Mechanical, magnetic and magnetostrictive properties of porous Fe-Ga films prepared by electrodeposition

Magnetostriction, known as the ability of magnetic materials to expand or contract in response to magnetic field, is a key property of Fe-Ga alloys exploited in various types of transducers. Usually, thin films of Fe-Ga deposited on rigid substrates suffer from a clamping effect that hinders the propagation of strain. Herein, Fe-Ga films with macroporous, not fully constrained, geometry are prepared by electrodeposition on metallized silicon substrates templated with sub-micrometer-sized polystyrene spheres. For comparison, fully-dense and inherently nanoporous films are prepared by sputtering and electrodeposition, respectively. The electrodeposition mechanism is discussed in terms of electrochemically active species distribution and partial current densities. The composition of the Fe-Ga films has been tuned (2-40 at.% Ga) by varying the electrodeposition parameters. A complete assessment of the nanomechanical and magnetic properties of the films with variable composition and porosity has been performed for an optimized performance. The magnetostriction has been studied by X-ray diffraction applying an in-situ magnetic field. The results demonstrate a larger magnetic-field-induced crystal deformation in templated (macroporous) films compared to the non-templated and fully-dense counterparts. The observed effects in porous Fe-Ga films are very appealing for the design of various strain-engineered nanomaterials, e.g., energy transducers or magnetoelectric composites

Design and packaging of an iron-gallium (Galfenol) nanowire acoustic sensor for underwater applications

A novel acoustic sensor incorporating cilia-like nanowires made of magnetostrictive iron-gallium (Galfenol) alloy has been designed and fabricated using micromachining techniques. The sensor and its package design are analogous to the structural design and the transduction process of a human-ear cochlea. The nanowires are sandwiched between a flexible membrane and a fixed membrane similar to the cilia between basilar and tectorial membranes in the cochlea. The stress induced in the nanowires due to the motion of the flexible membrane in response to acoustic waves results in a change in the magnetic flux in the nanowires. These changes in the magnetic flux are converted into electrical voltage changes by a GMR (giant magnetoresistive) sensor. As the acoustic sensor is designed for underwater applications, packaging is a key issue for the effective working of this sensor. A good package should provide a suitably protective environment to the sensor, while allowing sound waves to reach the sensing element with a minimal attenuation. In this thesis, design efforts aimed at producing this MEMS bio-inspired acoustic transducer have been detailed along with the process sequence for its fabrication. Package materials including encapsulants and filler fluids have been identified based on their acoustic performance in water by conducting several experiments to compare their impedance and attenuation characteristics and moisture absorption properties. Preliminary test results of the sensor without nanowires demonstrate the process is practical for constructing a nanowire based acoustic sensor, yielding potential benefits for SONAR applications and hearing implants

Characterization of Bending Magnetostriction in Iron-Gallium Alloys for Nanowire Sensor Applications

This research explores the possibility of using electrochemically deposited nanowires of magnetostrictive iron-gallium (Galfenol) to mimic the sensing capabilities of biological cilia. Sensor design calls for incorporating Galfenol nanowires cantilevered from a membrane and attached to a conventional magnetic field sensor. As the wires deflect in response to acoustic, airflow, or tactile excitation, the resultant bending stresses induce changes in magnetization that due to the scale of the nanowires offer the potential for excellent spatial resolution and frequency bandwidth. In order to determine the suitability for using Galfenol nanowires in this role, the first task was experimentally characterizing magnetostrictive transduction in bending beam structures, as this means of operation has been unattainable in previous materials research due to low tensile strengths in conventional alloys such as Terfenol-D. Results show that there is an appreciable sensing response from cantilevered Galfenol beams and that this phenomenon can be accurately modeled with an energy based formulation. For progressing experiments to the nanowire scale, a nanomanipulation instrument was designed and constructed that interfaces within a scanning electron microscope and allows for real time characterization of individual wires with diameters near 100 nm. The results of mechanical tensile testing and dynamic resonance identification reveal that the Galfenol nanowires behave similarly to the bulk material with the exception of a large increase in ultimate tensile strength. The magnetic domain structure of the nanowires was theoretically predicted and verified with magnetic force microscopy. An experimental methodology was developed to observe the coupling between bending stress and magnetization that is critical for accurate sensing, and the key results indicate that specific structural modifications need to be made to reduce the anisotropy in the nanowires in order to improve the transduction capabilities. A solution to this problem is presented and final experiments are performed

Strain Based Control of Magnetic Domain Walls

This work investigates the applicability of strain induced magnetic anisotropy in the control of magnetic domain walls in magnetically soft magnetostrictive nanostructures. The mechanical coupling of piezoelectric and magnetostrictive materials is investigated as a method of controlling the strain in the magnetic material. Two systems have been investigated. The first is a magnetic nanoring containing two transverse magnetic domain walls subjected to uniaxial strain and a rotating field. For this system an analytical model and finite element analysis are used to predict the effect of strain induced anisotropy on the domain walls. Experimental data for the system has also been presented. The second system is a notched magnetic nanowire subjected to uniaxial strain. The change depinning fields of domain walls from the notch at varying strains have been simulated and measured experimentally

Effect of lithographically-induced strain relaxation on the magnetic domain configuration in microfabricated epitaxially grown Fe81Ga19

We investigate the role of lithographically-induced strain relaxation in a micron-scaled device fabricated from epitaxial thin films of the magnetostrictive alloy Fe81Ga19. The strain relaxation due to lithographic patterning induces a magnetic anisotropy that competes with the magnetocrystalline and shape induced anisotropy to play a crucial role in stabilising a flux-closing domain pattern. We use magnetic imaging, micromagnetic calculations and linear elastic modelling to investigate a region close to the edges of an etched structure. This highly-strained edge region has a significant influence on the magnetic domain configuration due to an induced magnetic anisotropy resulting from the inverse magnetostriction effect. We investigate the competition between the strain-induced and shape-induced anisotropy energies, and the resultant stable domain configurations, as the width of the bar is reduced to the nanoscale range. Understanding this behaviour will be important when designing hybrid magneto-electric spintronic devices based on highly magnetostrictive materials

Study of an off-grid wireless sensors with Li-Ion battery and Giant Magnetostrisctive Material

L'abstract è presente nell'allegato / the abstract is in the attachmen

Elektrochemische Fe-Ga-Legierungsabscheidung zur Herstellung von Nanostrukturen

Eisen-Gallium-Legierungen sind aufgrund ihrer hohen Magnetostriktion und ihrer hervorragenden mechanischen Eigenschaften sehr interessant für Anwendungen sowohl in Form von Sensoren als auch Aktoren. Die fortschreitende Miniaturisierung erfordert die Herstellung von Bauteilen in eindimensionaler Struktur und komplexen Geometrien. Beide Herausforderungen sind mit templatbasierter elektrochemischer Abscheidung zugänglich. Es konnte gezeigt werden, dass dünne Fe-Ga-Schichten schon aus einfachen wässrigen Elektrolyten abgeschieden werden können. Gallium kann nur in Anwesenheit von Fe induziert reduziert werden. Gleichzeitig konnte nachgewiesen werden, dass durch die Hydrolyseneigung der Ga-Ionen immer Hydroxide gebildet und in das Deposit eingebunden werden. Durch die Einführung einer alternierenden potentiostatischen Abscheidung mit einem Reduktions- und einem Relaxationsschritt können dennoch dichte und homogene Fe80Ga20-Schichten mit wenigen Defekten und einem vernachlässigbar kleinen Sauerstoffgehalt hergestellt werden. Die Übertragung der so gefundenen Abscheideparameter zur templatbasierten Nanodrahtherstellung ist nur bis zu einem Porendurchmesser von 100nm möglich. Wird der Durchmesser der Porenkanäle weiter verringert, führt aufgrund eingeschränkter Diffusionsvorgänge die Abscheidung zu segmentierten und sauerstoffreichen Depositen. Die Modifizierung des Elektrolyten durch Komplexierung der Metallionen verhindert die Bildung und Einbindung der Hydroxide. Damit können auch für Porendurchmesser kleiner 100nm Drähte in AAO-Template abgeschieden werden. Diese sind dicht, defektfrei und weisen keinen Zusammensetzungsgradienten entlang der Wachstumsrichtung auf. Detaillierte TEM-Untersuchungen konnten zeigen, dass die Herstellung durch ein einfacheres potentiostatisches Abscheideregime zu weniger verspannten und dennoch homogenen und defektfreien Drähten führt. Für die Herstellung von magnetisch aktiven Drähten sollte daher die potentiostatische der gepulsten Abscheidung vorgezogen werden

Smart Materials and Devices for Energy Harvesting

This book is devoted to energy harvesting from smart materials and devices. It focusses on the latest available techniques recently published by researchers all over the world. Energy Harvesting allows otherwise wasted environmental energy to be converted into electric energy, such as vibrations, wind and solar energy. It is a common experience that the limiting factor for wearable electronics, such as smartphones or wearable bands, or for wireless sensors in harsh environments, is the finite energy stored in onboard batteries. Therefore, the answer to the battery “charge or change” issue is energy harvesting because it converts the energy in the precise location where it is needed. In order to achieve this, suitable smart materials are needed, such as piezoelectrics or magnetostrictives. Moreover, energy harvesting may also be exploited for other crucial applications, such as for the powering of implantable medical/sensing devices for humans and animals. Therefore, energy harvesting from smart materials will become increasingly important in the future. This book provides a broad perspective on this topic for researchers and readers with both physics and engineering backgrounds

Strain control of ferromagnetic thin films and devices

Magnetic memory and logic technologies promise greater energy efficiency and speed than conventional, semiconductor-based electronics. To date, electrical current has been used to operate such devices, although voltage-control may be a more efficient way to control magnetisation. One route to achieving voltage control of magnetisation is to use a hybrid piezoelectric/ferromagnetic device in which a voltage applied to the piezoelectric induces a strain in the ferromagnetic layer, which in turn induces a magnetic anisotropy. In this thesis such hybrid devices are used to investigate the control of magnetisation by inducing uniaxial anisotropy in the ferromagnetic layer. One material that shows promise for use as the ferromagnetic layer is Fe81Ga19. This material is attractive since it contains no rare earth elements, and in bulk crystals has been shown to be highly magnetically responsive to strain. This thesis investigates the magnetic properties of epitaxial Fe81Ga19 thin films grown by molecular beam epitaxy and it is demonstrated that these thin films retain the attractive magnetostrictive properties previously observed in bulk crystals. The presence of strong cubic magnetocrystalline anisotropy in the layers is exploited to demonstrate the non-volatile switching of magnetisation using strain-induced anisotropy in the absence of an applied magnetic field. This thesis shows also the manipulation of magnetic anisotropies and control of the configuration of magnetic domains and domain walls in Fe81Ga19 at a range of different lateral dimensions, from50 μm to 1 μm. It is shown that as the lateral dimensions of the device structures studied are reduced the domain configuration appears more regular, and that strain-induced anisotropy is more able to control these domains. In wires around 1 μm in width it is shown that growth strain relaxation by lithographic patterning induces sufficient anisotropy to cause a change in the domain configuration of the wire studied. Finally, this thesis begins to investigate how inverse magnetostriction can be used to tune the behaviour of domain walls in wires 1 μm wide and narrower. Experimental control of the field required to depin a vortex domain wall from a notch in a 1 μm wide Co wire is demonstrated. Using micromagnetic simulations it is shown that a large degree of control over the depinning of domain walls from notches in wires 1 μm wide and narrower is possible. The influence of in plane uniaxial magnetic anisotropy on the domain wall velocity in wires supporting in plane transverse domain walls driven by an external magnetic field is also investigated. Work previously done on the effect of uniaxial anisotropy on domain wall velocities close to Walker breakdown is extended in this thesis and in investigating the velocity and structure at driving magnetic fields far above walker Breakdown a second peak in domain wall velocity is observed, a phenomenon previously observed in wide wires, and wires under the influence of a transverse magnetic field