A magnetostrictive FeNi-coated surface acoustic wave (SAW)-based current sensor was proposed in this work. The weak remanence and hysteresis effect of the FeNi itself contributes to suppress the asymmetry in sensor response at increasing and decreasing current. The sensor response was simulated by solving the coupled electromechanical field equation in layered structure considering the magnetostrictive effect and an approach of effective dielectric constant. The effects from the aspect ratio and thickness of the FeNi film on sensor response were analyzed to determine the optimal design parameters. Differential oscillation structure was used to form the sensor, in which, the FeNi thin film was deposited along the SAW propagation of the sensor chip by using RF magnetron sputtering. The magnetostrictive effect of the FeNi coating induced by the magnetic loading generates the perturbation in SAW velocity, and corresponding oscillation frequency. High sensitivity of 10.7 KHz/A, good linearity and repeatability, lower hysteresis error of 0.97% were obtained from the developed prototype 150 MHz SAW FeNi coated current sensor.

Magnetostrictive thin-film coated Surface Acoustic Wave (SAW) devices were promising for sensing magnetic field owing to their superior features as micro-size, fast response, and high sensitivity originated from the magnetostrictive effect. However, the magnetostriction nature in magnetostrictive thin-film causes significantly mechanical fatigue in service, deteriorating the sensor performances. In this work, the fatigue phenomenon in magnetostrictive coating was underlined by characterizing the prepared FeCo thin-film coated magnetic device cyclically. Obvious shedding was observed in FeCo coating after cyclic testing and the magnetic-sensitivity decreases significantly. One of the reasons is the weak adhesion of FeCo thin-film towards the substrate. As an available way allowing enhancement of adhesion, a Cr thin-film was employed as the transition-layer to weaken the mechanical fatigue. However, it accompanied by the issue of the reduced magnetostrictive coefficient and the obstruction in magnetostrain-tranfer to piezoelectric substrate. As a result, the slump in sensitivity was observed. To address such issues, a design of dotted-pattern with Cr transition layer was employed to build the SAW based magnetic-device. High magnetic-sensitivity and excellent long-term stability were achieved because of the release of coercive force in FeCo dots and enhancement of the FeCo adhesion to the substrate.

A surface acoustic wave (SAW) device is proposed for sensing current by employing the patterned FeGa thin film as the sensitive interface. The layered media structure of FeGa/SiO₂/LiNbO₃ was established to reveal the working principle of the sensors, and an SAW chip patterned by delay-line and operating at 150 MHz was fabricated photolithographically on 128° YX LiNbO₃ substrate. The FeGa thin film with a larger magnetostrictive coefficient was sputtered onto the acoustic propagation path of the SAW chip to build the sensing device. The prepared device was connected into the differential oscillation loop to construct the current sensor. The FeGa thin film produces magnetostrictive strain and so-called ΔE effect at the magnetic field generated by the applied current, which modulates the SAW propagation velocity accordingly. The differential frequency signal was collected to characterize the measurand. Larger sensitivity of 37.9 kHz/A, low hysteresis error of 0.81%, excellent repeatability and stability were achieved in the experiments from the developed sensing device.

A new surface acoustic wave (SAW) magnetic field sensor was conducted in this contribution, which was composed of a differential delay-line oscillation configuration, and a magnetostrictive FeCo film coated onto the SAW propagation path of the sensing SAW device. The FeCo coating produces magnetostrictive strain under the magnetic field, then leading to changes in the SAW propagation velocity. The corresponding differential oscillation frequency shift was used to evaluate the applied magnetic field. By theoretical analysis, the sensor response from the FeCo coated magnetic field sensor was predicted. Using the Helmholtz coil system, the developed SAW sensor was characterized, high sensitivity of 8.3 kHz/mT and good linearity were achieved from the developed 300 MHz SAW magnetic field sensor.