Magnetostriction, elasticity, and D0(3) phase stability in Fe-Ga and Fe-Ga-Ge alloys

The contrast between the saturation tetragonal magnetostriction, lambda(gamma,2) = (3/2)lambda(100), of Fe1-xGax and Fe1-yGey, at compositions where both alloys exhibit D0(3) cubic symmetry (second peak region), was investigated. This region corresponds to x = 28 at. % Ga and y = 18 at. % Ge or, in terms of e/a = 2x + 3 y + 1, to an e/a value of similar to 1.55 for each of the alloys. Single crystal, slow-cooled, ternary Fe1-x-y GaxGey alloys with e/a similar to 1.55 and gradually increasing y/x were investigated experimentally (magnetostriction, elasticity, powder XRD) and theoretically (density functional calculations). It was found that a small amount of Ge (y = 1.3) replacing Ga in the Fe-Ga alloy has a profound effect on the measured lambda(gamma,2). As y increases, the drop in lambda(gamma,2) is considerable, reaching negative values at y/x = 0.47. The two shear elastic constants c' = (c(11) - c(12))/2 and c(44) measured for four compositions with 0.06 <= y/x <= 0.45 at 7 K range from 16 to 21 GPa and from 133 to 138 GPa, respectively. Large temperature dependence was observed for c' but not for c(44), a trend seen in other high-solute Fe alloys. The XRD analysis shows that the metastable D0(3) structure, observed previously in slow-cooled Fe-Ga at e/a = 1.55, is replaced with two phases, fcc L1(2) and hexagonal D0(19), at just 1.6 at. % Ge. The two are the stable phases of the assessed Fe-Ga phase diagram at x similar to 28. Notably, at y = 7.8, only the D0(3) phase (the equilibrium phase of Fe-Ge at e/a = 1.54) was found in the ternary alloy. The theory also shows that the D0(3) instability is removed for compositions with y >= 3.9, when D0(3) becomes the structure's ground-state phase. Thus, the high, positive lambda(gamma,2) value for Fe-Ga at x = 28 could be the result of the high sensitivity of its metastable D0(3) structure. VC 2011 American Institute of Physics. [doi:10.1063/1.3535444]