Printed, Flexible AMR Sensors for Detecting Weak Magnetic Fields

Weak magnetic fields in the range of approximately 0.1 to 5 mT can be detected very effectively using anisotropic magnetoresistive (AMR) sensors – ohmic electrical resistors that respond to an applied magnetic field with a defined change in electrical resistance. These consist of ferromagnetic materials, such as a nickel-iron alloy (permalloy) with very good magnet-izability. The anisotropic magnetoresistive effect (AMR) that occurs depends on the orientation of the current flow in the sensor resistor relative to the external magnetic field. For this reason, AMR sensors are used as position and rotation sensors in encoders to detect the motion of mechanical components in machines.

In the current state of the art, microelectronic AMR sensors are implemented as surface-mount devices (SMDs). These sensors have several disadvantages, however: a lack of mechanical flexibility, high costs for the fabrication of sensor arrays, and limited temperature stability up to 150 °C due to the packaging technology used. These disadvantages of SMD sensors can be avoided by printed AMR sensors fabricated using microprinting processes such as screen printing or inkjet printing. To this end, Fraunhofer IKTS develops magnetic material pastes and inks that can be precisely deposited on a wide range of substrates, including polymer and ceramic tapes. The printed layers are sintered within milliseconds using a diode laser array operating in continuous-wave mode and generating a line-shaped laser beam in the near-infrared range. This enables energy-efficient sensor production, including at industrial scale. Results demonstrate that AMR sensors printed on polymer tapes exhibit very good mechanical flexibility and durability (tested up to 30,000 bending cycles). Printed AMR sensors on polyimide tapes show significantly enhanced temperature stability compared with the state of the art, with resistance up to 300 °C. Fig. 1 shows screen-printed sensors produced as large-area, cost-effective arrays in a multilayer layout. Table 1 summarizes the key properties of these magnetic sensors.

The development was carried out in cooperation with Helmholtz-Zentrum Dresden-Rossendorf and Fraunhofer FEP in connection with the VIP+ project Mag4Ink (funding code: 03VP09092), funded by the Federal Ministry of Research, Technology and Space.

Services offered
 

• Development of pastes and inks based on magnetic powders
• Screen printing and inkjet printing of AMR sensors according to specified layouts, or scaled as AMR sensor arrays on a variety of substrate materials and tapes
• Laser sintering of printed AMR layers, particularly for thermally sensitive substrates
• Design and prototyping of magnetic sensors, including for position and rotation sensing
• Characterization of magnetoelectric effects up to 0.5 T over a temperature range from −100 to 600 °C, including characterization of electrical resistance noise