Chair for Multicomponent Materials

Magnetoelectric magnetic field sensors

Our work on magnetoelectric field sensors has been carried our within the framework of the Collaborative Research Center SFB 1261.

SFB1261

In recent years, interest in magnetoelectric (ME) composites has strongly increased. In addition to applications in the field of energy harvesting and other areas, ME composites turned out to be very attractive for the detection of very low AC magnetic fields. Single-phase ME materials exhibit only extremely small magnetoelectric coefficients αME = δEH and do not lend themselves to magnetic field sensors. Huge ME coefficients can be obtained with composite materials where the ME effect is a product property (see Fig.1). Here the strain of a magnetostrictive material in a magnetic field is transferred to a piezoelectric material via elastic coupling and can be measured as a voltage. With this composite approach, both the magnetostrictive and the piezoelectric components can be optimized independently. While bulk composites have been investigated for many years, our joint research with other groups in Kiel focuses on fully integrable thin film composites.

fig1

Fig. 1 Sketch of an ME sensor where the magnetostrictive and piezoelectric layers are deposited on a Si cantilever.

High ME coefficients are generally obtained by taking advantage of the mechanical resonance of a ME sensor on a vibrating cantilever. Depending on the quality factor Q of the resonator, amplifications of more than three orders of magnitude can be achieved. For the measurement of very low-frequency signals for medical applications this, however, causes two problems. On the one hand, the resonance frequencies of the current sensors are far above the relevant frequency range, and on the other hand, a high Q factor gives rise to a very narrow bandwidth.

Delta E effect sensor

In order to solve these problems, we developed a novel type of sensor which uses the so-called ΔE effect of a ferromagnetic material in conjunction with active electrical excitation of a cantilever with a resonance frequency far above the frequency range of measurement. The ΔE effect is directly related to the magnetostriction and originates from the change of the Young’s modulus of a ferromagnetic material in a magnetic field. The change in Young’s modulus due to the ΔE effect gives rise to detuning of the vibrating cantilever and hence to a resonance frequency shift.

In our first demonstrator of the ΔE-effect sensor, the vibrating cantilever of an atomic force microscope was coated with a magnetostrictive FeCoSiB layer, whose easy axis was oriented by field annealing (Fig. 2). The working point was located at the inflection point of the right edge of the resonance peak, and the change in amplitude caused by the detuning of the cantilever was measured by the laser deflection system of the AFM. Different from a conventional ME sensor, the ΔE-effect sensor allows broadband measurements down to the DC range and, as a result of the high resonance frequency, it is robust to microphonic noise and mechanical impact. Moreover, as an important step towards full integrability, the excitation of the vibrating cantilever occurred without an external AC magnetic field. After its publication in Applied Physics Letters, this ΔE sensor was presented in Nature as a “Research Highlight”.

fig2

Fig. 2 Our first ΔE-effect sensor prototype based on an AFM cantilever coated with a magnetostrictive amorphous FeCoSiB layer. The cantilever with a resonance frequency of 320 kHz, far above the measuring range, is excited piezoelectrically (see Nature 155 (2011) 480).

The sensor still showed a relatively high detection limit of 900 nT/Hz1/2, and the laser deflection method as well as the excitation by an external piezo crystal were disadvantageous with respect to integration. Since then, these problems have been solved, and the sensitivity of the sensor has been improved by almost five orders of magnitude. This was made possible by close cooperation with the groups of E. Quandt and B. Wagner from the Institute of Materials Science and of G. Schmidt and R. Knöchel from the Institute of Electrical Engineering in Kiel.

A crucial advancement also in terms of integrability was accomplished by using the piezoelectric layer of a conventional ME sensor for simultaneous mechanical excitation and readout. Currently, sensitivities well below 100 pT are achieved at 10 Hz with much room for further improvements (see Fig. 3).

fig3

Fig. 3 Progress in the development of the ΔE-effect sensor on the basis of the detection limit over the years. An improvement of almost 5 orders of magnitude was attained since our first demonstrator. The first two LODs were measured at 10 Hz, the third and fourth at 20 Hz magnetic signal frequency. b) and c) Photographs of the 1st and 2nd generation of MEMS sensors fabricated based on conventional ME sensor designs by the groups of E. Quandt and B. Wagner from the Institute of Materials Science in Kiel and the Fraunhofer Institute for Silicon Technology (ISIT) in Itzehoe.

 

Selected publications

Zabel, S.; Reermann, J.; Fichtner, S.; Kirchhof, C.; Quandt, E.; Wagner, B.; Schmidt, G.; Faupel, F.; Multimode delta-E effect magnetic field sensors with adapted electrodes, Applied Physics Letters 108 (2016) 222401.

Zabel, S.; Kirchhof, C; Yarar, E; Meyners, D; Quandt, E; S.; Marauska, S.; Gojdka, B.; Wagner, B.; Knöchel, R.; Adelung, R.; Faupel, F.: Phase Modulated Magnetoelectric ΔE Effect Sensor for Sub-nano Tesla Magnetic Fields, Appl. Phys. Letters; 107, 152402 (2015).

Reermann, J.; Schmidt, G.; Zabel, S.; Faupel, F.: Adaptive Multi-mode Combination for Magnetoelectric Sensors Based on the Delta-E Effect, Procedia Engineering 120, 536.

Jahns, R.; Zabel, S.; Marauska, S.; Gojdka, B.; Wagner, B.; Knöchel, R.; Adelung, R.; and Faupel, F.: Magnetic field sensor based on ΔE effect, Appl. Phys. Lett. 105 (2014) 052414.

Gojdka, B.; Jahns, R.; Meurisch, K.; Greve, H.; Adelung, R.; Quandt, E.; Knöchel, R., Faupel, F.: Fully integrable magnetic field sensor based on ΔE effect. Appl. Phys. Lett., 99 (2011) 223502.

Moving micro magnets, Nature Research Highlight, Nature 155 (2011) 480.

 

Magnetoelectric sensors with organic piezoelectric layer

Magnetoelectric layered composites involving piezoelectric polymers are very interesting particularly because of their low dielectric constants and resulting high piezoelectric voltage coefficients. We investigated sensors made up of a cantilever of a highly magnetostrictive metallic glass covered with a thin film of a piezoelectric co-polymer (Fig. 4). The sensor achieved a magnetoelectric coefficient of 850 Vcm-1 Oe-1, the highest value ever observed at low frequencies, at its fundamental bending mode resonance frequency at 27.8 Hz. In its second resonant mode at about 170 Hz, a detection limit as low as 10 pT/Hz1/2 was obtained.

fig4

Fig. 4 Sensitivity plot of a ME sensor with a spin-coated PVDF-TrFE copolymer on a Metglass cantilever showing a linear response down to magnetic fields as low as 10 pT at 168 Hz.

Selected publication

Kulkarni, A.; Meurisch, K.; Teliban, I.; Jahns, R.; Strunskus, T. ; Piorra, A.; Knöchel, R. and Faupel, F.: Giant magnetoelectric effect at low frequencies in polymer-based thin film composites, Appl. Phys. Lett. 104, (2014) 022904.