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

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

Converse Magnetoelectric Resonators for Biomagnetic Field Sensing

Contact-less biomagnetic sensing constitutes the next frontier for advanced healthcare, bringing novel diagnostic abilities using multichannel magnetocardiography (MCG) and magnetoencephalography (MEG) either as a single source of information for rapid patient screening or in combination with established methods such as electrocardiography (ECG) and electroencephalography (EEG) as a source for additional patient information. The combination of established electrical with magnetic patient information potentially leads to novel tools for deep knowledge generation towards pathologies and early prevention of such. The main obstacle towards biomagnetic diagnosis using magnetic imaging techniques is the lack of easy applicable sensor technology which offers extremely low magnetic noise floors; realtime MCG measurements demand for lower than 10 pT/sqrt(Hz), reaching below 100 fT/sqrt(Hz) enables even MEG signal acquisition. Such extremely minute amplitudes that are six to seven orders lower than earth's permanent magnetic field, demand lowest noise sensor technology as the low frequency signal regime below about 1 kHz is strongly affected by omnipresent 1/f-noise. Magnetoelectric (ME) thin film composites consisting of a sputtered piezoelectric (PE) and an amorphous magnetostrictive (MS) layer are usually employed for measurements of magnetic fields passively, i.e. an AC magnetic field directly generates an ME voltage by mechanical coupling of the MS deformation to the PE phase. In order to achieve high field sensitivities, a magnetic bias field is required to operate at the maximum piezomagnetic coefficient of the MS phase. Additionally using mechanical resonances further enhances this direct ME effect size. Despite being able to directly detect very small field amplitudes on the order of 1 pT/sqrt(Hz) for magnetic fields of a frequency exactly matching mechanical resonances comes at the expense of available signal bandwidth, because of rather high resonator quality factors. Strong 1/f noise prevalent in the low frequency regime, makes DC or low frequency magnetic fields tedious to record in that regime using direct ME detection scheme. In the presented work the PE phase is actively excited, thus exploiting the converse ME effect, remedying the shortcomings of the direct effect. ME composites are demonstrated for use as precision sensors, capable of magnetic signal detection in the low frequency, low amplitude biomagnetic regime. The combination of the converse ME effect with high frequency acoustic resonances leads to high piezoelectric stresses generated within the composite, leading to large inverse magnetostriction and thus high sensitivity. A limit of detection (LOD) of 70 pT/sqrt(Hz) at 10 Hz is obtained with composites based on amorphous films of Iron-Cobalt-Silicon-Boron (FeCoSiB). Exploiting advanced magnetoelectric composites based on exchange biased FeCoSiB films (EB-FeCoSiB) LOD values reaching down to 17 pT/sqrt(Hz) at 10 Hz are demonstrated. A trial recording a healthy subjects human MCG signal using an advanced ME composite demonstrates the practical feasibility of biomagnetic measurements and paves the way for routine, realtime biomagnetic measurements in the future.Kontaktlose biomagnetische Diagnostik stellt die nächste Generation von Patientenmonitoring und bildgebender Diagnostik dar, sie ist in der Lage einen schnellen, kontaktlosen Überblick der Vitalfunktionen zu liefern. In Kombination mit etablierten Methoden wie Elektrokardiografie (EKG) und Elektroenzephalografie (EEG) entsteht ein zusätzliches Werkzeug zur Erlangung tieferer Informationen über Pathogenesen und ermöglichen somit eine frühzeitige Erkennung solcher. Die größte technische Hürde der biomagnetischen Diagnose stellt die Entwicklung einer anwenderfreundlichen, wartungsarmen Sensortechnologie dar. Diese Technologie muss über ein extrem niedriges magnetisches Rauschen von kleiner als 10 pT/sqrt(Hz) für Echtzeit Magnetokardiografie (MKG) und bis unter 100 fT/sqrt(Hz) für Magnetoenzephalografie (MEG) verfügen. Derartige Feldstärken von biomagnetischem Niveau sind etwa sechs bis sieben Größenordnungen geringer als das statische Erdmagnetfeld und dabei ebenfalls stets niederfrequent, unterhalb etwa 1 kHz. Damit liegen die relevanten Magnetfelder im Bereich des omnipräsenten 1/f-Rauschens. Magnetoelektrische Dünnschicht-Komposite werden üblicherweise passiv betrieben, indem ein magnetisches Wechselfeld direkt zu einer proportionalen ME-Spannung führt. Dies geschieht mittels magnetostriktiver Dehnung welche durch mechanische Kopplung auf ein Piezoelektrikum übertragen wird und dort eine elektrische Spannung über den direkten piezoelektrischen Effekt erzeugt. Um den größtmöglichen piezomagnetischen Koeffizienten zu erhalten, kommt zusätzlich ein statisches magnetisches Haltefeld zum Einsatz. Durch die Ausnutzung mechanischer Resonanzen wird die Oszillation verstärkt, diese Verstärkung führt in gleichem Maße zu einer Verstärkung des ME-Effekts. Auf diese Weise ist es möglich, magnetische Detektionsgrenzen von etwa 1 pT/sqrt(Hz) zu erreichen, weit im erforderlichen Bereich für Echtzeit MKG Messungen. Diese direkte Ausnutzung mechanischer Resonanzen von hohem Gütefaktor, bringt den wesentlichen Nachteil, dass die Bandbreite des ME Oszillators auf wenige Herz beschränkt ist, welches einer praktischen, breitbandigen Signalerfassung entgegen steht. In dieser Arbeit wird die piezoelektrische Materialphase direkt elektrisch angeregt, es wird der inverse ME-Effekt ausgenutzt. Dieser inverse ME Effekt stellt sich als vorteilhaft im Bezug auf den direkten ME-Effekt heraus, da eine rauscharme Operation ermöglicht wird. Magnetoelektrische Dünnschicht-Komposite werden als Präzisionssensoren zur Detektion von niederfrequenten magnetischen Kleinstsignalen untersucht. Die Kombination aus inversem ME-Effekt und der Ausnutzung hochfrequenter mechanischer Oszillationen führt zu starken mechanischen Verspannungen in der magnetostriktiven Phase und dadurch zu hoher Empfindlichkeit des Sensor-Komposites. Eine Detektionsgrenze von 70 pT/sqrt(Hz) bei einer Frequenz von 10 Hz wird unter Verwendung von magnetostriktiven Einfachlagen erreicht. Die Verwendung fortgeschrittener Mehrlagen-Materialsysteme führt zu einer weiteren Verringerung der Detektionsgrenze auf 17 pT/sqrt(Hz) bei 10 Hz. Schließlich wird in einer Feldstudie am gesunden Probanden eine Machbarkeit zur Detektion humaner MKG Signale gezeigt

Master of Science

thesisThe design, working principle, fabrication, and characterization of ultrasensitive ferromagnetic and magnetoelectric magnetometer are discussed in this thesis. Different manufacturing techniques and materials were used for the fabrication of the two versions of the magnetometer. The ferromagnetic microelectromechanical systems (MEMS) magnetometer was fabricated using low-pressure chemical vapor deposition (LPCVD) of silicon nitride, yielding low compressive stress, followed by patterning. The built-in stress was found to be -14 Mpa using Tencor P-10 profilometer. A neodymium magnet (NdFeB) was used as a foot-mass to increase the sensitivity of the device. A coil (Ø=3 cm), placed at a distance from the sensor (2.5-15 cm), was used to produce the magnetic field. The response of the ferromagnetic MEMS magnetometer to the AC magnetic field was measured using Laser-Doppler vibrometer. The ferromagnetic sensor's average temperature sensitivity around room temperature was 11.9 pV/pT/-C, which was negligible. The resolution of the ferromagnetic sensor was found to be 27 pT (1 pT = 10-12 T). To further improve the sensitivity and eliminate the use of the optical detection method, we fabricated a Lead Zirconate titanate (PZT) based magnetoelectric sensor. The sensor structure consisted of a 9 mm long, and 0.17 mm thick PZT beam of varying widths. A neodymium permanent magnet was used as a foot-mass in this case as well. The magnetic field from the coil generated a driving force on the permanent magnet. The driving force displaced the free end of the PZT beam and generated a proportional voltage in the PZT layer. The magnetoelectric coupling, i.e., the coupling between mechanical and magnetic field, yielded a sensor resolution of ~40 fT (1 fT = 10-15 T); an improvement by three orders of magnitude. We used high permeability Mu sheets (0.003"") attached to copper plates (0.125"") to shield stray magnetic fields around the sensor. For both the ferromagnetic MEMS and the magnetoelectric magnetometer, the initial output was improved by using external bias and parametric amplification. By applying an external DC magnetic field bias to the sensor, the effective spring compliance of the sensor was modified. Electronic feedback reduced the active noise limiting the sensor's sensitivity. We used magnetic coupling to enhance the sensors' sensitivity and to reduce the electronic noise. Two identical sensors, with identical foot-mass (permanent magnet), was used to show coupling. The magnet on one of the sensors was mounted in NS polarity, whereas, on the other it was in SN polarity. When excited by the same external AC magnetic field (using coil), one of the sensors was pulled towards the coil and the other was pushed away from it. Adding the individual sensor output, using a preamplifier, an overall increase in sensors' output was observed. The techniques mentioned above helped to improve the output from the sensor. The sensitivity of the sensor can be improved further by using a 3-axis magnetic field cancellation system, by eliminating the AC and DC stray magnetic field, by using coupled-mode resonators and by increasing the surface field intensity of the foot-mass. The magnetometers, thus, developed can be used for mapping the magnetic print of the brain

Anisotropic behaviour of magneto-electric coupling in multiferroic composites

The anisotropy of the direct magnetoelectric effect in textured nickel ferrite/lead zirconate titanate strain mediated bilayer composites has been studied. The magnetic layers of these samples have been crystallographically textured in planes of the form {100}, {110} and {111}. In this study, it is shown that the optimum bias field and the maximum magnetoelectric coupling signal can be controlled by changing the alignment of the applied magnetic field with respect to the magnetocrystalline anisotropy directions. It is also shown that the product of the optimum bias field and the maximum magnetoelectric coupling signal are proportional to the theoretical saturation magnetostriction. The samples have been magnetically characterised using a recommissioned and developed biaxial vibrating sample magnetometer, capable of detecting the component of a sample’s magnetic moment in 2 perpendicular directions and thus determining the net magnetic moment vector of the sample. Coupled with sample rotation this allows insight into the magnetic anisotropy of the sample, which has been compared with a micromagnetic model. A specialist magnetoelectric coupling rig has also been developed to allow application of DC and AC magnetic fields to a sample simultaneously. As part of the magnetic anisotropy study, a modified torque magnetometry method has been developed to enhance the identification of the anisotropy directions in magnetically soft samples, as well as a method by which torque magnetometry can be approximated using the in-field direction component of magnetisation as measured using a standard vibrating sample magnetometer

Gradient metasurfaces: a review of fundamentals and applications

In the wake of intense research on metamaterials the two-dimensional analogue, known as metasurfaces, has attracted progressively increasing attention in recent years due to the ease of fabrication and smaller insertion losses, while enabling an unprecedented control over spatial distributions of transmitted and reflected optical fields. Metasurfaces represent optically thin planar arrays of resonant subwavelength elements that can be arranged in a strictly or quasi periodic fashion, or even in an aperiodic manner, depending on targeted optical wavefronts to be molded with their help. This paper reviews a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised to exhibit spatially varying optical responses resulting in spatially varying amplitudes, phases and polarizations of scattered fields. Starting with introducing the concept of gradient metasurfaces, we present classification of different metasurfaces from the viewpoint of their responses, differentiating electrical-dipole, geometric, reflective and Huygens' metasurfaces. The fundamental building blocks essential for the realization of metasurfaces are then discussed in order to elucidate the underlying physics of various physical realizations of both plasmonic and purely dielectric metasurfaces. We then overview the main applications of gradient metasurfaces, including waveplates, flat lenses, spiral phase plates, broadband absorbers, color printing, holograms, polarimeters and surface wave couplers. The review is terminated with a short section on recently developed nonlinear metasurfaces, followed by the outlook presenting our view on possible future developments and perspectives for future applications.Comment: Accepted for publication in Reports on Progress in Physic