Magnetic Field Sensor Based On Micro Structured Magnetoelastic Surface Acoustic Waves Devices

The last few decades have seen tremendous growth in the area of magnetic sensor technologies. The field has grown from simple micro-machined silicon based devices to more complex integrated microsystems combining high performance transducers as well as wireless interfaces. However, almost all of these devices operate with a complex mechanism while simultaneously being externally powered as well as expensive. Thus, there arises a deep need to develop a magnetic sensor that overcomes the challenges. This research work focused on the development of surface acoustic wave (SAW) sensors for the detection of magnetic field. Owing to the possibility of wireless interrogation, SAW devices of the resonator configuration have been considered in this study. The first part of our work aims to address the physics and interaction between the acoustic waves and magnetostrictive layers when subjected to a magnetic field. We investigated SAW resonators using LiNbO3 as the substrate and multi-layered [TbCo2/FeCo] as the electrode and sensitive material. We studied and showed the role of the shape effect in magnetism arising from the electrode geometry. A model experimental set-up was developed to demonstrate an application of the fabricated device as a sensor for detection of current along a cable. Subsequently, we developed a device that is self-compensated for the effects of temperature on the resonance frequency. The multi-layered sensor was based on ST-cut Quartz as the substrate whose positive temperature coefficient of frequency (TCF) was compensated for by the negative TCF of ZnO and CoFeB. Finally, we combine our understandings of the shape effects in magnetism and the multi-layered TCF compensated SAW structure to develop a device that is not only compensated for the effects of temperature on the resonance frequency but also on the magnetic anisotropy. In addition, this structure also presents the possibility of a proof-of-concept multi-sensory device because along with the temperature compensated resonance peak, there exist other resonances which are highly sensitive to any change in the temperature while at the same time immune to magnetic field.