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.

Magnetic nanowires, fabricated by template-assisted electrochemical deposition, have been proposed for a wide range of applications, including Fe-Ga alloy magnetostrictive nanosensors and actuators, Co/Cu multilayered CPP-GMR structures, cell labels and identification, MRI contrast agents, and nanowarming agents. The desired magnetic properties for each of these applications are described in this chapter. In addition, a simple and cost-effective double imprinting technique is described that yields long-range ordered anodic aluminum oxide templates for 3-D magnetic memory.

Galfenol (Fe1-xGax, 10 < x < 40) may be the only smart material that can be made by electrochemical deposition which enables thick film and nanowire structures. This article reviews the deposition, characterization, and applications of Galfenol thin films and nanowires. Galfenol films have been made by sputter deposition as well as by electrochemical deposition, which can be difficult due to the insolubility of gallium. However, a stable process has been developed, using citrate complexing, a rotating disk electrode, Cu seed layers, and pulsed deposition. Galfenol thin films and nanowires have been characterized for crystal structures and magnetostriction both by our group and by collaborators. Films and nanowires have been shown to be largely polycrystalline, with magnetostrictions that are on the same order of magnitude as textured bulk Galfenol. Electrodeposited Galfenol films were made with epitaxial texture on GaAs. Galfenol nanowires have been made by electrodeposition into anodic aluminum oxide templates using similar parameters defined for films. Segmented nanowires of Galfenol/Cu have been made to provide engineered magnetic properties. Applications of Galfenol and other magnetic nanowires include microfluidic sensors, magnetic separation, cellular radio-frequency identification (RFID) tags, magnetic resonance imaging (MRI) contrast, and hyperthermia.

We have demonstrated magnetostrictive nanowire (NW) array-based pressure sensing that utilizes magneto-mechanical transduction in an NW array and electro-magnetic transduction in a giant magnetoresistance (GMR) magnetic field sensor. The pressure sensing is performed by pressing the vertically aligned multilayered Fe 80 Ga 20 /Cu NW array against the GMR surface causing NW deflections. Micromagnetic simulations were used to visualize changes in NW magnetization state and resulting stray fields with applied mechanical strain. This magneto-mechanical transduction was similar to that measured experimentally by the GMR sensors with applied pressure.

Electron holograms allowed us to visualize the magnetic induction in a row of FeGa/Cu nanowires to see their neighboring effect, this observation demonstrate a limit to the density of individual bits in 3D memory.

A comprehensive three-dimensional picture of magnetic ordering in high-density arrays of segmented FeGa/Cu nanowires is experimentally realized through the application of polarized small-angle neutron scattering. The competing energetics of dipolar interactions, shape anisotropy, and Zeeman energy in concert stabilize a highly tunable spin structure that depends heavily on the applied field and sample geometry. Consequently, we observe ferromagnetic and antiferromagnetic interactions both among wires and between segments within individual wires. The resulting magnetic structure for our nanowire sample in a low field is a fan with magnetization perpendicular to the wire axis that aligns nearly antiparallel from one segment to the next along the wire axis. Additionally, while the low-field interwire coupling is ferromagnetic, application of a field tips the moments toward the nanowire axis, resulting in highly frustrated antiferromagnetic stripe patterns in the hexagonal nanowire lattice. Theoretical calculations confirm these observations, providing insight into the competing interactions and resulting stability windows for a variety of ordered magnetic structures. These results provide a roadmap for designing high-density magnetic nanowire arrays for spintronic device applications.

This paper reviews the synthesis of magnetic multilayerd nanowires that have a wide range of applications. Specifically of interest in this review are the applications of magnetic manipulation and separation of cells, which are important for potential cancer therapies. Compared to other magnetic nanoparticles in use today, the nanowires have the advantage of being ferromagnetic and also having high aspect ratios that enable barcoding. These nanobots are synthesized inside nanoporous oxide templates in large batches (1012 per square inch), and they can be composed of any magnetic metal, alloy, or multilayer that can be electroplated. Specific details for the electrochemistry of Galfenol deposition are given. Galfenol is an exciting new magnetostrictive material with durable mechanical properties. Next, a protocol is described for full removal of the growth contact prior to release of the nanobots from their oxide template. This mitigates aggregation which inhibits cellular uptake. Feasibility of manipulation and separation was shown using canine bone cancer (osteosarcoma) cells which internalized the nanobots, enabling magnetic cellular control. In addition, initial toxicity studies indicate that the nanobots are not cytotoxic. These studies merely scratch the surface of the potential use of nanobots for diagnosis and therapy in the near future.

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 15 nm was measured with and without an applied field which was then used to calculate the magnetostriction of the multilayered NW.

Metallic nanowires with low resistivity were grown inside insulating aluminum oxide matrices that contained very uniform columnar nanopores (10.6+/1.7 nm diameters). These nanopores can be made with large-scale order (cm 2 ), which is desirable in applications such as hard drive read sensors and random access memories. The nanowires are grown by electrochemical deposition directly inside the alumina to avoid sidewall damage compared to nanostructures that are defined from films by lithographical patterning and etching. Specifically, trilayers of [Co(15 nm)/Cu(5 nm)/Co(10 nm)] were synthesized and measured to have 30 Ω resistance and 19% magnetoresistance. These parameters are desirable for read head sensors, especially because the nanowires described here have 1:1 aspect ratios, and 10× smaller areas and 100× lower resistances than conventional read sensors based on lithographically produced magnetic tunnel junctions. A new nanostamping technique is introduced, in which linear stamps with ordered cm 2 areas are imprinted onto aluminum precursors to produce ordered nanoporous aluminum oxide upon anodization. These stamps are substantially less-time consuming and cheaper to make than dot type stamps, and the order enables closely spaced arrays of CPP-GMR sensors for one-pass 2-D recording and cross recording. Importantly, the GMR sensors are grown directly into aluminum oxide with 20 nm separation. Therefore, a relatively large pattern (30 × 100 nm) can be used to produce three 10 nm-diameter GMR sensors without roughening or redeposition on sidewalls. The sensors are also already embedded in alumina for subsequent device processing.

We have investigated nano-scale magnetic structures to learn how magnetic behaviors differ from what is observed in bulk materials. In this study, we have measured the magnetic hysteresis of individual multilayered nanowire using a magnetic force microscopy (MFM). The nanowires were composed of low-aspect ratio segments of iron-gallium (Galfenol, Fe80Ga20) and copper layers. Individual wires were released from nano-porous anodized aluminum oxide (AAO) templates and separated from other nanowires to avoid significant interactions between nearby nanowires. In this experiment, structures with aspects ratios c/a of similar to 3 were studied. Different MFM phase magnitudes in response to the interaction of magnetic film coated MFM tips and Fe-Ga/Cu nanowire were observed as a function of applied external magnetic field (-850 Oe < H < 850 Oe). Amplitude differences between bright and dark responses in MFM images were used to demonstrate hysteresis. Coercivity measured was about 125 Oe, which is almost same as the coercivity value (similar to 150 Oe) of a nanowire array measured with a vibrating sample magnetometer (VSM). When fields are applied perpendicular to the nanowires, the hysteresis loops sheared, indicating that the easy axis is along the nanowire axis. Remanence magnetizations of an individual nanowire and a nanowire array were compared to demonstrate inter-wire magnetostatic dipolar interaction. (C) 2013 American Institute of Physics.

In this study, using steady-state electrochemistry at a rotating disk electrode, a deposition mechanism for giant magnetostrictive Fe1−xGax alloys is proposed in which the formation of an adsorbed monovalent [Fe(I)]ads intermediate is determined to be the rate-determining step. In subsequent steps, this intermediate either gets reduced to iron or catalyzes the reduction of gallium by forming an adsorbed [Ga(III)–Fe(I)]ads intermediate. In line with the proposed mechanism, it was experimentally shown that the differences in the mass-transport rates of Fe(II) species determined the thin film composition. Therefore, this study has made possible a controllable and reproducible deposition of Fe1−xGax thin films with compositions in the entire range of interest (15%–30% Ga). As-grown Fe80Ga20 thin films were found to have magnetostriction constants of ~112ppm.

In this study, thin films of Fe83Ga17 (a giant magnetostrictive alloy) were grown on single-crystalline n-GaAs (001) and polycrystalline brass substrates via electrochemical synthesis from ferrous and gallium sulfate electrolytes. Extensive structural characterization using microdiffraction, high-resolution ω − 2θ, and rocking-curve analysis revealed that the films grown on GaAs(001) are highly textured with ⟨001⟩ orientation along the substrate normal, and the texture improved further upon annealing at 300 °C for 2 h in N2 environment. On the contrary, films grown on brass substrates exhibited ⟨011⟩ preferred orientation. Rocking-curve analysis done on Fe83Ga17/GaAs structures further confirmed that the ⟨001⟩ texture in the Fe83Ga17 thin film is a result of epitaxial nucleation and growth. The non-linear current−voltage plot obtained for the Fe−Ga/GaAs Schottky contacts was characteristic of tunneling injection, and showed improved behavior with annealing. Thus, this study demonstrates the feasibility of fabricating spintronic devices that incorporate highly magnetostrictive Fe(1−x)Gax thin films grown epitaxially via electrochemistry.

Resistivities of 5.4 μΩ·cm were measured in 10-nm-diameter metallic wires. Low resistance is important for interconnections of the future to prevent heating, electromigration, high power consumption, and long RC time constants. To demonstrate application of these wires, Co/Cu/Co magnetic sensors were synthesized with 20-30 Ω and 19% magnetoresistance. Compared to conventional lithographically produced magnetic tunnel junction sensors, these structures offer facile fabrication and over 2 orders of magnitude lower resistances due to smooth sidewalls from in situ templated chemical growth.

In this work, magnetization reversal mechanisms in various 35 nm diameter Fe80Ga20/Cu multilayered nanowire arrays were studied by a vibrating sample magnetometer (VSM) equipped with vector coils, making it possible to monitor both x- and y-components of the sample moment during reversal. When reversal fields were applied in the low angular range (0 – 60°), all nanowire structures, irrespective of Fe80Ga20 or Cu aspect ratios, experienced reversal by nucleation and propagation of a vortex domain wall. However, when fields were applied in the high angular range of 60–90°, reversal occurred by coherent rotation. Using vector-VSM, it was further shown that in structures with pancake-like Fe80Ga20 segments, the extent to which moments in adjacent segments rotate cooperatively decreased as the Cu thickness increased.

Arrays of nanowires are fabricated with alternating segments of the magnetostrictive alloy Fe1xGax and Cu using electrochemical deposition in nanoporous anodic aluminium oxide (AAO) templates. The difficult nature of Ga-alloy electrochemistry is overcome by controlling mass-transfer and hydrodynamic conditions using novel rotating disk electrode templates to obtain highly uniform segment lengths throughout the arrays. Extensive structural characterization by XRD, EBSD and TEM reveals a strong < 110 > textured Fe1xGax growth. Furthermore, using vibrating sample magnetometry (VSM), we demonstrate that control of magnetization reversal processes is possible once uniform aspect ratios are obtained for both the FeGa and Cu segments.