Magnetic Sensors
Principal Investigators
Dieter Suess and Claas Abert
Magnetic sensors enable robust and contactless measurements in a wide range of applications, from speed and position detection in modern vehicles to other systems requiring sensitive and reliable magnetic-field measurements. Our research combines micromagnetic modelling with advanced sensor design to understand and reduce non-linearity, hysteresis and magnetic noise in magnetoresistive devices.
Background and Motivation
Modern cars contain several magnetic sensors that measure quantities such as wheel speed, transmission gear speed and position, crankshaft position and engine speed. In addition to classical Hall-effect sensors, magnetoresistive technologies based on giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) provide enhanced sensitivity and can therefore extend the range of possible sensor applications.
However, the behaviour of magnetoresistive sensors is strongly influenced by the magnetic configuration within the sensor element. Domain-wall motion, magnetic switching events and other non-linear processes can lead to hysteresis, signal deviations and magnetic noise. Understanding and controlling these effects is therefore essential for designing reliable, sensitive and reproducible magnetic sensors.
Comparison of two magnetic sensor concepts: (a) an elliptical sensor element governed by shape anisotropy and (b) a circular vortex-based sensor element. The corresponding magnetisation curves illustrate their different hysteresis behaviour.
Research Approach and Computational Methods
The physical processes inside magnetic sensor elements can be investigated using micromagnetic simulations. These simulations describe the spatial distribution and evolution of the magnetisation and make it possible to analyse how different sensor geometries and material properties influence the sensor response.
Because typical sensor elements have dimensions in the micrometre range, accurate simulations may require discretisations with millions of degrees of freedom. Efficient numerical methods are therefore essential. Finite-difference micromagnetics combined with direct energy minimisation is used to calculate the hysteresis behaviour of these systems.
The simulations are used to investigate factors including:
- external and internal bias fields,
- surface roughness,
- the geometry and layout of the sensor,
- magnetic switching processes,
- non-linearity and hysteresis,
- and magnetic noise.
The resulting insights support the development and optimisation of magnetoresistive sensor concepts before they are implemented experimentally.
Key Results and Applications
One important result of this research is a magnetic sensor concept based on a topologically protected vortex structure. In conventional sensor elements, changes between different magnetic states can produce non-linear and hysteretic behaviour. By contrast, the vortex configuration provides a stable magnetic ground state in which the magnetisation curls around the centre of the sensor element.
Using this stable vortex state significantly reduces hysteresis while providing an extended linear operating range and reduced magnetic noise. These results demonstrate how controlling the internal magnetic structure of a sensor element can directly improve its performance.
Magnetoresistive sensors are particularly relevant for applications requiring reliable, contactless detection of position, speed or magnetic fields. Important examples include wheel-speed measurements, transmission speed and position sensing, crankshaft-position detection and engine-speed measurements.
- Suess, Dieter, et al. "Topologically protected vortex structures for low-noise magnetic sensors with high linear range." Nature Electronics 1.6 (2018): 362-370.
- Bachleitner-Hofmann, Anton, et al. "Magnetic sensor device and method for a magnetic sensor device having a magneto-resistive structure." U.S. Patent Application No. 15/375,995.
- Brueckl, H., et al. "Vortex magnetization state in a GMR spin-valve type field sensor." Magnetics Conference (INTERMAG), 2017 IEEE International. IEEE, 2017.
- Bachleitner-Hofmann, Anton, et al. "Unexpected width of minor magnetic hysteresis loops in nanostructures." IEEE Transactions on Magnetics 52.7 (2016): 1-4.
- Bruckner, Florian, et al. "A device model framework for magnetoresistive sensors based on the Stoner–Wohlfarth model." Journal of Magnetism and Magnetic Materials 381 (2015): 344-349.