Low symmetry short-range ordered nanoprecipitates directly observed by neutron diffuse scattering in highly magnetostrictive Galfenol Fe1-xGax alloys

Hu Cao, Virginia Tech

Giant magnetostriction in galfenol Fe-xat.%Ga alloys about 15 < x < 27, coupled with robust mechanical properties, make them attractive for potential applications as magneto-acoustic sensors, actuators and transducers. The addition of Ga into the body-centered cubic (bcc) . -Fe (or A2) phase results in a large increase in the magnetostriction 3.100/2=400ppm at room temperature with low saturation magnetic fields, whereas the saturation magnetostriction in pure Fe is only about 33ppm. A local short-range ordered DO3 (i.e. bcc Fe3Ga) nanoprecipitates, which were decomposed out of the A2 matrix in the two-phase region of DO3+A2, has been suggested to be responsible for this large increase in magnetostriction. Our neutron diffuse scattering measurements have shown that the crystal structure of DO3 nanoprecipitates is in fact strongly distorted from cubic symmetry. This is evidenced by the asymmetric radial lineshape of the diffuse scattering intensity observed near the (100), which is very broad and incommensurate with respect to the A2 lattice. Analysis of the diffuse intensity profile revealed a peak-splitting consistent with tetragonal or lower symmetry with a large lattice ratio of c/a . 1.2. Investigations as a function of Ga content also have shown that the diffuse scattering intensity is strongest near Fe-19at%Ga, where the maximum magnetostriction was observed. These results confirm that the enhanced magnetostriction in Fe-xat.%Ga alloys is directly related to the structural heterogeneity of the low symmetric DO3 nanoprecipitates.

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