Printable giant magnetoresistive devices

Printed magnetoresistive sensors for recyclable magnetoelectronics

We have developed an innovative recyclable printed magnetoresistive sensor using GMR microflakes and AMR microparticles as functional fillers, with PECH as the elastomer binder. Under saturation magnetic fields of 100 mT and 30 mT, these sensors respectively exhibit magnetoresistance values of 4.7% and 0.45%. The excellent mechanical properties and thermal stability of the PECH elastomer binder endow these sensors with outstanding flexibility and temperature stability. This flexibility, low cost, and scalability make these sensors highly suitable for integration into flexible electronic devices, such as smart security systems and home automation. Moreover, these sensors are fully recyclable and reusable, allowing the materials to be separated, reused, and remanufactured without loss of performance. The low energy consumption of the production process and the recyclability of the materials significantly reduce the environmental impact of these magnetic field sensors.

Advances in Flexible Magnetosensitive Materials and Devices for Wearable Electronics

Emerging fields, such as wearable electronics, digital healthcare, the Internet of Things, and humanoid robots, highlight the need for flexible devices capable of recording signals on curved surfaces and soft objects. In particular, flexible magnetosensitive devices garner significant attention owing to their ability to combine the advantages of flexible electronics and magnetoelectronic devices, such as reshaping capability, conformability, contactless sensing, and navigation capability. Several key challenges must be addressed to develop well-functional flexible magnetic devices. These include determining how to make magnetic materials flexible and even elastic, understanding how the physical properties of magnetic films change under external strain and stress, and designing and constructing flexible magnetosensitive devices. In recent years, significant progress is made in addressing these challenges. This study aims to provide a timely and comprehensive overview of the most recent developments in flexible magnetosensitive devices. This includes discussions on the fabrications and mechanical regulations of flexible magnetic materials, the principles and performances of flexible magnetic sensors, and their applications for wearable electronics. In addition, future development trends and challenges in this field are discussed.

Self-healable printed magnetic field sensors using alternating magnetic fields

We employ alternating magnetic fields (AMF) to drive magnetic fillers actively and guide the formation and self-healing of percolation networks. Relying on AMF, we fabricate printable magnetoresistive sensors revealing an enhancement in sensitivity and figure of merit of more than one and two orders of magnitude relative to previous reports. These sensors display low noise, high resolution, and are readily processable using various printing techniques that can be applied to different substrates. The AMF-mediated self-healing has six characteristics: 100% performance recovery; repeatable healing over multiple cycles; room-temperature operation; healing in seconds; no need for manual reassembly; humidity insensitivity. It is found that the above advantages arise from the AMF-induced attraction of magnetic microparticles and the determinative oscillation that work synergistically to improve the quantity and quality of filler contacts. By virtue of these advantages, the AMF-mediated sensors are used in safety application, medical therapy, and human-machine interfaces for augmented reality.

Giant Magnetoresistance Biosensors in Biomedical Applications

The giant magnetoresistance (GMR) effect has seen flourishing development from theory to application in the last three decades since its discovery in 1988. Nowadays, commercial devices based on the GMR effect, such as hard-disk drives, biosensors, magnetic field sensors, microelectromechanical systems (MEMS), etc., are available in the market, by virtue of the advances in state-of-the-art thin-film deposition and micro- and nanofabrication techniques. Different types of GMR biosensor arrays with superior sensitivity and robustness are available at a lower cost for a wide variety of biomedical applications. In this paper, we review the recent advances in GMR-based biomedical applications including disease diagnosis, genotyping, food and drug regulation, brain and cardiac mapping, etc. The GMR magnetic multilayer structure, spin valve, and magnetic granular structure, as well as fundamental theories of the GMR effect, are introduced at first. The emerging topic of flexible GMR for wearable biosensing is also included. Different GMR pattern designs, sensor surface functionalization, bioassay strategies, and on-chip accessories for improved GMR performances are reviewed. It is foreseen that combined with the state-of-the-art complementary metal-oxide-semiconductor (CMOS) electronics, GMR biosensors hold great promise in biomedicine, particularly for point-of-care (POC) disease diagnosis and wearable devices for real-time health monitoring.

New Dimension in Magnetism and Superconductivity: 3D and Curvilinear Nanoarchitectures

Traditionally, the primary field, where curvature has been at the heart of research, is the theory of general relativity. In recent studies, however, the impact of curvilinear geometry enters various disciplines, ranging from solid-state physics over soft-matter physics, chemistry, and biology to mathematics, giving rise to a plethora of emerging domains such as curvilinear nematics, curvilinear studies of cell biology, curvilinear semiconductors, superfluidity, optics, 2D van der Waals materials, plasmonics, magnetism, and superconductivity. Here, the state of the art is summarized and prospects for future research in curvilinear solid-state systems exhibiting such fundamental cooperative phenomena as ferromagnetism, antiferromagnetism, and superconductivity are outlined. Highlighting the recent developments and current challenges in theory, fabrication, and characterization of curvilinear micro- and nanostructures, special attention is paid to perspective research directions entailing new physics and to their strong application potential. Overall, the perspective is aimed at crossing the boundaries between the magnetism and superconductivity communities and drawing attention to the conceptual aspects of how extension of structures into the third dimension and curvilinear geometry can modify existing and aid launching novel functionalities. In addition, the perspective should stimulate the development and dissemination of research and development oriented techniques to facilitate rapid transitions from laboratory demonstrations to industry-ready prototypes and eventual products.

Magnetic materials: a journey from finding north to an exciting printed future

The potential implications/applications of printing technologies are being recognized worldwide across different disciplines and industries. Printed magnetoactive smart materials, whose physical properties can be changed by the application of external magnetic fields, are an exclusive class of smart materials that are highly valuable due to their magnetically activated smart and/or multifunctional response. Such smart behavior allows, among others, high speed and low-cost wireless activation, fast response, and high controllability with no relevant limitations in design, shape, or dimensions. Nevertheless, the printing of magnetoactive materials is still in its infancy, and the design apparatus, the material set, and the fabrication procedures are far from their optimum features. Thus, this review presents the main concepts that allow interconnecting printing technologies with magnetoactive materials by discussing the advantages and disadvantages of this joint field, trying to highlight the scientific obstacles that still limit a wider application of these materials nowadays. Additionally, it discusses how these limitations could be overcome, together with an outlook of the remaining challenges in the emerging digitalization, Internet of Things, and Industry 4.0 paradigms. Finally, as magnetoactive materials will play a leading role in energy generation and management, the magnetic-based Green Deal is also addressed.

Clusters of Spin Valve Sensors in 3D Magnetic Field of a Label

Magnetic field sensors based on the giant magnetoresistance (GMR) effect have a number of practical current and future applications. We report on a modeling of the magnetoresistive response of moving spin-valve (SV) GMR sensors combined in certain cluster networks to an inhomogeneous magnetic field of a label. We predicted a large variety of sensor responses dependent on the number of sensors in the cluster, their types of interconnections, the orientation of the cluster, and the trajectory of sensor motion relative to the label. The model included a specific shape of the label, producing an inhomogeneous magnetic field. The results can be used for the optimal design of positioning devices.

Printable and Stretchable Giant Magnetoresistive Sensors for Highly Compliant and Skin-Conformal Electronics

Highly compliant electronics, naturally conforming to human skin, represent a paradigm shift in the interplay with the surroundings. Solution-processable printing technologies are yet to be developed to comply with requirements to mechanical conformability of on-skin appliances. Here, it is demonstrated that high-performance spintronic elements can be printed on ultrathin 3 µm thick polymeric foils enabling the mechanically imperceptible printed magnetoelectronics, which can adapt to the periodic buckling surface to be biaxially stretched over 100%. They constitute the first example of printed and stretchable giant magnetoresistive sensors, revealing 2 orders of magnitude improvements in mechanical stability and sensitivity at small magnetic fields, compared to the state-of-the-art printed magnetoelectronics. The key enabler of this performance enhancement is the use of elastomeric triblock copolymers as a binder for the magnetosensitive paste. Even when bent to a radius of 16 µm, the sensors printed on ultrathin foils remain intact and possess unmatched sensitivity for printed magnetoelectronics of 3 T-1 in a low magnetic field of 0.88 mT. The compliant printed sensors can be used as components of on-skin interactive electronics as it is demonstrated with a touchless control of virtual objects including zooming in and out of interactive maps and scrolling through electronic documents.

Advances in Magnetoresistive Biosensors

Magnetoresistance (MR) based biosensors are considered promising candidates for the detection of magnetic nanoparticles (MNPs) as biomarkers and the biomagnetic fields. MR biosensors have been widely used in the detection of proteins, DNAs, as well as the mapping of cardiovascular and brain signals. In this review, we firstly introduce three different MR devices from the fundamental perspectives, followed by the fabrication and surface modification of the MR sensors. The sensitivity of the MR sensors can be improved by optimizing the sensing geometry, engineering the magnetic bioassays on the sensor surface, and integrating the sensors with magnetic flux concentrators and microfluidic channels. Different kinds of MR-based bioassays are also introduced. Subsequently, the research on MR biosensors for the detection of protein biomarkers and genotyping is reviewed. As a more recent application, brain mapping based on MR sensors is summarized in a separate section with the discussion of both the potential benefits and challenges in this new field. Finally, the integration of MR biosensors with flexible substrates is reviewed, with the emphasis on the fabrication techniques to obtain highly shapeable devices while maintaining comparable performance to their rigid counterparts.

Magnetization Dynamics of an Individual Single-Crystalline Fe-Filled Carbon Nanotube

The magnetization dynamics of individual Fe-filled multiwall carbon-nanotubes (FeCNT), grown by chemical vapor deposition, are investigated by microresonator ferromagnetic resonance (FMR) and Brillouin light scattering (BLS) microscopy and corroborated by micromagnetic simulations. Currently, only static magnetometry measurements are available. They suggest that the FeCNTs consist of a single-crystalline Fe nanowire throughout the length. The number and structure of the FMR lines and the abrupt decay of the spin-wave transport seen in BLS indicate, however, that the Fe filling is not a single straight piece along the length. Therefore, a stepwise cutting procedure is applied in order to investigate the evolution of the ferromagnetic resonance lines as a function of the nanowire length. The results show that the FeCNT is indeed not homogeneous along the full length but is built from 300 to 400 nm long single-crystalline segments. These segments consist of magnetically high quality Fe nanowires with almost the bulk values of Fe and with a similar small damping in relation to thin films, promoting FeCNTs as appealing candidates for spin-wave transport in magnonic applications.

Self-assembly of highly sensitive 3D magnetic field vector angular encoders

Novel robotic, bioelectronic, and diagnostic systems require a variety of compact and high-performance sensors. Among them, compact three-dimensional (3D) vector angular encoders are required to determine spatial position and orientation in a 3D environment. However, fabrication of 3D vector sensors is a challenging task associated with time-consuming and expensive, sequential processing needed for the orientation of individual sensor elements in 3D space. In this work, we demonstrate the potential of 3D self-assembly to simultaneously reorient numerous giant magnetoresistive (GMR) spin valve sensors for smart fabrication of 3D magnetic angular encoders. During the self-assembly process, the GMR sensors are brought into their desired orthogonal positions within the three Cartesian planes in a simultaneous process that yields monolithic high-performance devices. We fabricated vector angular encoders with equivalent angular accuracy in all directions of 0.14°, as well as low noise and low power consumption during high-speed operation at frequencies up to 1 kHz.

Flexible Sensors—From Materials to Applications

Flexible sensors have the potential to be seamlessly applied to soft and irregularly shaped surfaces such as the human skin or textile fabrics. This benefits conformability dependant applications including smart tattoos, artificial skins and soft robotics. Consequently, materials and structures for innovative flexible sensors, as well as their integration into systems, continue to be in the spotlight of research. This review outlines the current state of flexible sensor technologies and the impact of material developments on this field. Special attention is given to strain, temperature, chemical, light and electropotential sensors, as well as their respective applications.

Ionic Gel Modulation of RKKY Interactions in Synthetic Anti-Ferromagnetic Nanostructures for Low Power Wearable Spintronic Devices

To meet the demand of developing compatible and energy-efficient flexible spintronics, voltage manipulation of magnetism on soft substrates is in demand. Here, a voltage tunable flexible field-effect transistor structure by ionic gel (IG) gating in perpendicular synthetic anti-ferromagnetic nanostructure is demonstrated. As a result, the interlayer Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction can be tuned electrically at room temperature. With a circuit gating voltage, anti-ferromagnetic (AFM) ordering is enhanced or converted into an AFM-ferromagnetic (FM) intermediate state, accompanying with the dynamic domain switching. This IG gating process can be repeated stably at different curvatures, confirming an excellent mechanical property. The IG-induced modification of interlayer exchange coupling is related to the change of Fermi level aroused by the disturbance of itinerant electrons. The voltage modulation of RKKY interaction with excellent flexibility proposes an application potential for wearable spintronic devices with energy efficiency and ultralow operation voltage.

Three-dimensional nanomagnetism

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.

Nanoscale magnetic devices play a key role in modern technologies but current applications involve only 2D structures like magnetic discs. Here the authors review recent progress in the fabrication and understanding of 3D magnetic nanostructures, enabling more diverse functionalities.

High Performance MgO-barrier Magnetic Tunnel Junctions for Flexible and Wearable Spintronic Applications

The magnetic tunnel junction (MTJ) using MgO barrier is one of most important building blocks for spintronic devices and has been widely utilized as miniaturized magentic sensors. It could play an important role in wearable medical devices if they can be fabricated on flexible substrates. The required stringent fabrication processes to obtain high quality MgO-barrier MTJs, however, limit its integration with flexible electronics devices. In this work, we have developed a method to fabricate high-performance MgO-barrier MTJs directly onto ultrathin flexible silicon membrane with a thickness of 14 μm and then transfer-and-bond to plastic substrates. Remarkably, such flexible MTJs are fully functional, exhibiting a TMR ratio as high as 190% under bending radii as small as 5 mm. The devices‘ robustness is manifested by its retained excellent performance and unaltered TMR ratio after over 1000 bending cycles. The demonstrated flexible MgO-barrier MTJs opens the door to integrating high-performance spintronic devices in flexible and wearable electronics devices for a plethora of biomedical sensing applications.

Integration of GMR Sensors with Different Technologies

Less than thirty years after the giant magnetoresistance (GMR) effect was described, GMR sensors are the preferred choice in many applications demanding the measurement of low magnetic fields in small volumes. This rapid deployment from theoretical basis to market and state-of-the-art applications can be explained by the combination of excellent inherent properties with the feasibility of fabrication, allowing the real integration with many other standard technologies. In this paper, we present a review focusing on how this capability of integration has allowed the improvement of the inherent capabilities and, therefore, the range of application of GMR sensors. After briefly describing the phenomenological basis, we deal on the benefits of low temperature deposition techniques regarding the integration of GMR sensors with flexible (plastic) substrates and pre-processed CMOS chips. In this way, the limit of detection can be improved by means of bettering the sensitivity or reducing the noise. We also report on novel fields of application of GMR sensors by the recapitulation of a number of cases of success of their integration with different heterogeneous complementary elements. We finally describe three fully functional systems, two of them in the bio-technology world, as the proof of how the integrability has been instrumental in the meteoric development of GMR sensors and their applications.

Stretchable Spin Valve with Stable Magnetic Field Sensitivity by Ribbon-Patterned Periodic Wrinkles

A strain-relief structure by combining the strain-engineered periodic wrinkles and the parallel ribbons was employed to fabricate flexible dual spin valves onto PDMS substrates in a direct sputtering method. The strain-relief structure can accommodate the biaxial strain accompanying with stretching operation (the uniaxial applied tensile strain and the induced transverse compressive strain due to the Poisson effect), thus significantly reducing the influence of the residual strain on the giant magnetoresistance (GMR) performance. The fabricated GMR dual spin-valve sensor exhibits the nearly unchanged MR ratio of 9.9%, magnetic field sensitivity up to 0.69%/Oe, and zero-field resistance in a wide range of stretching strain, making it promising for applications on a conformal shape or a movement part.

Flexible Hall sensors based on graphene

The excellent electronic and mechanical properties of graphene provide a perfect basis for high performance flexible electronic and sensor devices. Here, we present the fabrication and characterization of flexible graphene based Hall sensors. The Hall sensors are fabricated on 50 μm thick flexible Kapton foil using large scale graphene grown by chemical vapor deposition technique on copper foil. Voltage and current normalized sensitivities of up to 0.096 V VT(-1) and 79 V AT(-1) were measured, respectively. These values are comparable to the sensitivity of rigid silicon based Hall sensors and are the highest values reported so far for any flexible Hall sensor devices. The sensitivity of the Hall sensor shows no degradation after being bent to a minimum radius of 4 mm, which corresponds to a tensile strain of 0.6%, and after 1000 bending cycles to a radius of 5 mm.

Wearable magnetic field sensors for flexible electronics

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.

Flexible and stretchable micromagnet arrays for tunable biointerfacing

A process to surface pattern polydimethylsiloxane (PDMS) with ferromagnetic structures of varying sizes (micrometer to millimeter) and thicknesses (>70 μm) is developed. Their flexibility and magnetic reach are utilized to confer dynamic, additive properties to a variety of substrates, such as coverslips and Eppendorf tubes. It is found that these substrates can generate additional modes of magnetic droplet manipulation, and can tunably steer magnetic-cell organization.

High-performance magnetic sensorics for printable and flexible electronics

High-performance giant magnetoresistive (GMR) sensorics are realized, which are printed at predefined locations on flexible circuitry. Remarkably, the printed magnetosensors remain fully operational over the complete consumer temperature range and reveal a giant magnetoresistance up to 37% and a sensitivity of 0.93 T(-1) at 130 mT. With these specifications, printed magnetoelectronics can be controlled using flexible active electronics for the realization of smart packaging and energy-efficient switches.

Imaging of buried 3D magnetic rolled-up nanomembranes

Increasing performance and enabling novel functionalities of microelectronic devices, such as three-dimensional (3D) on-chip architectures in optics, electronics, and magnetics, calls for new approaches in both fabrication and characterization. Up to now, 3D magnetic architectures had mainly been studied by integral means without providing insight into local magnetic microstructures that determine the device performance. We prove a concept that allows for imaging magnetic domain patterns in buried 3D objects, for example, magnetic tubular architectures with multiple windings. The approach is based on utilizing the shadow contrast in transmission X-ray magnetic circular dichroism (XMCD) photoemission electron microscopy and correlating the observed 2D projection of the 3D magnetic domains with simulated XMCD patterns. That way, we are not only able to assess magnetic states but also monitor the field-driven evolution of the magnetic domain patterns in individual windings of buried magnetic rolled-up nanomembranes.

Magnetic microstructure of rolled-up single-layer ferromagnetic nanomembranes

The magnetic microstructure of rolled-up magnetic nanomembranes is revealed both theoretically and experimentally. Two types of nanomembranes are considered, one with a non-magnetic spacer layer and the other without. Experimentally, by using different materials and tuning the dimensions of the rolled-up nanomembranes, domain patterns consisting of spiral-like and azimuthally magnetized domains are observed, which are in qualitative agreement with the theoretical predictions.

Lab-on-a-Chip Magneto-Immunoassays: How to Ensure Contact between Superparamagnetic Beads and the Sensor Surface

Lab-on-a-chip immuno assays utilizing superparamagnetic beads as labels suffer from the fact that the majority of beads pass the sensing area without contacting the sensor surface. Different solutions, employing magnetic forces, ultrasonic standing waves, or hydrodynamic effects have been found over the past decades. The first category uses magnetic forces, created by on-chip conducting lines to attract beads towards the sensor surface. Modifications of the magnetic landscape allow for additional transport and separation of different bead species. The hydrodynamic approach uses changes in the channel geometry to enhance the capture volume. In acoustofluidics, ultrasonic standing waves force µm-sized particles onto a surface through radiation forces. As these approaches have their disadvantages, a new sensor concept that circumvents these problems is suggested. This concept is based on the granular giant magnetoresistance (GMR) effect that can be found in gels containing magnetic nanoparticles. The proposed design could be realized in the shape of paper-based test strips printed with gel-based GMR sensors.

Flexible and stretchable polymers with embedded magnetic nanostructures

A novel pathway is presented to transfer and embed functional patterned magnetic nanostructures into flexible and stretchable polymeric membranes. The geometrical and magnetic properties are maintained through the process, realized even directly inside a microfluidic channel. These results pave the way to the realization of smart biomedical systems and devices based on the integration of magnetic nanostructures into new classes of substrates.

Stretchable spin valves on elastomer membranes by predetermined periodic fracture and random wrinkling

The first highly stretchable and sensitive spin valve sensor on elastomeric membranes are demonstrated. The sensor elements exhibit stable GMR behavior up to tensile strains of 29% in in situ stretching experiments and show no fatigue over 500 loading cycles. This remarkable stretchability is achieved by a predetermined periodic fracture mechanism that creates a meander-like pattern upon stretching.