Flexible Magnetic Sensors

Abstract

With the merits of high sensitivity, high stability, high flexibility, low cost, and simple manufacturing, flexible magnetic field sensors have potential applications in various fields such as geomagnetosensitive E-Skins, magnetoelectric compass, and non-contact interactive platforms. Based on the principles of various magnetic field sensors, this paper introduces the research progress of flexible magnetic field sensors, including the preparation, performance, related applications, etc. In addition, the prospects of flexible magnetic field sensors and their challenges are presented.

Keywords: Hall sensors; flexible electronics; flexible magnetic sensors; magnetoelectric (ME) sensors; magnetoimpedance (MI) sensors; magnetoresistance (MR) sensors; shapeable magnetoelectronics.

Figure 1

Working principles of Hall sensor.

Figure 2

The device architecture of the flexible Hall sensor is based on graphene. Adapted with permission from Ref. [17]. Copyright 2016, copyright Royal Society of Chemistry.

Figure 3

Working principles of AMR sensor.

Figure 4

(a) Schematics on the fabrication process of printed AMR sensors; (b) Physical picture of the printed AMR sensor; (c) Photographs revealing the bending state of the sensor; (d) the respective AMR response measured at ±400 mT. Adapted with permission from Ref. [33] and licensed under CC BY 4.0. Copyright 2021, copyright Springer Nature Limited.

Figure 5

(a) Vision of the active matrix integrated micro-origami magnetic sensor for E-skin application; (b) Deterministic self-folding process; (c) Layout design of the self-foldable platform; (d) Photograph of the magnetic hair embedded e-skin system. Scale bar, 5 mm; (e) One pixel in the 3D sensor. Scale bar, 200 µm. Adapted with permission from Ref. [34] and licensed under CC BY 4.0. Copyright 2022, copyright Springer Nature Limited.

Figure 6

(a) Schematics on the fabrication process of flexible PHE sensor; (b) Magnetoelectrical characterization of flexible PHE sensors in flat state and bent to 1 mm. Adapted with permission from Ref. [36] and licensed under CC BY 4.0. Copyright 2019, copyright Springer Nature Limited.

Figure 7

Working principles of GMR sensor.

Figure 8

(a) Schematics of stretchable GMR sensors; (b) Stretching experiments results of the stretchable GMR sensor. Adapted with permission from Ref. [66] and licensed under CC BY 4.0. Copyright 2015, copyright Springer Nature Limited. (c) Schematics of printed GMR sensors; (d) GMR performance of the printed sensor in the relaxed and stretched state; (e) Photograph of a printed GMR sensor from 100% of stretching to 0% of relaxed state. Adapted with permission from Ref. [72] and licensed under CC BY 4.0. Copyright 2021, copyright John Wiley & Sons, Inc.

Figure 9

Working principles of TMR sensor.

Figure 10

Schematic diagrams of transferring the MTJ stack. Adapted with permission from Ref. [91]. Copyright 2016, copyright John Wiley & Sons, Inc.

Figure 11

(a) Schematics on the fabrication process of flexible TMR sensors; (i) MgO-barrier MTJ stacks on thin thermally oxidized silicon wafer; (ii) MgO-barrier MTJ array after patterning; (iii) A layer of S1813 photoresist spin-coated onto the device surface; (iv) The sample was turned over, mounted onto a four-inch silicon wafer covered with photoresist; (v) Thin the back side of the silicon; (vi) Flexible MgO-barrier MTJs after removing the photoresist; (b) Resistance in the parallel (R_P) and antiparallel (R_AP) states and TMR ratios varying with bending radius. Adapted with permission from Ref. [92] and licensed under CC BY 4.0. Copyright 2017, copyright Springer Nature Limited.

Figure 12

Working principles of MI sensor.

Figure 13

(a) The device architecture of the flexible GMI sensor based on a Kapton substrate; (b1,b2) Physical picture of the flexible GMI sensor; (c) Schematic diagram of deflection measuring device for flexible GMI sensor. Adapted with permission from Ref. [97]. Copyright 2014, copyright Elsevier B.V.

Figure 14

Magnetic properties of flexible GMI sensors induced by strain by self-assembly rolling approach. Adapted with permission from Ref. [101] and licensed under CC BY 4.0. Copyright 2022, copyright Springer Nature Limited.

Figure 15

Working principles of ME sensor.

Figure 16

(a) Schematics of PVDF/CI composite structure; (b)Tensile stress-strain curves of PVDF/CI films of CI particles in different content; (c) Piezoelectric charge variation of PVDF/CI-10% films at different bending displacement. Adapted with permission from Ref. [112]. Copyright 2018, copyright Elsevier B.V.

Figure 17

Schematics of the cellulose based flexible ME sensors. (a) Scheme of cellulose crystal II, the most common crystalline type in regenerated cellulose materials; (b) Illustration of cellulose fibril alignment at the cross-section of cellulose film; (c) cellulose based ME laminate structure. Adapted with permission from Ref. [123] and licensed under CC BY 4.0. Copyright 2017, copyright Springer Nature Limited.

Figure 18

Outdoor geomagnetic exploration by E-skin compass. (a) E-skin compass attached to the finger; (b) Time evolution of the output voltage of the e-skin compass when the person rotates back and forth from the magnetic north (N) to magnetic south (S) via west (W); (c–e) Pictures of people facing N, W, and S. Adapted with permission from Ref. [31]. Copyright 2018, copyright Springer Nature Limited.

Figure 19

Highly flexible printed GMR sensors for Non-contact Interactive Platform. (a) A compliant printed GMR sensor attached to the finger which can read displacement of a permanent magnet; (b) The time evolution of the normalized sensor read out dependent on the distance between the finger and the magnet; The sensor signal is used to navigate through the document (c) or zoom in/out the map (d). Adapted with permission from Ref. [72] and licensed under CC BY 4.0. Copyright 2021, copyright John Wiley & Sons, Inc.

Figure 20

Flexible Hall sensor attached to the human body for interactive pointing devices. (a) Magnified view of the flexible Hall sensor; (b,c) Flexible Hall sensor attached to the human body; (d,e) The relative position of the finger with respect to a permanent magnet is displayed in real time by monitoring the sensor output. Adapted with permission from Ref. [18]. Copyright 2015, copyright John Wiley & Sons, Inc.