Schematic of an magnetoelectric cantilever sensor the magnified region represents the layer stacking of piezolectric (AlN), and the self biased magnetostrictive stack (Ta, Cu, MnIr, FeCoSiB). The arrow represents the bias field orientation.

For his work about magnetoelectric sensors in medical engineering Dr. Enno Lage, post doc at Kiel University, received an award for young researchers from the German Society for Materials Science (DGM, Deutsche Gesellschaft für Materialkunde). The prize was awarded at the annual conference of the DGM in Darmstadt on September 26th 2016. It honors outstanding achievements of young researchers in material science.

During his PhD, Enno Lage conducted his research on highly sensitive magnetoelectric sensors within the collaborative research center “Magnetoelectric Composites” at the Faculty of Engineering.

Those sensors show their highest sensitivity in presence of well-defined magnetic bias fields. For vector sensors and in densely arranged sensor-arrays each component needs an individually oriented bias field. Thus, the sources of bias fields, either generated with electromagnetic coils or with permanent magnets, are detrimental in terms of miniaturization.

In order to overcome these limitations, the researchers utilized the exchange bias effect which is well established in magneto-resistive sensing. The challenge in their approach was the precise adjustment of the bias field. The orientation of the internal bias field is crucial for the sensitivity. If one would simply substitute the external bias field by an internal field with same orientation and strength, the sensors would show a vanishingly small response and hence an alternative orientation had to be considered. Additionally, if the field is too large the sensitivity decreases, if it is too small the sensor behaves partially unbiased.

The followed approach led to the successful realization of self-biased magnetoelectric sensors and was suitable to combine sensor elements for vector sensing.

Corresponding publication:

Lage, E., Kirchhof, C., Hrkac, V., Kienle, L., Jahns, R., Knöchel, R., Quandt, E. and Meyners, D., 2012. Exchange biasing of magnetoelectric composites. Nature materials, 11(6), pp.523-529.

Lage, E., Woltering, F., Quandt, E. and Meyners, D., 2013. Exchange biased magnetoelectric composites for vector field magnetometers. Journal of Applied Physics, 113(17), p.17C725.

Lage, E., Urs, N.O., Röbisch, V., Teliban, I., Knöchel, R., Meyners, D., McCord, J. and Quandt, E., 2014. Magnetic domain control and voltage response of exchange biased magnetoelectric composites. Applied Physics Letters, 104(13), p.132405.

Today (September 15th, 2016) the inventors Robert Jahns, Holger Runkowske, and Reinhard Knöchel were informed from the European Patent Office that their idea on the "tuning fork" sensor principle is protected now by the European patent EP 2 811 314 B1. Congratulations to the inventors!

A "tuning fork" sensor basically consists of two ME sensors, which are arranged on top and at the bottom of a mounting block. The inverse orientation of the individual sensors with respect to the suspension point (FR-4 substrate in the picture below) leads to distinguishable output signals for magnetic and vibrational excitations. If a magnetic field is applied to the tuning fork, the cantilevers are bent in opposite directions (e.g. both away from the centre, black arrows). The outputs of the upper and lower cantilevers are opposite in phase with respect to a common ground and thus produce a differential-mode signal. In the case of vibrational coupling, the cantilevers are predominantly bent in the same direction (green arrows) and produce co-phase signals of the two sensor outputs (common mode). The comparison of a tuning fork sensor with a single cantilever sensor (see figure below) reveals that the tuning fork shows a limit of detection of approximately 500 fT/Hz1/2 – a very good LOD for magnetoelectric thin film sensors - whereas the individual magnetoelectric cantilevers, similar to those of which the tuning fork is composed, have sensitivities of approximately 5 pT/Hz1/2. With superimposed white noise the effect of the tuning fork is even more distinct. Whereas the tuning fork experiences an increase in noise level of about a factor of roughly 4, the single magnetoelectric sensor shows a rise of approximately two decades.

   
Magnetoelectric tuning fork sensor (a). LOD plots of tuning fork setup (c) in comparison to a single sensor (b) with and without additional superimposed wideband noise. The dashed auxiliary lines indicate the noise level and the LOD. The resonance frequency was 958 Hz.

 

Recently a new AlN deposition process was sucessfully established within the CRC 1261. Conventional sensors follow a certain deposition order, i.e. AlN-FeCoSiB, dictated by the temperature limitations of the magnetostrictive phase. This was overcome with the implementation of a low temperature AlN deposition process, which in turn allows the deposition of FeCoSiB as the first layer on the polished surface of a Si-wafer. The inversed deposition order further allows the optimization of the top electrode size.

There are several important results;

  • Low temperature AlN displayed comparable/superior microstructural and piezoelectric properties compared to the high temperature AlN thin films found in the literature.
  • The structural integrity i.e., amorphous structure of FeCoSiB, is preserved.
  • Measured sensors exhibit a higher signal-to-noise-ratio (SNR) compared to conventional AlN-FeCoSiB sensors.
  • The resulting SNR originates from the combined effect of the improved piezoemagnetic coefficient of FeCoSiB (when deposited on the smooth surface of a Si-wafer) and the much smaller dielectric loss tangent of AlN (compared to high temperature AlN).
  • An average magnetic sensitivity of 400 ± 37 fT/Hz1/2 is determined at the mechanical resonance of the sensor. This value corresponds to the lowest sensitivity ever obtained with thin film resonant sensors up-to-date.

Corresponding publication:

E. Yarar, S. Salzer, V. Hrkac, A. Piorra, M. Höft, R. Knöchel, L. Kienle, and E. Quandt: Inverse Bilayer Magnetoelectric Thin Film Sensor; Appl. Phys. Lett. 109, 022901 (2016); http://dx.doi.org/10.1063/1.4958728

Contact

sfb1261@tf.uni-kiel.de

Chairman:

Prof. Dr. Eckhard Quandt

Kiel University
Institute for Materials Science

 

Internal server

 

CAU

Christian-Albrechts-Universität zu Kiel (CAU)

Christ.-Albrechts-Platz 4
D-24118 Kiel

UKSH

University Hospital Schleswig-Holstein, Campus Kiel (UKSH)

Arnold-Heller-Straße 3
D-24105 Kiel

ISIT

Fraunhofer Institute for Silicon Technology, Itzehoe (ISIT)

Fraunhoferstrasse 1
D-25524 Itzehoe  

IPN

IPN - Leibniz-Institut für die Pädagogik der Naturwissenschaften und Mathematik 

Olshausenstraße 62 
D-24118 Kiel

Cookies make it easier for us to provide you with our services. With the usage of our services you permit us to use cookies.
Ok