Magnetoelastic coupling in Fe100−xGex single crystals with 4

Abstract

<p>In this paper we examine the elastic (<em>c</em>′ and <em>c</em>₄₄) and magnetostrictive (<em>λ</em>₁₀₀ and <em>λ</em>₁₁₁) behaviors of Fe_{100−<em>x</em>}Ge_<em>x</em> for 4<<em>x</em><18, quantities used further to find the fundamental magnetoelastic coupling constants <em>b</em>₁ and <em>b</em>₂ at room temperature. The <em>x</em> dependence of<em>b</em>₁ and <em>b</em>₂ for Fe_{100−<em>x</em>}Ge_<em>x</em> is contrasted to those of Fe_{100−<em>x</em>}Ga_<em>x</em> and Fe_{100−<em>x</em>}Al_<em>x</em>. While the rhombohedral shear elastic constant <em>c</em>₄₄ is almost insensitive to the type and amount of solute, the tetragonal shear constant <em>c</em>′ shows a pronounced and rapid softening with increasing <em>x</em> for all three alloys but with different decreasing slopes. Similarly, while the rhombohedral magnetostriction <em>λ</em>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 <em>λ</em>₁₀₀ behavior differs. For the Ga and Al alloys, <em>λ</em>₁₀₀ maintains positive values over the entire <em>x</em> range, both curves showing large peak values, whereas<em>λ</em>₁₀₀ of Fe_{100−<em>x</em>}Ge_<em>x</em> exhibits a moderate positive peak followed by a negative dip, both of comparable magnitude. Finally the tetragonal coupling constant −<em>b</em>₁ of Fe–Ge shows a marked, sharp decrease as chemical order occurs at <em>x</em> ∼ 12 at. % Ge. The decline continues until the ordered <em>D</em>0₃ phase is fully established at <em>x</em> ∼ 18 at. % Ge. The peak value of |<em>b</em>₁| for Fe–Ge is approximately half of those for Fe–Ga and Fe–Al. This smaller value of |<em>b</em>₁|, obtained for the higher electron concentration Ge alloy, is consistent with predictions based on band structure calculations. The rhombohedral coupling constant−<em>b</em>₂ shows a consistent sign change at the occurrence of chemical ordering in both Fe–Ga and Fe–Ge.</p>