The behavior of magnetostrictive nanowires and their potential for use in the design of devices for actuation and sensing applications is explored in this chapter. An introduction to magnetostrictive materials is presented first. This includes MOKE images of the response of magnetic domains in a magnetostrictive material to applied magnetic fields and to applied mechanical compression, as well as presentation of typical sensor and actuator characterization data from bulk samples of the magnetostrictive alloy Fe100−xGax (10 ≤ x ≤ ~35 atomic %) (also known as Galfenol or Fe-Ga). Next, models of magnetostriction at the macro- and microscale are presented. Micromagnetic simulations are used to visualize the significant role of shape anisotropy on magnetostriction in nanowires and to explain the challenges associated with achieving magnetic domain rotation in high aspect ratio nanowires. Methods for fabrication of Fe-Ga nanowires and Fe-Ga/X multilayer nanowires are discussed. Structural and magnetic characterization data are presented from Fe-Ga and Fe-Ga/Cu nanowires with of diameters of 100–200 nm and aspect ratios ranging from as low as 0.5 in some of the multilayer nanowire segments to over 100 in Fe-Ga single alloy nanowires. The last two sections of the chapter present experimental studies into use of the nanowires for actuation and sensing. This includes demonstration of magnetic domain rotation and magnetostriction in Fe-Ga/Cu multilayer nanowires in response to an applied magnetic field, as well as the use of a GMR sensor to detect magnetic domain rotation in response to application of a compressive mechanical load to an array of Fe-Ga/Cu nanowires.

Developing protocols for making thin sheet FeGa (Galfenol) with abnormally grown Goss or Cube grains, which provide maximum magnetostriction, is challenging because the mechanisms that regulate grain boundary mobility and texture development in these alloys are not yet understood. Grain boundary energy models do not account for forces caused by the control of surface energy from atmospheric annealing conditions. By characterizing the surface energy of specific Galfenol grains, we can develop a more accurate thermodynamic framework for modeling abnormal grain growth and texture development. To non-destructively measure surface energy of specific crystal orientations and overcome passivation layer difficulties in previous studies, a two-liquid-phase contact angle method is utilized. A single-crystal (1 0 0) Fe82Ga18 is used as a proof of concept for its isotropic surface crystal orientation. The resultant contact angle data shows a high dependence on the use of Ar-plasma surface preparation to remove native oxides exposing a true interaction between a sessile drop and the Galfenol surface. Experimentally measured surface energy values are in agreement with density functional theory simulations. Surface texture and composition are confirmed using EBSD and XPS measurements. This non-destructive technique paves the way towards studying surface energies of bare metal surfaces.

There is a growing interest in 3D printing to fabricate culture substrates; however, the surface properties of the scaffold remain pertinent to elicit targeted and expected cell responses. Traditional 2D polystyrene (PS) culture systems typically require surface functionalization (oxidation) to facilitate and encourage cell adhesion. Determining the surface properties which enhance protein adhesion from media and cellular extracellular matrix (ECM) production remains the first step to translating 2D PS systems to a 3D culture surface. Here we show that the presence of carbonyl groups to PS surfaces correlated well with successful adhesion of ECM proteins and sustaining ECM production of deposited human mesenchymal stem cells, if the surface has a water contact angle between 50° and 55°. Translation of these findings to custom-fabricated 3D PS scaffolds reveals carbonyl groups continued to enhance spreading and growth in 3D culture. Cumulatively, these data present a method for 3D printing PS and the design considerations required for understanding cell-material interactions. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res B Part B, 2019.

Alfenol (Fe-Al) and Galfenol (Fe-Ga) are iron-based structural magnetostrictive alloys that, for compositions of ∼81% iron, are increasingly being used in sensing, actuating, and energy harvesting devices [Park et al., AIP Advances 6(5), 056221 (2016)]. Recent improvements in the development of magnetostrictive materials using the deformation processing methods of rolling to produce highly textured thin sheet [Park et al., AIP Advances 6(5), 056221 (2016)] and ball milling to produce (001)-oriented micron-size flakes [S. M. Na, J. Galuardi, and A. B. Flatau, IEEE Transactions on Magnetics 53(11), 1–4 (2017)] provide the opportunity to develop a non-contact torque sensor. Torque-induced shear forces at the surface of a shaft lead to a measurable change in the flux passing through the air above the surface of a shaft to which a magnetostrictive layer has been bonded. The current study builds on prior work which demonstrated that torque influenced the magnitude of magnetic flux in the air gaps located between a piece of Galfenol and the rest of a magnetic circuit [Raghunath et al., Proceedings of the ASME, 2013]. The current work overcomes limitations of the prior work. This work demonstrates that using a magnetostrictive layer made of a patch of Alfenol, an alloy that is less expensive, more ductile, and less magnetostrictive than Galfenol, but has almost the same saturation magnetization of ∼1.5T, slightly outperformed the patches made of Galfenol. Additional contributions of the present work include a first look at the application to a shaft of an epoxy-based paint containing micron-sized flakes of (001)-oriented Galfenol, and a comparison of square and ring-shaped patches (aspect ratios of 1 and of ∼4). Data are presented from quasi-static testing and from dynamic tests at rotational rates of up to 1000 rpm.

Galfenol is a magnetostrictive alloy used in sensing, actuating, and energy harvesting devices. Developing protocols for making thin sheet Galfenol with abnormally grown Goss or Cube grains is challenging because the mechanisms that regulate grain boundary mobility and texture development in these alloys are not well understood. Grain boundary energy models do not account for extraneous driving forces caused by the control of surface energy from atmospheric annealing conditions. By characterizing the surface energy of specific Galfenol grains at room temperature, we can develop a more accurate thermodynamic-based framework for modeling abnormal grain growth and texture development. This will be used to understand why a high temperature atmospheric anneal under 0.5%H2S in Ar transforms myriad grains into highly textured, single-crystal-like polycrystalline material while pure Ar does not. To experimentally measure surface energy, we have developed a non-destructive process to directly probe highly-textured and single-crystal Galfenol. The process involves high quality polishing to sub-nanometer roughness, Ar plasma cleaning to remove native oxides, and preserving the bare metal surface by immersion in hydrocarbon liquid. In this bulk hydrocarbon liquid, we use the two-liquid-phase contact angle method to measure surface energy. Experimental surface energy values on single crystals agree with DFT calculations, confirming the validity of this process. We use this method to observe a decrease in surface energy for sulfur contaminated Galfenol.