100-x)Ge_x exhibits a moderate positive peak followed by a negative dip, both of comparable magnitude. Finally the tetragonal coupling constant -b_1 of Fe-Ge shows a marked, sharp decrease as chemical order occurs at x～12 at. % Ge. The decline continues until the ordered D0_3 phase is fully established at x～ 18 at. % Ge. The peak value of ∣b_1∣ for Fe-Ge is approximately half of those for Fe-Ga and Fe-Al. This smaller value of ∣b_1∣, obtained for the higher electron concentration Ge alloy, is consistent with predictions based on band structure calculations. The rhombohedral coupling constant -b_2 shows a consistent sign change at the occurrence of chemical ordering in both Fe-Ga and Fe-Ge.