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We have investigated a method for measuring the dimensions of an individual multilayered Fe- Ga/Cu nanowire (NW) as it changes with induced magnetization. In this study, we demonstrate the proposed approach and establish this as a viable method for measuring the magnetostrictive behavior of an individual Fe-Ga/Cu NW using atomic force microscopy (AFM). When an external magnetic field (~300 Oe) was applied perpendicular to the NW axis, the NW length appeared minimized. When a field (~1000 Oe) was applied parallel to the NW axis, the height profile of the NW was found to be higher than in the case with no parallel external field. Since both ends of the NW were welded to the substrate, the magnetic field induced dimensional change of the NW caused deflection of the NW in the upward direction, which was significant enough to be detected by AFM. An average height difference of 15nm was measured with and without an applied field which was then used to calculate the magnetostriction of the multilayered NW. [ABSTRACT FROM AUTHOR]

We investigate the magnetic properties of magnetostrictive iron-gallium (Galfenol, Fe100-xGax, 10⩽x⩽25 at. %) nanowires with magnetic force microscopy (MFM) and using micromagnetic modeling software (magpar). Wires with diameters of 150 nm were fabricated in alternating multilayered structures with Fe-Ga and Cu having aspect ratios of ∼2 and ∼0.5, respectively, with the goal of minimizing the relative contribution of shape anisotropy to magnetic domain alignment. Micromagnetic simulations of isolated Fe-Ga segments with these dimensions predict that (1) at remanence, two opposing vortex structures will form at the ends of a Fe-Ga segment, with a single domain wall in the middle of the segment and (2) traditional magnetic dipoles will form at the ends of the segment to align with a large (saturation) external magnetic field. MFM results are presented that support these models. At remanence, no contrast is observed in the MFM phase images. Magnetic poles become evident at the ends of the Fe-Ga segments when a magnetic field exceeding ∼300 Oe is applied along the length of the nanowire. The direction of the pole alignment is readily flipped by changing the direction of applied field by 180°. Additionally, MFM images show rotation of the magnetic poles in each Fe-Ga segment as they align with fields of ∼550 Oe applied at angles of ∼55° and ∼105° relative to the length of the nanowire. The MFM results support the simulation results and demonstrate that an aspect ratio of ∼2 will reduce shape anisotropy effects sufficiently in Fe-Ga nanowire that magnetization can lie off of the nanowire axis. [ABSTRACT FROM AUTHOR]

Highly textured (011)[100] Goss-oriented rolled sheet Fe-Al alloy is a promising magnetostrictive material for use in bending mode sensors and vibrational energy harvesters. In this paper, we performed magnetic field annealing (FA) to induce magnetic anisotropy in strips of highly textured Fe-Al. Prior work suggests FA as a viable alternative to stress annealing (SA), which leads to buckling of Fe-Al rolled sheet samples. The Fe-Al strips studied here exhibited tetragonal magnetostriction values ((3/2)€sat˔=˔€€€ sat˔=˔€€ - €€˔) of ˔136 ppm along their length, which corresponds to ˔78% of the single crystal value along a <100> orientation. The effectiveness of FA on magnetic moment rotation was inferred by comparing post-FA magnetostriction measurements with the maximum possible yield, where €€€€= 0 and €€˔= (3/2)€sat€sat. Strain gauge data from the middle of the strip indicates that FA achieved ˔27% of the desired built-in uniaxial anisotropy along the parallel direction of the strip length, decreasing €€€€ by 25 ppm of the 95 ppm. Hall effect sensor data was used to assess the potential effect of FA on sensing and energy harvesting performance. FA improved the bending-stress-induced changes in magnetization near the clamped end of the strips by ˔45%. These results suggest that the FA was more effective in the region near the end of the strip than toward the middle of the strip, which we explain may be a result of the use of high temperature permanent magnets at the ends of the strips for the FA protocol. [ABSTRACT FROM AUTHOR]

Magnetostrictive Fe-Ga and Fe-Al alloys are promising materials for use in bendingmode vibrational energy harvesters. For this study, 50.8 mm x 5.0 mm x 0.5 mm strips of Fe-Ga and Fe-Al were cut from 0.50-mm thick rolled sheet. An atmospheric anneal was used to develop a Goss texture through an abnormal grain growth process. The anneal lead to large (011) grains that covered over 90% of sample surface area. The resulting highly-textured Fe-Ga and Fe-Al strips exhibited saturation magnetostriction values (λsat= λ|| - λ⊥) of ~280 ppm and ~130 ppm, respectively. To maximize 90° rotation of magnetic moments during bending of the strips, we employed compressive stress annealing (SA). Samples were heated to 500°C, and a 100-150 MPa compressive stress was applied while at 500°C for 30 minutes and while being cooled. The effectiveness of the SA on magnetic moment rotation was inferred by comparing post-SA magnetostriction with the maximum possible yield of rotated magnetic moments, which is achieved when λ|| = λsat and λ⊥ = 0. The uniformity of the SA along the sample length and the impact of the SA on sensing/energy harvesting performance were then assessed by comparing pre- and post-SA bending-stress-induced changes in magnetization at five different locations along the samples. The SA process with a 150 MPa compressive load improved Fe-Ga actuation along the sample length from 170 to 225 ppm (from ~60% to within ~80% of λsat). The corresponding sensing/energy harvesting performance improved by as much as a factor of eight in the best sample, however the improvement was not at all uniform along the sample length. The SA process with a 100 MPa compressive load improved Fe-Al actuation along the sample length from 60 to 73 ppm (from ~46% to ~56% of λsat, indicating only a marginally effective SA and suggesting the need for modification of the SA protocol. In spite of this, the SA was effective at improving the sensing/energy harvesting performance by a factor of ~2.5 in the best sample. As with the Fe-Ga strip, improvement in performance was quite varied along the strip length. [ABSTRACT FROM AUTHOR]