Probing of local atomic structure using EXAFS and study of magnetostriction in iron-alloy single crystals.

摘要

Ga, W, Mo and other nonmagnetic atom additions lead to large enhancements in the magnetostriction of the alpha-Fe phase. However, the magnetostrictive behavior of Fe, as well as how the solute additions modify the magnetostrictive behavior of Fe are not fully understood. The magnetostrictive strain arises from the magnetoelastic coupling of the atoms. The nature of magnetoelastic coupling is thought to depend on the interatomic spacing of magnetic ion cores, though the exact nature of this dependence is not fully understood. This work examined the local atomic structure through extended x-ray absorption fine structure (EXAFS) measurements that allow an estimation of near-neighbor interatomic distances and near neighbor solute distribution. EXAFS measurements were made on as-grown, long-term annealed, and ordered Fe-27.5 at.% Ga alloy single crystals at the Fe and Ga Kalpha-edges. The EXAFS data were processed using AthenaRTM software and analyzed using Artemis RTM software. The results suggest that a lower level of Ga-Ga covalent bonding in the long-term annealed alloy and its larger magnetostriction value ( 32 lambda100 of 340 x 10-6) are attributed to the decrease in covalently bonded Ga-Ga dimers in the alloy.;A detailed study of magnetostriction in Fe-W alloy single crystals was also made extending previous work that showed a large increase in the magnetostriction of Fe with W additions of 4.4 and 10 at.%. Single crystals of Fe-3 at.% W, Fe-6 at.% W, Fe-7.5 at.% W were obtained using vertical Bridgman technique. Magnetostriction measurements made on as-grown and annealed [001]-oriented single crystal samples showed that annealing enhanced the magnetostriction levels and the magnetostriction constant lambda100 increases rapidly up to 4.4 wt.% W and then decreases at a much lower rate at higher W contents due to increasing propensity to form second phases. The addition of W to Fe decreases the saturation magnetization much more rapidly than that in Fe-Ga alloys.;Magnetostriction and magnetization in Fe-10 at.% Mo-10 at.% Ga, and Fe-15 at.% Mo-5 at.% Ga and Fe-20 at.% Ga single crystals were also examined. The results showed that the substitution of Ga with Mo in Fe-20 at.% Ga alloys drastically decreases the magnetostriction and that magnetization rapidly decreased with the substitution of Ga with Mo.