Love Wave Magnetic Field Sensor Operating Without External Bias Field

Surface acoustic wave (SAW) technology is widely utilized in electronics as e.g. filters and sensors. One notable application is their implementation for magnetic field detection [1]. Currently, magnetic field sensors have achieved a limit of detection (LOD) below 40 pT*Hz^-1/2 within the frequency range of 1 Hz to 1 kHz. However, these sensors require an external bias field of approximately 0.4 mT to achieve the desired sensitivity [2]. Alternatively, newly developed SAW sensors incorporate a sensing layer of FeCoSiB deposited through ion beam deposition. These sensors can achieve a LOD of 71 pT*Hz^-1/2 at 10 Hz without the need for an external bias field. This eliminates the requirement for an additional magnetic field source during operation.<br/>The SAW sensor is constructed on a substrate of piezoelectric ST-cut quartz with gold interdigital transducers (IDTs) for generating the acoustic wave. A waveguiding layer made of amorphous SiO₂ is placed on top to confine the wave to the surface, resulting in Love waves. They exhibit an in-plane displacement perpendicular to the direction of propagation. The IDTs are arranged sequentially to create a delay line, and a magnetostrictive sensing element is positioned between them. This sensing layer consists of a single-layer amorphous (Fe₈₀Co₂₀)₇₈Si₁₂B₁₀ thin film deposited using ion beam deposition. If a magnetic field is present, it induces changes in the elastic properties of the FeCoSiB due to the ΔE effect. This alteration in properties leads to a phase shift in the Love wave. The sensitivity of the sensor to the magnetic field is determined by measuring this phase shift. This measurement is carried out using a Zurich Instruments UHFLI lock-in amplifier, which employs dynamic phase detection. The phase noise analysis is performed using a Rhode & Schwarz FSWP phase noise analyzer.<br/>The FeCoSiB coating on the new sensor does not exhibit ideal soft magnetic behavior, but instead displays a more hysteretic behavior with a coercivity field of 0.5 mT. Consequently, this leads to a shift in the sensitivity curve, wherein the maximum sensitivity of 1410 °/mT is achieved at a bias field of 0.75 mT. But, even in the remanence state, the sensor retains a sensitivity of 640 °/mT, whereas other sensors with a more ideal soft magnetic film have a sensitivity below 100 °/mT. To determine the LOD, the phase noise is divided by the sensitivity. The best sensor, in this case, achieves a LOD of 71 pT*Hz^-1/2 at 10 Hz and 24 pT*Hz^-1/2 at 100 Hz. The magnetic properties, as well as the findings when considering the sensitivities, will be presented. Furthermore, advances in utilizing LiNbO₃ single crystal substrate, which possesses a significantly higher electromechanical coupling factor than quartz, will be discussed. LiNbO₃ also allows higher excitation frequencies to be used, which is expected to result in a further increase in sensitivity.<br/><br/>[1] A. Kittmann, P. Durdaut, S. Zabel, J. Reermann, J. Schmalz, B. Spetzler, D. Meyners, N. X. Sun, J. McCord, M. Gerken, G. Schmidt, M. Höft, R. Knöchel, F. Faupel, and E. Quandt, “Wide Band Low Noise Love Wave Magnetic Field Sensor System” Scientific Reports, 8, 278, 2018.<br/>[2] V. Schell, E. Spetzler, N. Wolff, L. Bumke, D. Meyners, L. Kienle, J. McCord, E. Quandt, “Exchange biased surface acoustic wave magnetic field sensors” Scientific Reports, 13, 8446, 2023.