Growth, characterisation and modelling of novel magnetic thin films for engineering applications

Magnetic materials, especially thin films, are being exploited today in many engineering applications such as magnetic recording heads and media, magnetic sensors and actuators and even magnetic refrigeration due to their smaller form factor or to thin film effects that do not occur in bulk material. Hence there is a need for optimised growth of thin films to suit the requirements of applications. The aim of this research work is two-fold: 1. Growth and characterisation of optimised magnetic thin films using pulsed-laser deposition and 2. Extension of Jiles-Atherton (JA) theory of hysteresis. A series of magnetoelastic thin films based on cobalt ferrite were deposited on SiO2/Si(100) substrates using pulsed-laser deposition at different substrate temperatures and different reactive oxygen pressures. The crystal structure, composition, magnetic properties, microstructure and magnetic domains of cobalt ferrite thin films were investigated. The optimised growth conditions of poly crystalline spinel cobalt ferrite thin films were determined from characterisation results. The Curie point of the optimised cobalt ferrite thin film was determined from moment vs. temperature measurement. The optimised thin film was magnetically annealed in order to induce an in-plane uniaxial anisotropy. The magnetostriction of the optimised sample was determined in the vibrating sample magnetometer using the inverse measurement technique. A special 3-point bender was designed and built for this purpose. The first successful thin film of Gd5Si2Ge2, a magnetocaloric rare earth intermetallic alloy, was deposited on a polycrystalline AlN substrate. The crystal structure, composition and magnetic phase transformation of Gd5Si2Ge2 thin film were investigated. The preliminary results are furnished in this thesis. The JA model of hysteresis was extended to incorporate thermal dependence of magnetic hysteresis. The extended model was validated against measurements made on substituted cobalt ferrite material. A functional form of anhysteretic magnetisation was derived. The JA theory was also extended to model magnetic two-phase materials. This proposed model was qualitatively compared with measured data published in the literature. The JA theory was applied to magnetoelastic thin films. The cobalt ferrite thin films deposited on SiO2/Si(100) substrates at different substrate temperatures and oxygen pressures have been modelled based on JA theory and were validated against measurements. This model would help in understanding the influence of deposition parameters on properties of thin films. The calculated and measured data were in excellent agreement

Lämpökäsiteltyjen terästen magneettiset ominaisuudet

Mechanical and magnetic properties of materials are closely coupled, because both properties arise from the structure of the material. Magnetoelasticity describes the inter-action between the elastic and magnetic properties of the material. The influence of stress on the magnetic properties of steels is of interest because magnetoelasticity can be exploited in the condition monitoring of structural components. The most important phenomena related to magnetoelasticity are magnetostriction and magnetomechanical effect. Magnetostriction is a fractional change in length due to the applied magnetic field. Magnetomechanical effect describes the stress-induced change in the magnetization of ferromagnetic materials. Both phenomena occur simul-taneously. The aim of this work was to examine the magnetic and magnetoelastic properties of steel bars. The literature survey revealed that the interaction between stress and magnet-ic properties of steel is complicated because it is non-linear and exhibits hysteresis. It was found that the microstructural features influence the basic magnetic and magneto-mechanical properties of steels. The experimental part of this work consisted of two parts. In the first part the hyste-resis curves of different steel grades were measured in order to investigate the basic magnetic properties of the samples. It was found that gas carburizing and prior cold working decrease the permeability of the material. No simple relationship was revealed between steel composition and magnetic properties. The purpose of the second part of the experimental work was to build a test device in order to study the magnetoelastic properties and behaviour of the steel bars under impact loading. The main goal of this work was to find out the scatter of the measure-ment results between the samples. The impact tests revealed that there are differences in the results obtained on samples of the same material. The factors of uncertainty were identified and discussed. The main challenge in the magnetic measurements is the inter-pretation of the results because there are several factors influencing the results. Future work requires more detailed study of the microstructure of the samples in order to inter-pret the results with more accuracy

Low-temperature plastic deformation of BCC metals with internal magnetic order

Přestože plastická deformace kovů s prostorově střeďenou kubickou mřížkou za nízkých teplot je dnes dobře vysvětlena, vliv vnitřního magnetického uspořádání na mechanické vlastnosti a vliv plastické deformace na vnitřní magnetické uspořádání zůstávají z větší časti nevysvětleny. Cílem této práce je popsat nízkoteplotní plastickou deformaci -železa a chromu pomocí analýzy skluzových čar, elektronovou zpětně odraženou difrakcí a transmisní elektronovou mikroskopií (TEM). Vnitřní magnetické uspořádání chromu je zkoumáno pomocí magnetické neutronové difrakce v neutronovém centru na Institutu Paula Scherrera ve Švýcarsku. Vibrační magnetometrie je využita pro hledání změn v hmotnostní susceptibilitě, aby byly rozeznány různé magnetické stavy. Anomální skluz byl poprvé pozorován v deformovaném vzorku chromu. Dislokační síť zodpovědná za anomální skluz je charakterizována pomocí g·b analýzy v TEM. Síť protínajících se 1/2111 šroubových dislokací a jejich 100 spoje se tvoří na nízkozatížených {110} rovinách. Dvojčata vytvořená antidvojčatovým smykem byla pozorována v chromu, ale nikoliv v -železe. Tento jev je vysvětlen pomocí atomárních simulací využívajících semiempirické interakční potenciály. Magnetické domény v -železe byly studovány pomocí Kerrovy mikroskopie. Výsledky ukazují, že dvojčata mohou efektivně blokovat pohyb doménových stěn a nukleovat nové.While plastic deformation of body-centered cubic (bcc) metals at low temperatures is now well understood, the effect of internal magnetic order on mechanical properties and the effect of plastic deformation of internal magnetic order remain largely unexplained. The aim of this thesis is to elucidate the low-temperature plastic deformation of -iron and chromium with focus on magnetism. It is investigated by slip-trace analysis of deformed specimens, electron backscattered diffraction, and transmission electron microscopy (TEM) where necessary. The internal magnetic order in chromium is explored by means of magnetic neutron diffraction at SINQ facility of the Paul Scherrer Institute in Switzerland. The vibrating vample magnetometry is utilized to look for changes in mass susceptibility in order to discriminate between different magnetic states. The anomalous slip was found in a deformed chromium specimen for the first time. The dislocation network responsible for the anomalous slip is characterized by the g · b analysis in TEM, where a network of 1/2111 intersecting screw dislocations and their 100 junctions were observed to form on low-stressed {110} planes. Furthermore, twins produced by antitwinning shear were found in chromium, but not in -Fe. This phenomenon is explained using atomistic simulations employing semi-empirical interatomic potentials. The magnetism in -iron is studied by the vibrating sample magnetometry and the results are reconciled using the Jiles-Atherton model. The magnetic domains in -Fe were imaged by the Kerr microscopy and the results show that twins can effectively block the movement of domain walls and nucleate new ones.

Size and temporal distributions in nanoscale magnetic materials via advanced extraction methods

Magnetic nanoparticles possess a multitude of fields of application, for example in biotechnology and utilization as (magnetically) easily separable catalysts. Among the diverse fabrication methods that allow the production of nanoparticles with magnetic properties respectively a chemical composition tailored for a specific task one can find the bottom-up sol-gel dip-coating technique, with which the CoNi nanoparticles embedded in silica were created. Those nanoparticles exhibit a wavelength dependent coercivity if irradiated with laser light. The mentioned behaviour was modeled via an extended, single domain ferromagnetic Stoner-Wohlfarth model implemented in Mathematica. Therein the amount of permitted angles between the nanoparticles easy axis and the external applied field can be limited to arbitrary starting and ending angles. Furthermore, a particle size distribution dependent model for superparamagnetic magnetisation measurements is discussed and its implementation in Mathematica documented. To complement the models programed in Mathematica the extended Jiles-Atherton model that rules out unphysical behaviour and allows the simulation of hystereses was implemented, such that the models available comprise superparamagnetic, single and multi domain ferromagnetic behaviour. Another simple to set up, top-down and „green“ production method is laser ablation sythesis in solution (LASiS). Nitinol (NiTi) nanoparticles created with this technique are of great interest as nitinol shows some exceptional properties, exempli gratia a shape memory, a high resistance to material fatigue and biocompatibility. Thus an extensive (magnetic) characterisation of these nanoparticles is carried out that should ease further production of task specific nanoparticles. Hence the influence of the LASiS liquid on the magnetic properties of the nanoparticles was also analysed. The impact of the laser power on the magnetic behaviour was scrutinized on iron nanoparticles, such that in conclusion some of the „knobs“ that allow tailoring of the properties of the nanoparticles are better understood and therefore grant a more precise parameter choice at the time of fabrication.Magnetische Nanopartikel besitzen einen Durchmesser, welcher unter 100 nm liegt und die Fabrikation entsprechend anspruchsvoll gestaltet. Trotz der damit einhergehenden Herausforderungen gelang es in den letzten Jahren die Herstellungsmethoden soweit zu verbessern, dass sich die magnetischen Eigenschaften wie auch die chemische Zusammensetzung anwendungsspezifisch maßschneidern lassen. Für einen effektiven Einsatz in einer Vielzahl von Anwendungsgebieten sind magnetische Nanopartikel ideal geeignet, beispielsweise in der Hyperthermiekrebsbehandlung, die bei Mäusen eine Überlebensrate von bis zu 90% erzielt, oder als magnetisch leicht trennbare Katalysatoren, die aufgrund des hohen Oberflächen-Volumen-Verhältnisses sehr effizient sind. Des Weiteren wird erwartet, dass die Datenspeicherdichte mittels magnetischer Nanopartikel auf einige Terabit pro Quadratzentimeter erhöht werden kann. Ein weiterer Bereich in dem magnetische Nanopartikel von eminenter Bedeutung sind, ist die Grundlagenforschung, in welcher sie zum Verständnis magnetischer Mechanismen beitragen. Ferner können Phänomene, wie beispielsweise Superparamagnetismus, die erst unterhalb einer bestimmten Maximalgröße auftreten, ihrerseits wieder technologischen Nutzen besitzen. Aus diesen Gründen wurden und werden weltweit zahlreiche Herstellungsmethoden für Nanopartikel untersucht und die Produktionsparameter, die eine feinere Justage der Eigenschaften der resultierenden Nanopartikel ermöglichen, analysiert. Eine ökonomische wie auch ökologische Herstellungsmethode ist durch die Laserablationssynthese in Lösung (LASiS) gegeben. Mit dieser Verfahrensweise erzeugte Nitinolnanopartikel sind von hohem Interesse, da Nitinol (NiTi) über etliche außergewöhnliche Eigenschaften, wie zum Beispiel ein Formgedächtnis, einen hohen Widerstand gegen Materialermüdung und eine gewisse Biokompatibilität verfügt. Darum wird in der vorliegenden Arbeit eine ausführliche magnetische Charakterisierung (die feldabhängige Magnetisierungs-, zero field cooling field cooling- und ac-Messungen umfasst) dieser Nanopartikel vorgenommen, die es erleichtern soll entsprechende Nanopartikel mit aufgabenspezifischen Eigenschaften zu produzieren. Des Weiteren wurde analysiert, wie sich der Einfluss der eingesetzten Mutterlösung und die eingestellte Laserleistung während des Herstellungsprozesses auf die magnetischen Eigenschaften von Nitinolnanopartikeln auswirken. Der Einfluss der Laserleistung wurde zusätzlich an Eisennanopartikeln untersucht. Dadurch wurden einige der Einstellmöglichkeiten mit deren Hilfe sich die Eigenschaften der Nanopartikel regulieren lassen, besser verstanden und somit eine gezieltere Parameterwahl bei der Herstellung ermöglicht wird, wodurch das Optimierungspotential besser ausgeschöpft werden kann. Unter den mannigfaltigen weiteren Herstellungsmethoden, welche heutzutage die Produktion von Nanopartikeln mit anwendungsspezifischen magnetischen Eigenschaften beziehungsweise chemischer Zusammensetzung erlauben, findet sich auch der Sol-Gel-Tauchbeschichtungsprozess, mit welchem sich in Siliciumdioxid eingebettete CoNi-Nanopartikel herstellen lassen. Diese weisen bei Bestrahlung mit Laserlicht eine wellenlängenabhängige Koerzitivität auf. Um dieses besondere Phänomen zu beschreiben und zu simulieren, wurde ein erweitertes, einzeldomänenferromagnetisches Stoner-Wohlfarth-Modell in Mathematica implementiert. Dabei wird die Möglichkeit genutzt, nicht alle möglichen Winkel zwischen der leichten Richtung und dem extern angelegten Feld zuzulassen. Darüber hinaus wird ein teilchengrößenverteilungsabhängiges Modell für superparamagnetische, feldabhängige Magnetisierungsmessungen diskutiert und seine Implementation in Mathematica dokumentiert. In den gebräuchlichen Magnetismusmodellen sind anspruchsvolle Parameter die den Magnetismus beeinflussen, wie beispielsweise die Form der Nanopartikel, noch nicht implementiert. Da es einen Mangel an frei und online verfügbaren Implementationen von Magnetismusmodellen gibt, vermögen die während dieser Arbeit entwickelten Modelle als Ausgangspunkt für weitere Entwicklungen zu dienen

Development of magnetostrictive active members for control of space structures

The goal of this Phase 2 Small Business Innovative Research (SBIR) project was to determine the technical feasibility of developing magnetostrictive active members for use as truss elements in space structures. Active members control elastic vibrations of truss-based space structures and integrate the functions of truss structure element, actively controlled actuator, and sensor. The active members must control structural motion to the sub-micron level and, for many proposed space applications, work at cryogenic temperatures. Under this program both room temperature and cryogenic temperature magnetostrictive active members were designed, fabricated, and tested. The results of these performance tests indicated that room temperature magnetostrictive actuators feature higher strain, stiffness, and force capability with lower amplifier requirements than similarly sized piezoelectric or electrostrictive active members, at the cost of higher mass. Two different cryogenic temperature magnetostrictive materials were tested at liquid nitrogen temperatures, both with larger strain capability than the room temperature magnetostrictive materials. The cryogenic active member development included the design and fabrication of a cryostat that allows operation of the cryogenic active member in a space structure testbed