ΔE-Effect Magnetic Field Sensors

Many conceivable biomedical and diagnostic applications require the detection of small-amplitude and low-frequency magnetic fields. Against this background, a magnetometer concept is investigated in this work based on the magnetoelastic ΔE effect. The ΔE effect causes the resonance frequency of a magnetoelastic resonator to detune in the presence of a magnetic field, which can be read-out electrically with an additional piezoelectric phase. Various microelectromechanical resonators are experimentally analyzed in terms of the ΔE effect and signal-and-noise response. This response is highly complex because of the anisotropic and nonlinear coupled magnetic, mechanical, and electrical properties. Models are developed and extended where necessary to gain insights into the potentials and limits accompanying sensor design and operating parameters. Beyond the material and geometry parameters, we analyze the effect of different resonance modes, spatial property variations, and operating frequencies on sensitivity. Although a large ΔE effect is confirmed in the shear modulus, the sensitivity of classical cantilever resonators does not benefit from this effect. An approach utilizing surface acoustic shear-waves provides a solution and can detect small signals over a large bandwidth. Comprehensive analyses of the quality factor and piezoelectric material parameters indicate methods to increase sensitivity and signal-to-noise ratio significantly. First exchange-biased ΔE-effect sensors pave the way for compact setups and arrays with a large number of sensor elements. With an extended signal-and-noise model, specific requirements are identified that could improve the signal-to-noise ratio. The insights gained lead to a new concept that can circumvent previous limitations. With the results and models, important contributions are made to the understanding and development of ΔE-effect sensors with prospects for improvements in the future

Magnetoelectric Sensor Systems and Applications

In the field of magnetic sensing, a wide variety of different magnetometer and gradiometer sensor types, as well as the corresponding read-out concepts, are available. Well-established sensor concepts such as Hall sensors and magnetoresistive sensors based on giant magnetoresistances (and many more) have been researched for decades. The development of these types of sensors has reached maturity in many aspects (e.g., performance metrics, reliability, and physical understanding), and these types of sensors are established in a large variety of industrial applications. Magnetic sensors based on the magnetoelectric effect are a relatively new type of magnetic sensor. The potential of magnetoelectric sensors has not yet been fully investigated. Especially in biomedical applications, magnetoelectric sensors show several advantages compared to other concepts for their ability, for example, to operate in magnetically unshielded environments and the absence of required cooling or heating systems. In recent years, research has focused on understanding the different aspects influencing the performance of magnetoelectric sensors. At Kiel University, Germany, the Collaborative Research Center 1261 “Magnetoelectric Sensors: From Composite Materials to Biomagnetic Diagnostics”, funded by the German Research Foundation, has dedicated its work to establishing a fundamental understanding of magnetoelectric sensors and their performance parameters, pushing the performance of magnetoelectric sensors to the limits and establishing full magnetoelectric sensor systems in biological and clinical practice

Modeling of Magnetoelectric Microresonator Using Numerical Method and Simulated Annealing Algorithm

A comprehensive understanding of the linear/nonlinear dynamic behavior of wireless microresonators is essential for micro-electromechanical systems (MEMS) design optimization. This study investigates the dynamic behaviour of a magnetoelectric (ME) microresonator, using a finite element method (FEM) and machine learning algorithm. First, the linear/nonlinear behaviour of a fabricated thin-film ME microactuator is assessed in both the time domain and frequency spectrum. Next, a data driven system identification (DDSI) procedure and simulated annealing (SA) method are implemented to reconstruct differential equations from measured datasets. The Duffing equation is employed to replicate the dynamic behavior of the ME microactuator. The Duffing coefficients such as mass, stiffness, damping, force amplitude, and excitation frequency are considered as input parameters. Meanwhile, the microactuator displacement is taken as the output parameter, which is measured experimentally via a laser Doppler vibrometer (LDV) device. To determine the optimal range and step size for input parameters, the sensitivity analysis is conducted using Latin hypercube sampling (LHS). The peak index matching (PIM) and correlation coefficient (CC) are considered assessment criteria for the objective function. The vibration measurements reveal that as excitation levels increase, hysteresis variations become more noticeable, which may result in a higher prediction error in the Duffing array model. The verification test indicates that the first bending mode reconstructs reasonably with a prediction accuracy of about 92 percent. This proof-of-concept study demonstrates that the simulated annealing approach is a promising tool for modeling the dynamic behavior of MEMS systems, making it a strong candidate for real-world applications

Exchange biased delta-E effect enables the detection of low frequency pT magnetic fields with simultaneous localization

Delta-E effect sensors are based on magnetoelectric resonators that detune in a magnetic field due to the delta-E effect of the magnetostrictive material. In recent years, such sensors have shown the potential to detect small amplitude and low-frequency magnetic fields. Yet, they all require external magnetic bias fields for optimal operation, which is highly detrimental to their application. Here, we solve this problem by combining the delta-E effect with exchange biased multilayers and operate the resonator in a low-loss torsion mode. It is comprehensively analyzed experimentally and theoretically using various kinds of models. Due to the exchange bias, no external magnetic bias fields are required, but still low detection limits down to [Formula: see text] at 25 Hz are achieved. The potential of this concept is demonstrated with a new operating scheme that permits simultaneous measurement and localization, which is especially desirable for typical biomedical inverse solution problems. The sensor is localized with a minimum spatial resolution of 1 cm while measuring a low-frequency magnetic test signal that can be well reconstructed. Overall, we demonstrate that this class of magnetic field sensors is a significant step towards first biomedical applications and compact large number sensor arrays

Signal-to-noise ratio enhanced electrode configurations for magnetoelectric cantilever sensors

Magnetoelectric cantilevers consisting of strain-coupled magnetostrictive and piezoelectric (PE) layers are applicable to magnetic-fi eld sens- ing. For the fi rst bending mode, the magnetic fi eld-induced stress distribution is of equal sign along the cantilever length. Thus, a plate- capacitor electrode configuration encompassing the complete PE layer may be used for collecting the strain-induced charge. For higher order modes, stress regions of the opposite sign occur in the cantilever length direction. To prevent charge cancellation and to harvest the piezo- electric induced charge effi ciently, segmented electrodes are employed. This study investigates the effect of the electrode confi gurationon the signal-to-noise ratio (SNR) for higher order bending modes. The charges collected by the electrodes are calculated using a fi nite element method simulation considering the mechanical, electrical, and magnetic properties of the cantilever. By combination with an analytic noise model, taking into account the sensor and amplifi er noise sources, the SNR is obtained. We analyze a 3 mm long, 1 mm wide, and 50 μm thick silicon cantilever with layers of 2 μm magnetostrictive soft amorphous metal (FeCoSiB) and 2 μm piezoelectric aluminum nitride. We demonstrate that an SNR-optimized electrode design yields an SNR improvement by 2.3 dB and 2.4 dB for the second and third bending modes compared to a signal optimized design

Imaging of Magnetic Nanoparticles using Magnetoelectric Sensors

Imaging of magnetic nanoparticles offers a variety of promising medical applications for therapeutics and diagnostics. Using magnetic nanoparticles as tracer material for imaging allows for the non-invasive detection of spatial distributions of nanoparticles that can give information about diseases or can be used in preventive medicine. Imaging biodistributions of magnetically labeled cells offers applicability for tissue engineering, as a means to monitor cell growth within artificial scaffolds non-destructively. In the presented work, the capabilities of an imaging system for magnetic nanoparticles via magnetoelectric sensors are investigated. The investigated technique, called Magnetic Particle Mapping, is based on the detection of the nonlinear magnetic response of magnetic nanoparticles. A resonant magnetoelectric sensor is used for frequency selective measurements of the nanoparticles magnetic response. Extensive modeling was performed that enabled proper imaging of magnetic nanoparticle distributions. Fundamental limitations of the imaging system were derived to describe resolution in correspondence to signal-to-noise ratios. Incorporation of additional parameters in the imaging system for the data analysis resulted in an algorithm for a more robust reconstruction of spatial particle distributions, increasing its imaging capabilities. Experimental investigations of the imaging system show the capabilities for imaging of cell densities using magnetically labeled cells. Furthermore, resolution limitations were investigated and differentiation of different particle types in imaging was shown, referred to as ”colored” imaging. The imaging of biodistributions of magnetically labeled cells thus enable exciting perspectives on further research and possible applications in tissue engineering

Charakterisierung funktionaler Nanomaterialien für biomagnetische Sensoren und Atemanalyse

The presented thesis is covering materials aspects for the development of magnetoelectric sensors for biomagnetic sensing and solid state sensors for breath monitoring. The electrophysiological signals of the human body and especially their irregularities provide extremely valuable information about the heart, brain or nerve malfunction in medical diagnostics. Similar and even more detailed information is contained in the generated biomagnetic fields which measurement offers improved diagnostics and treatment of the patients. A new type of room temperature operable magnetoelectric composite sensors is developed in the framework of the CRC1261 Magnetoelectric Sensors: From Composite Materials to Biomagnetic Diagnostics. This thesis focuses on the individual materials structure-property relations and their combination in magnetoelectric composite sensors studied by electron beam based techniques, at lengths scales ranging from micrometers to atomic resolution. The first part of this thesis highlights selected studies on the structural and analytic aspects of single phase materials and their composites using TEM as the primary method of investigation. With respect to the piezoelectric phase, alternatives to AlN have been thoroughly investigated to seek for improvement of specific sensor approaches. In this context, the alloying of Sc into the AlN matrix has been demonstrated to yield high quality films with improved piezoelectric and unprecedented ferroelectric properties grown under the control of deposition parameters. Lead-free titanate films with large piezo-coefficients at the verge of the morphotropic phase boundary as alternative to PZT films have been investigated in terms of crystal symmetry, defect structure and domains of cation ordering. New morphologies of ZnO and GaN semiconductors envisioned for a piezotronic-based sensor approach were subject of in-depth defect and analytical studies describing intrinsic defects and lattice strains upon deposition as well as hollow composite structures. When the dimensions of a materials are reduced, novel exciting properties such as in-plane piezoelectricity can arise in planar transition-metal dichalcogenides. Here, the turbostratic disorder in a few-layered MoSe2 film has been investigated by nanobeam electron diffraction and Fast Fourier Transformations. From the perspective of magnetic materials, the atomic structure of magnetostrictive multilayers of FeCo/TiN showing stability up to elevated temperatures has been analyzed in detail regarding the crystallographic relationship of heteroepitaxy in multilayer composites exhibiting individual layer thicknesses below 1 nm. Further, magnetic hard layers have been investigated in the context of exchange spring concepts and ME composites based on shape memory alloy substrates have been studied regarding structural changes implied by different annealing processes. The second part of this thesis introduces materials aspects and sensor studies on gas detection in the clinical context of breath analysis. The detection of specific vapors in the human breath is of medical relevance, since certain species can be enriched depending on the conditions and processes within the human body. Hence, they can be regarded as biomarkers for the patients condition of health. The selection of suitable materials and the gas measurement working principle are considered and selected studies on solid state sensors with different surface functionalization or targeted application on basis of ZnO or CuO-oxide and Fe-oxide species are presented

Next Generation Acoustic and Magnetic Devices for Radio Frequency Communication

This dissertation primarily focuses on utilizing low wave speed acoustic waves coupled with electromagnetics to increase performance of radio frequency front end architectures and reduce device dimensions. Chapter 1 begins with the history of communication technology beginning with Maxwell’s equations. Next brief introductions into the piezoelectricity, magnetism, and multiferroics are given to lay the groundwork for the following Chapters.Chapter 2 of this dissertation aims at improving the capability of communicating in lossy RF-denied media such as seawater. First, magnetic antennas are theoretically analyzed and compared to electric antennas showing that magnetic antennas perform better when surrounded by lossy conductive media. Next, a prototype multiferroic antenna is developed that uses piezoelectric PZT and magnetostrictive FeGa. The PZT applies a time varying stress to the FeGa causing the FeGa’s internal flux density to dynamically vary resulting in a time-varying magnetic near field. Magnetic near field measurements are compared to an analytical model showing good agreement.In Chapter 3 Lamb wave devices are investigated for filtering and frequency conversion applicationsin RF-front ends. Leveraging micro-fabrication techniques two Lamb wave delay lines are fabricated out of piezoelectric aluminum nitride (AlN). Interdigitated transducers (IDTs) are used to launch and receive Lamb waves as well as generate a time and space varying mechanical compliance. A circuit model is developed to compare to the experimental results and determine the magnitude of the compliance nonlinearity present in the AlN. Results show that acoustic devices can be developed that simultaneously filter and down-convert or up-convert a signal.Chapter 4 numerically analyzes strain tunable magnetic filters for applications in software defined radio and cognitive radio. For these applications filters with a tunable bandpass are necessary. The design relies on two CoFeB ellipses deposited on piezoelectric PMN-PT. An electric field is applied through the thickness of the PMN-PT resulting in a strain applied to the CoFeB ellipses. The electric field can be applied to either strain one ellipse or both ellipses. Straining both ellipses results in a tunable susceptibility from 6 GHz to 8 GHz, while straining only one ellipse results in a broadening of the bandpass response. These results show a potential solution for dynamic filters for next generation communication architectures

Untersuchung von Magnetostriktiven und Piezotronischen Mikrostrukturen und Materialien für biomagnetische Sensoren mittels Röntgenstrahlen

Detecting electric potential differences from the human physiology is an established technique in medical diagnosis, e.g., as electrocardiogram. It arises from a changing electrical polarization of living cells. Simultaneously, biomagnetism is induced and can be utilized for medical examinations, as well. Benefits in using magnetic signals are, no need for direct skin contact and an increased spatial resolution, e.g., for mapping brain activity, especially in combination with electrical examinations. But biomagnetic signals are very weak and, thus, highly sensitive devices are necessary. The development of small and easy to use biomagnetic sensors, with a sufficient sensitivity, is the goal of the Collaborative Research Centre 1261 - Magnetoelectric Sensors: From Composite Materials to Biomagnetic Diagnostics. This thesis was written as part of this collaboration, with the main focus on the investigation of crystalline structures and structure related properties of piezotronic and magnetostrictive materials by utilizing a selection of X-ray techniques, i.e., X-ray diffraction (XRD), X-ray reflectivity (XRR) and coherent X-ray diffraction imaging (CXDI). Piezotronics, realized by combining piezoelectricity and Schottky contacts in one structure, provides a promising path to enhance sensor sensitivity. A first study investigated the crystalline structure of three piezotronic ZnO rods, spatially resolved by scanning nano XRD and combined with electrical examinations of their Schottky contact properties. It is found that the crystalline quality has a clear impact on the electrical properties of the related Schottky contact, probably due to crystalline defects. A complementary transmission electron microscopy (TEM) and XRD study performed on hybride vapor phase epitaxy (HVPE) grown GaN showed a slight, photoelectrochemical etching related relaxion of strain originating from crystal growth. In a separate study, CXDI was utilized for three-dimensional visualization of strain in a gold coated ZnO rod, with spatial resolution below 30 nm. A distinct strain distribution was found inside the rod, denoted to depletion and screening effects occurring in bent piezotronic structures, and a high strain at the interface may be related to Schottky contact formation. This interface strain agrees with results obtained from TEM. A succeeding CXDI study was conducted on a ZnO rod coated with magnetostrictive FeCoSiB and the possibility for the investigation of the Schottky contacts electrical properties. It was found that FeCoSiB sputtered on ZnO results in an ohmic contact and that an external magnetic field causes a change of the electrical properties, probably due to a strain change, visualized by CXDI. In a fifth study, magnetostrictive FeCo/TiN multilayer structures were investigated by a combined TEM and XRD/XRR approach, showing a relaxation of the structure due to an annealing process and a cube-on-cube structure of the FeCo and TiN layers

Auslesemethoden für magnetoelektrische Sensoren

The detection of weak magnetic fields has the potential to provide additional, non-redundant information in scientific fields such as medical diagnostics, geomagnetic investigations, data storage, amongst others. Many substances feature a low permeability and magnetic fields can penetrate them nearly unhindered which yields the possibility to detect signals that originate from within a volume without contact. Thin-film magnetoelectric sensors are mm-sized magnetometers that transform magnetic fields into a measurable polarisation via a mechanical coupling of a magnetostrictive and a piezoelectric layer. They do not need to be cooled and their high dynamic range allows them to be operated in unshielded environments. The output signal of the cantilever-shaped sensors is enhanced at their resonance frequency which can be exploited to increase the signal-to-noise ratio. This dissertation treats the signal processing for thin-film magnetoelectric sensors from a system point of view. Four main readout methods are investigated, modelled, and evaluated with the aim to lower the limit of detection: The direct detection, magnetic frequency conversion, electric frequency conversion, and a completely novel method utilising the sensor as a microwave resonator. With the focus on the signal-to-noise ratio, the noise sources of the measurement systems are discussed in depth and the dominant noise sources identified. The ultimate noise limit is given by the thermal-mechanical noise of the sensors. Acoustic environmental interference can be reduced with a tuning fork assembly that discriminates magnetic and mechanical excitation of two cantilevers clamped face-to-face. The best limit of detection for thin-film magnetoelectric sensors at 10 Hz is 50 pT/Hz^1/2 achieved with the magnetic frequency conversion leading the way towards measurements of biomagnetic signals.Die Detektion von schwachen magnetischen Feldern hat das Potential in Forschungsfeldern wie der medizinischen Diagnostik, geomagnetischen Untersuchungen, Datenspeicherung, usw. zusätzliche, nicht redundante Informationen verfügbar zu machen. Viele Stoffe haben eine geringe Permeabilität und sind daher für Magnetfelder nahezu uneingeschränkt durchlässig. Dadurch entsteht die Möglichkeit tieferliegende Signale kontaktlos zu detektieren. Dünnfilm magnetoelektrische Sensoren sind millimeter große Magnetometer, welche magnetische Felder über die mechanische Kopplung eines magnetostriktiven und eines piezoelektrischen Materials in eine messbare Polarisation transformieren. Die Sensoren brauchen nicht gekühlt werden und weisen einen hohen Dynamikbereich auf, wodurch sie in ungeschirmten Umgebungen betrieben werden können. In der mechanischen Resonanzfrequenz der balkenförmigen Sensoren wird deren Ausgangssignal erhöht, wodurch das Signal-zu-Rausch Verhältnis verbessert wird. In dieser Dissertation wird die Signalverarbeitung von diesen Sensoren aus Systemsicht behandelt. Mit dem Ziel das Detektionslimit zu senken werden vier Ausleseverfahren untersucht, modelliert und bewertet: Die direkte Detektion, die magnetische Frequenzumsetzung, die elektrische Frequenzumsetzung und ein komplett neues Verfahren, in dem der Biegebalken als Mikrowellen Resonator verwendet wird. Mit dem Fokus auf dem Signal-zu-Rausch Verhältnis werden die einzelnen Rauschquellen der Messsysteme diskutiert und die jeweils dominanten Rauschquellen ermittelt. Das grundlegende Rauschlimit ist das thermisch-mechanische Rauschen des Sensors. Akustische Umweltstörungen können mit einer Stimmgabelanordnung bestehend aus zwei Einzelsensoren unterdrückt werden, welche magnetische und mechanische Einkopplungen bauartbedingt unterscheiden kann. Als bestes Detektionslimit für dünnfilm magnetoelektrische Sensoren bei 10 Hz wird 50 pT/Hz^1/2 gemessen