Methodological Reduction of Magnetically Induced Noise in Magnetic Multilayers for Sensor Applications

Current composite magnetoelectric (ME) sensors of cantilever design have shown promising properties and positive tendencies toward sensor applications in biomagnetic sensing. Despite great signal performance, the sensors have limitations, resulting from noise generated by the magnetic phase. The magnetically induced noise dominates the sensors performance even further when low frequency signals are probed, using magnetic frequency conversion (MFC). The work orients around this specific impediment and provides various techniques to suppress the noise sources, originating from the magnetic phase. In the beginning, the work reviews the effectiveness of multilayer exchange bias (EB) systems that were used to reduce magnetic noise. An improvement of the system is introduced that allowed increased signal yield. However, the noise response could not be improved. For this reason, a new magnetic multilayer system dubbed antiparallel exchange bias (APEB) is introduced, which is formed with a specialized post-production heating scheme. The developed coupling allows formation of a stable magnetic configuration, leading to a significant reduction of the magnetic noise. Incorporated into the ME sensors, the APEB yields improvement in their limit of detection (LOD) and reproducibility of their sensing properties. The sensors were characterized electrically as well as magnetically using magneto-optical Kerr effect microscopy. In their finalized form, the sensors exhibit a significantly decreased noise behavior, demonstrating a voltage noise threshold below 10-7 V/Hz1/2 at optimal working parameters at 10 Hz during MFC operation. The sensors demonstrate more than two orders of magnitude and one order of magnitude lower noise level in comparison to conventional single layer sensors and multilayer exchange biased sensors respectively. The sensors demonstrate noise limitations only by the thermo-mechanical noise level with application of MFC. Thickness increase of individual ferromagnetic layers and variation of anisotropy alignment was performed to counteract the reduction of the signal output, whilst upholding the low noise behavior. The new sensors with a total ferromagnetic thickness of 4 µm provided an LOD of 50 pT/Hz1/2 at 10 Hz. Within this work also MOKE analysis of sensor behavior during electrical excitation was performed. The effect and mechanism of the electrical excitation on the domain construct is investigated for single layer as well as multilayer (AP)EB systems. Preliminary measurements of such sensors already indicated an LOD of 30 pT/Hz1/2 at 10 Hz. The final part of the dissertation revolves around the minimization of edge effects that plague the sensor designs and form a major noise source in the magnetic phase. To circumvent this effect, an investigation of various patterned edges was employed in order to induce the stress effect complementary to the desired magnetic anisotropy direction. In conclusion, this work provides significant research in the field of magnetic domain manipulation and magnetic domain stabilization that is not limited in applicability to ME sensors. It spans also to other sensors that utilize magnetic layers for the sensing of magnetic fields. Furthermore, the newly developed concepts could be implemented into other devices utilizing magnetic thin film components such as energy harvesters, magnetic shielding and magnetic recordings.Gegenwärtige magnetoelektrische (ME) Kompositsensoren, die auf einem Biegebalkendesign beruhen, besitzen vielversprechende Eigenschaften bezüglich der Anwendung als Magnetfeldsensor zur Detektion biomagnetischer Signale. Trotz der hohen Signalamplitude weisen diese Sensoren Einschränkungen auf, die auf von der ferromagnetischen Phase erzeugten Rauschbeiträgen beruhen. Diese magnetisch induzierten Rauschbeiträge dominieren die Sensorleistung, insbesondere wenn Niederfrequenzsignale mit der sogenannten magnetischen Frequenzumwandlung (MFC) abgetastet werden. Der Fokus der Arbeit liegt daher auf diesem spezifischen Problem, und es werden verschiedene Techniken aufgezeigt, um die von der magnetischen Phase herrührenden Rauschquellen zu unterdrücken. Zu Beginn der Arbeit wurde die Wirksamkeit von mehrschichtigen Exchange-Bias (EB) Systemen untersucht, die bereits zur Reduktion des magnetischen Rauschens eingesetzt werden. Durch eine Verbesserung des Systems konnte eine erhöhte Signalausbeute ermöglicht, jedoch das Rauschverhalten nicht verbessert werden. Aus diesem Grund wurde ein magnetisches Mehrschichtsystem, "Antiparallel Exchange Bias" (APEB), eingeführt. Das neue System wird mittels einer speziellen Temperaturnachbehandlung hergestellt. Das neu entwickelte Kopplungsschema ermöglicht die Bildung einer stabilen magnetischen Konfiguration, welche zu einer signifikanten Reduzierung des magnetischen Rauschens führt. Zusätzlich verbessert das APEB die Nachweisgrenze (LOD) und die Reproduzierbarkeit der Sensoreigenschaften. Die Charakterisierung der Sensoren wurden sowohl elektrisch als auch magnetisch mittels magneto-optischer Kerr-Effekt (MOKE) Mikroskopie durchgeführt. In ihrer endgültigen Form zeigen die Sensoren ein signifikant verringertes Rauschverhalten unter 10- 7 V/Hz1/2 bei 10 Hz bei optimalen Arbeitsparametern während der Anwendung von MFC. Die Sensoren weisen damit einen mehr als zwei Größenordnungen bzw. eine Größen-ordnung niedrigeren Rauschpegel im Vergleich zu herkömmlichen Einzelschicht-sensoren bzw. herkömmlichen mehrschichtigen EB Sensoren auf. Die Performance der Sensoren ist damit auch bei der Verwendung von MFC nur durch das thermomechanische Rauschen begrenzt. Eine Erhöhung der Schichtdicke der einzelnen ferromagnetischen Schichten sowie eine Variation der magnetischen Anisotropieausrichtung wurden durchgeführt, um der einhergehenden Verringerung der Signalamplitude entgegenzuwirken und dabei trotzdem das rauscharme Verhalten beizubehalten. Die Sensoren mit einer ferromagnetischen Gesamtdicke von 4 µm und angeregt mit MFC lieferten ein LOD von 50 pT/Hz1/2 bei 10 Hz. Im Rahmen dieser Arbeit wurden zusätzliche MOKE-Analysen des Sensorverhaltens während elektrischer Modulation durchgeführt. Der Effekt und Mechanismus der elektrischen Anregung auf das Domänenverhalten wurden sowohl für Einzelschicht-systeme als auch für mehrschichtige EB-Systeme untersucht. Für diese Anregungsmethode wurden auch spezielle APEB Sensoren entwickelt. Vorläufige Messungen der Sensoren zeigten bereits ein LOD von 30 pT/Hz1/2 bei 10 Hz. Der letzte Teil der Dissertation befasst sich mit der Minimierung von Randeffekten, die die Hauptstörquellen in der magnetischen Phase bilden. Um diese Effekte zu umgehen, wurde eine Untersuchung verschiedener Kantenstrukturen zur Induktion des Spannungseffektes komplementär zur gewünschten Anisotropieausrichtung durchgeführt. Zusammenfassend liefert diese Arbeit einen bedeutenden Fortschritt auf dem Gebiet der magnetischen Domänenmanipulation und Domänenstabilisierung, die nicht nur auf ME-Sensoren anwendbar ist, sondern auch auf andere Sensortypen, die Magnetschichten zum Erfassen von Magnetfeldern verwenden. Darüber hinaus könnten die neu entwickelten Konzepte in andere Gebieten implementiert werden, die magnetische Dünnschichtkomponenten verwenden. Dazu gehören sogenannte Energy Harvester, magnetische Abschirmungen und magnetische Datenspeicher

Revealing the Pb Whisker Growth Mechanism from Al-Alloy Surface and Morphological Dependency on Material Stress and Growth Environment

Spontaneous metallic Pb whisker formation from Pb and Bi containing Al-alloy’s surfaces is a newly discovered phenomenon. The whiskers display unique formations, growth and morphology, which give the opportunity to be applied for specialized sensor and electronics applications. Within this work, the impact of environmental conditions (gas composition and moisture) is investigated and correlated with the modification of whisker evolution and growth dynamics. Furthermore, the residual stress state of the aluminum matrix using deep cryogenic treatment is modified and used to further increase whisker nucleation and growth by up to three- and seven-fold, respectively, supported by quantitative results. The results of this paper indicate the possibility to manipulate the whisker not only in terms of their kinetics but also their morphology (optimal conditions are 20% O2 and 35% humidity). Such features allow the tailoring of the whisker structure and surface to volume ratio, which can be optimized for different applications. Finally, this research provides new insight into the growth dynamics of the whiskers through in situ and ex situ measurements, providing further evidence of the complex nucleation and growth mechanisms that dictate the spontaneous growth of Pb whiskers from Al-alloy 6026 surfaces with growth velocities up to 1.15 µm/s

Extraordinary Nanocrystalline Pb Whisker Growth from Bi-Mg-Pb Pools in Aluminum Alloy 6026 Moderated through Oriented Attachment

The elucidation of spontaneous growth of metal whiskers from metal surfaces is still ongoing, with the mainstream research conducted on Sn whiskers. This work reports on the discovery of Pb whisker growth from Bi-Mg-Pb solid pools found in common machinable aluminum alloy. The whiskers and hillocks display unique morphologies and complex growth that have not been documented beforehand. In contrast to typical understanding of whisker growth, the presented Pb whiskers show a clear nanocrystalline induced growth mechanism, which is a novel concept. Furthermore, the investigated whiskers are also found to be completely composed of nanocrystals throughout their entire length. The performed research gives new insight into nucleation and growth of metal whiskers, which raises new theoretical questions and challenges current theories of spontaneous metal whisker growth. Additionally, this work provides the first microscopic confirmation of recrystallization growth theory of whiskers that relates to oriented attachment of nanocrystals formed within an amorphous metallic matrix. The impact of mechanical stress, generated through Bi oxidation within the pools, is theoretically discussed with relation to the observed whisker and hillock growth. The newly discovered nanocrystalline growth provides a new step towards understanding spontaneous metal whisker growth and possibility of developing nanostructures for potential usage in sensing and electronics applications

Unravelling the role of nitrogen in surface chemistry and oxidation evolution of deep cryogenic treated high-alloyed ferrous alloy

The role of nitrogen, introduced by deep cryogenic treatment (DCT), has been investigated and unraveled in relation to induced surface chemistry changes and improved corrosion resistance of high-alloyed ferrous alloy AISI M35. The assumptions and observations of the role of nitrogen were investigated and confirmed by using a multitude of complementary investigation techniques with a strong emphasis on ToF-SIMS. DCT samples display modified thickness, composition and layering structure of the corrosion products and passive film compared to a conventionally heat-treated sample under the same environmental conditions. The changes in the passive film composition of a DCT sample is correlated to the presence of the so-called ghost layer, which has higher concentration of nitrogen. This layer acts as a precursor for the formation of green rust on which magnetite is formed. This specific layer combination acts as an effective protective barrier against material degradation. The dynamics of oxide layer build-up is also changed by DCT, which is elucidated by the detection of different metallic ions and their modified distribution over surface thickness compared to its CHT counterpart. Newly observed passive film induced by DCT successfully overcomes the testing conditions in more extreme environments such as high temperature and vibrations, which additionally confirms the improved corrosion resistance of DCT treated high-alloyed ferrous alloys

Influence of deep cryogenic treatment on natural and artificial aging of Al-Mg-Si alloy EN AW 6026

This paper discusses the effect of deep cryogenic treatment (DCT) on the evolution of natural and artificial aging of Al-Mg-Si alloy EN AW 6026. This study provides the first research and evidence of DCT effect on dispersoids and their development in aluminum alloys. DCT induces reformation and regrowth of dispersoids during natural aging from a preferential cuboidal shape to spherical, which is also distinguishable by the change in chemical composition. Additionally, with DCT the dispersoids form in a denser manner, which consequently influences the hardness evolution with aging time. The study also reveals that the exposure duration to DCT (from 24 to 48 h) increases the DCT impact on the hardness evolution during natural aging of selected alloy. The influence of homogenization temperature on the DCT performance is also researched in connection to modified natural aging, which is correlated to the presence and formation of secondary phases during homogenization. Furthermore, DCT also promotes the formation of β'' precipitates and at the same time retards the formation of larger β' precipitates during artificial aging. After artificial aging, both dispersoids and precipitates display a denser population and more elongated shape aligned along the axis of the aluminum matrix with DCT compared to conventionally heat-treated samples without DCT. The microstructural changes during DCT application are strongly linked to the modification of dispersoids with homogenization and artificial aging that influence the content of alloying elements in the matrix

Effect of Deep Cryogenic Treatment on Wear and Galling Properties of High-Speed Steels

New approaches to improving wear resistance with an affordable and noncomplex technology, such as deep cryogenic treatment, (DCT0), are receiving attention. The aim of this study is to investigate the effect of DCT on the friction and wear performance of high-speed steels. AISI M2, AISI M3:2 and AISI M35 were heat-treated under different conditions, and then investigated under dry sliding conditions. Tribological testing involved different contact conditions, prevailing wear mechanisms and loading conditions. The DCT effect on sliding wear resistance depends on HSS steel grade, as well as contact conditions and wear mode, whereas it improves the dynamic impact of the wear and galling resistance

Complex Interdependency of Microstructure, Mechanical Properties, Fatigue Resistance, and Residual Stress of Austenitic Stainless Steels AISI 304L

Stainless steels are important in various industries due to their unique properties and durable life cycle. However, with increasing demands for prolonged life cycles, better mechanical properties, and improved residual stresses, new treatment techniques, such as deep cryogenic treatment (DCT), are on the rise to further push the improvement in stainless steels. This study focuses on the effect of DCT on austenitic stainless steel AISI 304L, while also considering the influence of solution annealing temperature on DCT effectiveness. Both aspects are assessed through the research of microstructure, selected mechanical properties (hardness, fracture and impact toughness, compressive and tensile strength, strain-hardening exponent, and fatigue resistance), and residual stresses by comparing the DCT state with conventionally treated counterparts. The results indicate the complex interdependency of investigated microstructural characteristics and residual stress states, which is the main reason for induced changes in mechanical properties. The results show both the significant and insignificant effects of DCT on individual properties of AISI 304L. Overall, solution annealing at a higher temperature (1080 °C) showed more prominent results in combination with DCT, which can be utilized for different manufacturing procedures of austenitic stainless steels for various applications