Jens Reermann

Electrically modulated magnetoelectric sensors

Magnetoelectric thin film composites have demonstrated their potential to detect sub-pT magnetic fields if mechanical resonances (typically few hundred Hz to a few kHz) are utilized. At low frequencies (1–100 Hz), magnetic field-induced frequency conversion has enabled wideband measurements with resonance-enhanced sensitivities by using the nonlinear characteristics of the magnetostriction curve. Nevertheless, the modulation with a magnetic field with a frequency close to the mechanical resonance results in a number of drawbacks, which are, e.g., size and energy consumption of the sensor as well as potential crosstalk in sensor arrays. In this work, we demonstrate the feasibility of an electric frequency conversion of a magnetoelectric sensor which would overcome the drawbacks of magnetic frequency conversion. This magnetoelectric sensor consists of three functional layers: an exchange biased magnetostrictive multilayer showing a high piezomagnetic coefficient without applying a magnetic bias field, a non...

Adaptive Readout Schemes for Thin-Film Magnetoelectric Sensors Based on the delta-E Effect

Thin-film magnetoelectric sensors, i.e., composites of magnetostrictive and piezoelectric materials, are able to measure very low magnetic fields. As a consequence, an application of such sensors could be, e.g., the measurement of biomagnetic fields in the near future. To measure these signals, typically characterized by low-frequency components, techniques, such as the delta-E effect, are utilized. The limit of detection (LoD) of such sensor systems did not reach the required level until now. In order to improve this, an adaptive readout scheme is proposed for sensor systems based on the delta-E effect. The basis is a simultaneous measurement with a single sensor at different frequency ranges close to the resonance frequencies. The signals are combined optimally in regard to their signal-to-noise ratio. Two combination approaches are presented and evaluated. An improvement up to 6 dB in terms of LoD is achieved. Due to an adaption of the weighting coefficients with time, the proposed method can be interpreted as a noise reduction technique, which increases the usability of such sensors in realistic measurement scenarios.

Multimode delta-E effect magnetic field sensors with adapted electrodes

We present an analytical and experimental study on low-noise piezoelectric thin film resonators that utilize the delta-E effect of a magnetostrictive layer to measure magnetic fields at low frequencies. Calculations from a physical model of the electromechanical resonator enable electrode designs to efficiently operate in the first and second transversal bending modes. As predicted by our calculations, the adapted electrode design improves the sensitivity by a factor of 6 and reduces the dynamic range of the sensor output by 16 dB, which significantly eases the requirements on readout electronics. Magnetic measurements show a bandwidth of 100 Hz at a noise level of about 100 pTHz−0.5.

Adaptive Acoustic Noise Cancellation for Magnetoelectric Sensors

Sensors based on the magnetoelectric (ME) effect have the potential to be genuine alternatives for measuring bio-magnetic signals. Unfortunately, the sensor structure usually inhibits the problem that several non-magnetic types of noise couple mechanically into the sensor: in this contribution, we will focus on undesired acoustic coupling. Therefore, an adaptive cancellation approach based on a computationally efficient gradient estimation algorithm with a pseudo-optimally control scheme is proposed. The approach is using a microphone as a noise reference sensor and is implemented in real time. An evaluation in terms of measurements is performed inside a magnetically shielded chamber. For a particular scenario, which is characterized by double excitation, an algorithm with binary control-scheme improves the signal-to-noise ratio (SNR) only by around 4dB. If the proposed control scheme is used instead, an improvement of the SNR of around 13dB is achieved.

Thermal-Mechanical Noise in Resonant Thin-Film Magnetoelectric Sensors

Thin-film magnetoelectric sensors, i.e., composites of magnetostrictive and piezoelectric materials, are able to measure very low magnetic flux densities in the picotesla range. In order to further improve the limit of detection it is of high importance to understand and quantify the relevant noise sources. In this paper, a common model for the deflection noise in vibrational structures is applied to the cantilever structure of resonant magnetoelectric sensors. By means of deflection and noise measurements the existence of thermal-mechanical noise even in sensor structures with a size in the centimeter range is proven. Based on these findings a noise equivalent circuit is suggested which allows not only the distinction between the impact of different sensor-intrinsic noise sources and also the involvement of the preamplifier noise. We found that the thermal-mechanical noise is the dominant noise source if direct signal detection is performed at the first bending resonance frequency of the sensor. However, this kind of noise is not the limiting influence when applying magnetic frequency-conversion techniques.

Wide Band Low Noise Love Wave Magnetic Field Sensor System

We present a comprehensive study of a magnetic sensor system that benefits from a new technique to substantially increase the magnetoelastic coupling of surface acoustic waves (SAW). The device uses shear horizontal acoustic surface waves that are guided by a fused silica layer with an amorphous magnetostrictive FeCoSiB thin film on top. The velocity of these so-called Love waves follows the magnetoelastically-induced changes of the shear modulus according to the magnetic field present. The SAW sensor is operated in a delay line configuration at approximately 150 MHz and translates the magnetic field to a time delay and a related phase shift. The fundamentals of this sensor concept are motivated by magnetic and mechanical simulations. They are experimentally verified using customized low-noise readout electronics. With an extremely low magnetic noise level of ≈100 pT/

Modeling and Analysis of Noise Sources for Thin-Film Magnetoelectric Sensors Based on the Delta-E Effect

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Detection of steering direction using EEG recordings based on sample entropy and time-frequency analysis

Hz, a bandwidth of 50 kHz and a dynamic range of 120 dB, this magnetic field sensor system shows outstanding characteristics. A range of additional measures to further increase the sensitivity are investigated with simulations.