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

In this paper we examine the elastic (c' and c(44)) and magnetostrictive (lambda(100) and lambda(111)) behaviors of Fe100-xGex 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 Fe100-xGex is contrasted to those of Fe100-xGax and Fe100-xAlx. 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 lambda(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 lambda(100) behavior differs. For the Ga and Al alloys, lambda(100) maintains positive values over the entire x range, both curves showing large peak values, whereas lambda(100) of Fe100-xGex 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 similar to 12 at. % Ge. The decline continues until the ordered D0(3) phase is fully established at x similar to 18 at. % Ge. The peak value of vertical bar b(1)vertical bar for Fe-Ge is approximately half of those for Fe-Ga and Fe-Al. This smaller value of vertical bar b(1)vertical bar, 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. (c) 2009 American Institute of Physics. [DOI: 10.1063/1.3061864]