Magnetoelastic coupling in Fe100−xGex single crystals with 4<x<18

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In this paper we examine the elastic (c′ and c44) and magnetostrictive (λ100 and λ111) behaviors of Fe100−xGex for 4<x<18, quantities used further to find the fundamental magnetoelastic coupling constants b1 and b2 at room temperature. The x dependence of b1 and b2 for Fe100−xGex is contrasted to those of Fe100−xGax and Fe100−xAlx. While the rhombohedral shear elastic constant c44 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 λ111 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 λ100 behavior differs. For the Ga and Al alloys, λ100 maintains positive values over the entire x range, both curves showing large peak values, whereas λ100 of Fe100−xGex exhibits a moderate positive peak followed by a negative dip, both of comparable magnitude. Finally the tetragonal coupling constant −b1 of Fe–Ge shows a marked, sharp decrease as chemical order occurs at x∼12 at. % Ge. The decline continues until the ordered D03 phase is fully established at x∼18 at. % Ge. The peak value of |b1| for Fe–Ge is approximately half of those for Fe–Ga and Fe–Al. This smaller value of |b1|, obtained for the higher electron concentration Ge alloy, is consistent with predictions based on band structure calculations. The rhombohedral coupling constant −b2 shows a consistent sign change at the occurrence of chemical ordering in both Fe–Ga and Fe–Ge.