# Development of a Magnetostrictive FeNi Coated Surface Acoustic Wave Current Sensor
## 1 Introduction  
Surface acoustic wave (SAW)-based devices attracts increasing attention for various physical and chemical sensing because they offer some unique performances such as high sensitivity, excellent reproducibility, small size, simple structure, robustness, being passive (batteryless), and remotely queried (wireless). Many proven SAW-based sensors were developed for sensing gas species, temperature, force, torque, and so on [1; 2; 3; 4; 5]. Recently, referring to the magnetostrictive thin film as the sensing material or magnetoresistor as the external loaded sensor, a new concept of SAW-based magnetic/current sensor were proposed, allowing possible features of fast response, high sensitivity, strong anti-interference, and small size. The functional magnetostrictive thin-film materials are favored and very promising for high-precision magnetic/current field measurement owing to their large magnetostrictive coefficient, high energy conversion efficiency, fast response and non-contact driving. [PERSON] et al. subsequently presented a new prototype of SAW magnetic sensor using interdigitaltransducers (IDTs) made by magnetostrictive Ni electrodes on a quartz substrate and wireless and passive measurement was offered. Obtained magnetic sensitivity was 730 Hz/mT [6,7]. [PERSON] et al. proposed a similar SAW magnetic sensor with a magnetostrictive-piezoelectric layered structure of Ni/ZnO/IDT/LiNbO\({}_{3}\), and the corresponding finite element modeling (FEM) simulation was performed to predict the sensor performance [8,9]. Using the Galfenol thin film as the sensitive interface, a SAW magnetic field sensor was developed by [PERSON] et al. a maximum change of 0.64% in SAW velocity was obtained with a 500 nm Galfenol thick film, but not including further details about the experimental results [10]. [PERSON] et al. addresses the experimental and theoretical investigations of SAW magnetic sensor structured by TbCo\({}_{2}\)/FeCo/Y-cut LiNbO\({}_{3}\)[11]. Just recently, a magnetostrictive FeCo-coated SAW magnetic/current sensor using the differential oscillation structure was proposed in our previous work, where high sensitivity of 16.6 KHz/A was obtained [12]. As for the magnetoresistor loaded magnetic sensor, [PERSON] et al. developed a wirelessly interrogable micro-magnetic field sensor using the giant magneto-impedance (GMI) effect in a 30 \(\upmu\)m diameter amorphous FeCoSiBNd wire of zero magnetostriction, and a SAW transponder devices, a relative signal sensitivity of 80 dB/T at low magnetic flux density B less than 30 mT, quick response, and a high temperature stability were obtained [13]. Utilizing a reflective delay line pattern loaded a tri-layer GMI sensor, an autonomous magnetic field sensor was realized by [PERSON] et al. and a sensitivity of 0.34 dB/Oe are reported experimentally [14,15]. Also, [PERSON] et al. reported a dexterous design of the configuration of the SAW magnetic/current sensor loaded with magnetoresistance sensor, where wireless and passive measurement for current was realized [16,17], and the obtained current resolution is approximately 5% of full scale (\(-\)800-800 A). Obviously, the magnetostrictive film coated SAW sensor exhibits simpler design, higher magnetic sensitivity and better temperature compensation in sensor structure over the magnetoresistor loaded sensor, and becomes the future research focus. Some magnetostrictive materials with larger magnetostrictive coefficient as FeCo, TbCo\({}_{2}\) were advised as the sensitive coating, which produce higher magnetic sensitivity. However, due to the large magnetic remanence and hysteresis effect of these materials mentioned above, there occurs obvious asymmetry in sensor response at increasing and decreasing current, which degrades significantly the sensor performance.