Magnetic nanostructures are being developed for use in many aspects of our daily life, spanning areas such as data storage, sensing and biomedicine. Whereas patterned nanomagnets are traditionally two-dimensional planar structures, recent work is expanding nanomagnetism into three dimensions; a move triggered by the advance of unconventional synthesis methods and the discovery of new magnetic effects. In three-dimensional nanomagnets more complex magnetic configurations become possible, many with unprecedented properties. Here we review the creation of these structures and their implications for the emergence of new physics, the development of instrumentation and computational methods, and exploitation in numerous applications.

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Three-dimensional nanomagnetism

This paper provides a review on some of the significant research work done on wearable flexible sensors (WFSs). Sensors fabricated with the flexible materials have been attached to a person along with the embedded system to monitor a parameter and transfer the significant data to the monitoring unit for the further analyses. The use of wearable sensors has played a quite important role to monitor the physiological parameters of a person to minimize any malfunctioning happening in the body. This paper categorizes the work according to the materials used for designing the system, the network protocols, and different types of activities that were being monitored. The challenges faced by the current sensing systems and future opportunities for the WFSs regarding its market values are also briefly explained in this paper.

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Wearable Flexible Sensors: A Review

High-gauge-factor stretchable strain sensors are developed by utilizing a new strategy of thickness-gradient films with high durability, and high uniaxial/isotropic stretchability based on the self-pinning effect of SWCNTs. The monitoring of detailed damping vibration modes driven by weak sound based on such sensors is demonstrated, making a solid step toward real applications.

Thickness-Gradient Films for High Gauge Factor Stretchable Strain Sensors.

Author(s): Streubel, R; Fischer, P; Kronast, F; Kravchuk, VP; Sheka, DD; Gaididei, Y; Schmidt, OG; Makarov, D | Abstract: Extending planar two-dimensional structures into the three-dimensional space has become a general trend in multiple disciplines, including electronics, photonics, plasmonics and magnetics. This approach provides means to modify conventional or to launch novel functionalities by tailoring the geometry of an object, e.g. its local curvature. In a generic electronic system, curvature results in the appearance of scalar and vector geometric potentials inducing anisotropic and chiral effects. In the specific case of magnetism, even in the simplest case of a curved anisotropic Heisenberg magnet, the curvilinear geometry manifests two exchange-driven interactions, namely effective anisotropy and antisymmetric exchange, i.e. Dzyaloshinskii-Moriya-like interaction. As a consequence, a family of novel curvature-driven effects emerges, which includes magnetochiral effects and topologically induced magnetization patterning, resulting in theoretically predicted unlimited domain wall velocities, chirality symmetry breaking and Cherenkov-like effects for magnons. The broad range of altered physical properties makes these curved architectures appealing in view of fundamental research on e.g. skyrmionic systems, magnonic crystals or exotic spin configurations. In addition to these rich physics, the application potential of three-dimensionally shaped objects is currently being explored as magnetic field sensorics for magnetofluidic applications, spin-wave filters, advanced magneto-encephalography devices for diagnosis of epilepsy or for energy-efficient racetrack memory devices. These recent developments ranging from theoretical predictions over fabrication of three-dimensionally curved magnetic thin films, hollow cylinders or wires, to their characterization using integral means as well as the development of advanced tomography approaches are in the focus of this review.

Magnetism in curved geometries

Detection of nanoscale objects is highly desirable in various fields such as early-stage disease diagnosis, environmental monitoring and homeland security. Optical microcavity sensors are renowned for ultrahigh sensitivities due to strongly enhanced light-matter interaction. This review focuses on single nanoparticle detection using optical whispering gallery microcavities and photonic crystal microcavities, both of which have been developing rapidly over the past few years. The reactive and dissipative sensing methods, characterized by light-analyte interactions, are explained explicitly. The sensitivity and the detection limit are essentially determined by the cavity properties, and are limited by the various noise sources in the measurements. On the one hand, recent advances include significant sensitivity enhancement using techniques to construct novel microcavity structures with reduced mode volumes, to localize the mode field, or to introduce optical gain. On the other hand, researchers attempt to lower the detection limit by improving the spectral resolution, which can be implemented by suppressing the experimental noises. We also review the methods of achieving a better temporal resolution by employing mode locking techniques or cavity ring up spectroscopy. In conclusion, outlooks on the possible ways to implement microcavity-based sensing devices and potential applications are provided.

Single Nanoparticle Detection Using Optical Microcavities.

Highly flexible bismuth Hall sensors on polymeric foils are fabricated, and the key optimization steps that are required to boost their sensitivity to the bulk value are identified. The sensor can be bent around the wrist or positioned on the finger to realize an interactive pointing device for wearable electronics. Furthermore, this technology is of great interest for the rapidly developing market of -eMobility, for optimization of eMotors and magnetic bearings.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/adma.201405027

Wearable magnetic field sensors for flexible electronics.

A flexible bismuth-based Hall sensor is realized with a total thickness of only 80 $\mu $ m including encapsulation. After wiring, the sensors have a maximum height of 280 $\mu $ m and could be mounted onto curved surfaces with radii down to 5 mm without degradation of its performance. These flexible sensors allow one to measure time-varying magnetic fields and are applied to realize flux-based control strategies for magnetically suspended systems, which is demonstrated by means of a single-axis magnetic levitation setup.

Flexible Hall Sensorics for Flux-Based Control of Magnetic Levitation

We realized ultra-thin and flexible magnetic field sensors based on the Hall Effect in 2 μm thick Bismuth films prepared on commercial flexible double-layer polyimide PCB with a total thickness of 150 μm. These sensors could be potentially applied in active magnetic bearing (AMB) systems. Therefore, here we address reliability aspects of flexible Hall sensors with emphasis on the proper choice of adhesive to mount the sensor to a stator of the AMB. The thermal shock tests in the standard automotive temperature range of -40°C to 165°C as well as the vibration test of the sensor mounted to the stator has proven the sensor's stability.

Packaging technologies for (Ultra-)thin sensor applications in active magnetic bearings

The invention concerns the field of electrical, materials and mechanical engineering and relates to the use of flexible magnetic thin layer sensor elements, which can be used for measuring magnetic flux density in electromagnetic energy converters and magnetomechanical energy converters. The aim of the invention is to specify the use of flexible magnetic thin layer sensor elements in electric machines and magnetic bearings, which can be placed in air gaps without substantially limiting the air gap widths. Said aim is achieved by the use of at least one flexible magnetic thin layer sensor element, which is attached to non-planar surfaces in the air gap on at least one of the main elements of electromagnetic energy converters and magnetomechanical energy converters and at least partially covers the non-planar surface of at least one of the main elements in the air gap in order to measure the magnetic flux density in the air gap and/or to regulate and/or monitor electromagnetic energy converters and magnetomechanical energy converters.

Use of flexible magnetic thin layer sensor elements

Verwendung von flexiblen magnetischen Dunnschichtsensorelementen Die Erfindung bezieht sich auf die Gebiete der Elektrotechnik, der Werkstofftechnik und des Maschinenbaus und betrifft die Verwendung von flexiblen magnetischen Dunnschichtsensorelementen, welche fur die Messung magnetischer Feldgrosen, vorzugsweise der magnetischen Felddichte, in elektromagnetischen Energiewandlern und magnetomechanischen Energiewandlern eingesetzt werden konnen. Aufgabe der vorliegenden Losung ist es, die Verwendung von flexiblen magnetischen Dunnschichtsensorelementen in elektrischen Maschinen und Magnetlagern anzugeben, welche in den Luftspalten platziert werden konnen, ohne die Luftspaltbreite wesentlich zu begrenzen. Die Aufgabe wird gelost durch die Verwendung von mindestens einem flexiblen magnetischen Dunnschichtsensorelement, das auf nichtplanaren Oberflachen im Luftspalt elektromagnetischer Energiewandler und magnetomechanischer Energiewandler angebracht ist und die nichtplanare Oberflache im Luftspalt mindestens teilweise bedeckt, zur Messung von magnetischen Feldgrosen im Luftspalt.

Falk Bahr

Papers

Wearable magnetic field sensors for flexible electronics.

Flexible Hall Sensorics for Flux-Based Control of Magnetic Levitation

Packaging technologies for (Ultra-)thin sensor applications in active magnetic bearings

Use of flexible magnetic thin layer sensor elements

Verwendung von flexiblen magnetischen dünnschichtsensorelementen