From Magnetostrictive Composites to Precision Magnetoelectric Field Sensing

The demand for magnetic field sensors is rapidly increasing due to various factors, including the requirements for navigation, orientation, and motion tracking systems, as well as their growing utilization in the automotive sector and notably in biomedical applications. There are two magnetoelectric (ME) sensors discussed in this context. The first employs a magnetostrictive polymer composite (MPC) as its magnetosensitive material, which consists of ferromagnetic particles embedded within a soft polymer matrix. This capacitive sensor, featuring an electret [1], is specifically well suited for biomedical applications due to its low resonance frequency, typically around 100 Hz. To evaluate the sensor's performance, three measurements were conducted. Subsequently, a DC magnetic bias field measurement was carried out to identify the optimal operating point. In the final measurement, the sensor operated at its resonance frequency with the optimal bias field. In each measurement, the amplitude of the resonance frequency was systematically reduced to determine the minimum field the sensor can reliably detect. The resonance frequency is in the range of 50-150 Hz, largely influenced by the MPC's geometry. The sensor requires a bias field of several mT. Furthermore, these measurements confirmed that no remagnetization occurs within the sensor during the bias sweep. Even without optimization, the sensor displayed linear behavior across four orders of magnitude. The magnetic characterization of the sensor revealed that it fundamentally operates as intended, with its magnetic properties remaining stable when exposed to magnetic fields. The sensor could provide valuable insights into the common challenge faced by all magnetic field sensors, which is noise. The mechanical characteristics of the sensor are derived from the polymer matrix, while the magnetic properties are predominantly determined by the particles. Both components can be independently adjusted or altered, allowing an investigation of the individual noise components. To understand all the mechanics of the sensor a second simplified approach was adopted.<br/>The second sensor is the most elementary version of the first sensor, where only two macroscopic ferromagnets are used emulating two particles in the composite, to understand the underlying mechanisms of the MPC. One of these magnets is mounted to a silicon cantilever, while the other remains fixed beneath it. The magnets oppose each other. When an external magnetic field is applied, superposition takes place, and the cantilever starts to oscillate. On the top part of the cantilever a piezoelectric AlN thinfilm is placed between two electrode layers. The mechanical stress of the oscillation is transduced into an electrical voltage and generates a signal proportional to the applied magnetic field. In this composition magnetic noise amplitude densities as low as 47 pT/√Hz have been measured [2]. It is possible to measure within the earth’s magnetic field without sacrificing any performance. The sensor shows a sensitivity of 2170 V/T and doubles as an energy harvester. Without any optimization it was possible to generate 1.31 µW/cm³Oe² with a magnetic AC field of 20 µT in resonance.<br/><br/>[1] Schröder, S., Strunskus, T., Rehders, S. <i>et al.</i> Tunable polytetrafluoroethylene electret films with extraordinary charge stability synthesized by initiated chemical vapor deposition for organic electronics applications. <i>Sci Rep</i> <b>9</b>, 2237 (2019). https://doi.org/10.1038/s41598-018-38390-w<br/>[2] L. Zimoch et al., Self-powered elementary hybrid magnetoelectric sensor, Nano Energy, 2023. https://doi.org/10.1016/j.nanoen.2023.108720<br/><br/>The authors acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-ID 286471992 – SFB 1261