# Enhanced Sensitivity of FeGa Thin-Film Coated SAW Current Sensor
## 1 Introduction  
Current sensors are widely used in current monitoring applications in smart grid line testing, metallurgical and power supplies, rail transit safety warnings and rescue, and power relay protection in industrial automation [1]. Among the available sensing technologies, the surface acoustic wave (SAW) current sensor features fast response, simple structure, low cost, excellent resistance to interference, low power consumption, and long service life [2, 3, 4]. Especially, it can realize wireless and passive measurement means to improve system security [5, 6, 7]. The specialized current sensing prototype employing the magnetoresistance effect was proposed firstly by [PERSON] et al. The obtained current resolution was approximately 5% of full scale (\(-\)800 A-800 A) [6]. Another typical SAW current sensing device is built by depositing magnetostrictive thin film along the acoustic propagation path of the SAW chip. The magnetostrictive thin film produces magnetostrictive strain and so-called \(\Delta E\) effect at the magnetic field generated by the applied current, which modulates the SAW propagation velocity accordingly. Then the corresponding shifts in oscillation frequency are collected to evaluate the applied current information [1, 8, 9]. Since the pioneering work was conducted by [PERSON] et al. in 1975 [10], the SAW devices for sensing current/magnetic field have attracted more interests because of their unique advantages, and meaningful results were reported from some prototypes. Using the FeCo thin film as the sensitive interface, the SAW current sensor prototype was constructed successfully, a larger sensitivity of 16.6 kHz/A was achieved, and the patterned design was considered to improve the hysteresis error [1, 8, 9]. A similar structure was also proposed by [PERSON] et al. ; the current sensitivity of 10.7 kHz/A was achieved by using the FeNi as thesensing interface [11]. [PERSON] et al. developed a SAW sensing chip constructed using a magnetostrictive Ni electrode on ST-cut 90\({}^{\circ}\)X quartz substrate [12]. [PERSON] et al. obtained a maximum SAW velocity shift close to 20% from a multilayered sensing structure of TbCo\({}_{2}\)/FeCo for the shear horizontal wave as a ratio close to 1 between magneto-elastic film thickness and wavelength [13]. [PERSON] et al. proposed a SAW magnetic sensor using Polyvinyl Alcohol (PVA) bound magnetostrictive nanopowder thin film, and a sensor response of up to 678.05 kHz was obtained towards a magnetic field of 120 mT [14]. [PERSON] et al. demonstrated that laminate composed of longitudinally magnetized magnetostrictive Terfenol-D and a transversely poled piezoelectric Pb(Mg\({}_{1/3}\)Nb\({}_{2/3}\))O\({}_{3}\)-PbTiO\({}_{3}\) crystal offered extremely high magnetic field sensitivity of 10\({}^{-11}\) T [15]. [PERSON] et al. analyzed the influence of the deposition process and heat treatment on the performance of devices to improve the limits of detection [16]. [PERSON] et al. deposited the FeGaB film on the AlN piezoelectric materials, which enabled a measurement range of up to 300 pT in the presence of a DC-biased magnetic field [17]. Taking the magnetostriction effect, \(\Delta E\) effect, and the third-order material constants into account, [PERSON] et al. investigated the sensing mechanism of SAW magnetic field sensors [18], and they also explored a grooved sensing surface structure to improve sensitivity. Obviously, the performance of an SAW-based current/magnetic sensor is significantly determined by the magnetostrictive coatings [19]. Terfenol-D features a larger magnetostrictive coefficient. Therefore, very high magnetic sensitivity will be expected. However, an easily oxidized nature prevents its application. Meanwhile, FeGa features lower coercivity, higher Curie temperature, lower cost, and excellent mechanical properties [20], and maximum magnetostriction coefficient is up to 400 ppm [21], which is much larger than that of Fe, Co, Ni and their alloys [22; 23].