Magnetoelectric (ME) magnetic field sensors commonly rely on one of the two modulation principles: the nonlinear dependence of magnetostrictive strain on the applied field or the stress-induced change in magnetization susceptibility. While both effects coexist in any ME device, different readout schemes can be chosen to utilize one or the other effect for magnetic field sensing. This work demonstrates that both principles can be simultaneously implemented in a single electrically modulated ME sensor with inductive readout (a converse ME sensor). This mixed modulation approach significantly enhances low-frequency sensitivity while not affecting the sensitivity at higher frequencies. This leads to a nontrivial dependency of the sensor sensitivity on the frequency of the magnetic field to be measured and can effectively decrease the sensor bandwidth by up to an order of magnitude. We show that the contribution of the modulation from the nonlinearity of the magnetostrictive strain to the sensor sensitivity can be changed by applying a magnetic bias field, offering an additional dimension to the design of ME sensors, especially for potential applications in the unshielded environment.

In principle, electrode-based bioelectrical signal acquisition can be complemented by biomagnetic sensing and therefore requires a more detailed assessment, especially because of the availability of novel noncryogenic sensor technologies. The current development of thin-film magnetoelectric (ME) sensors ensures that ME technology is becoming a prospective candidate for biomagnetometry. The main obstacle for large-scale usage is the lack of extremely low noise floors at the final sensor system output. This article highlights the current state of ME sensor development based on a magnetocardiography (MCG) pilot study involving a healthy volunteer in a magnetically shielded chamber. For assessment, an ME prototype (converse ME thin-film sensors) will be applied for the first time. This sensor type ensures a noise amplitude spectral density below 20 pT / $\sqrt {\text {Hz}}$ at 10 Hz by using a sophisticated magnetic layer system. The main aim of this pilot study is to evaluate the applicability of this promising sensor for the detection of a human heart signal and to evaluate the sensor output with competitive optical magnetometry technology. A magnetic equivalent of a human R wave could be successfully detected within a 1-min measurement period with the sensor presented here. Finally, the article will provide an outlook on future ME perspectives and challenges, especially for cardiovascular applications.

Abstract Some magnetoelectric sensors require predefined external magnetic fields to satisfy optimal operation depending on their resonance frequency. While coils commonly generate this external magnetic field, a microelectromechanical systems (MEMS) resonator integrated with permanent magnets could be a possible replacement. In this proof-of-concept study, the interaction of a MEMS resonator and the ME sensor is investigated and compared with the standard approach to achieve the best possible sensor operation in terms of sensitivity. The achievable sensor sensitivity was evaluated experimentally by generating the magnetic excitation signal by a coil or a small-sized MEMS resonator. Moreover, the possibility of using both approaches simultaneously was also analysed. The MEMS resonator operated with 20Vppat 1.377 kHz has achieved a sensor sensitivity of 221.21mV/T. This sensitivity is comparable with the standard approach, where only a coil for sensor excitation is used. The enhanced sensitivity of 277.0mV/T could be identified by generating the excitation signal simultaneously by a coil and the MEMS resonator in parallel. In conclusion, these MEMS resonator methods can potentially increase the sensitivity of the ME sensor even further. The unequal excitation frequency of the MEMS resonator and the resonance frequency of the ME sensor currently limit the performance. Furthermore, the MEMS resonator as a coil replacement also enables the complete sensor system to be scaled down. Therefore, optimizations to match both frequencies even better are under investigation.

Biomagnetic field sensors based on AlN/FeCoSiB magnetoelectric (ME) composites desire a resonant frequency that can be precisely tuned to match the biomagnetic signal of interest. A tunable mechanical resonant frequency is achieved when ME composites are integrated onto shape memory alloy (SMA) thin films. Here, high-quality c -axis growth of AlN is obtained on (111) Pt seed layers on both amorphous and crystallized TiNiCu SMA thin films on Si substrates. These composites show large piezoelectric coefficients as high as d 33,f = 6.4 pm/V ± 0.2 pm/V. Annealing the AlN/Pt/Ta/amorphous TiNiCu/Si composites to 700 °C to crystallize TiNiCu promoted interdiffusion of Ti into the Ta/Pt layers, leading to an enhanced conductivity in AlN. Depositing AlN onto already crystalline TiNiCu films with low surface roughness resulted in the best piezoelectric films and hence is found to be a more desirable processing route for ME composite applications.