High-sensitivity surface acoustic wave magnetic field sensors enabled by amorphous Fe-Ga-B films with optimized boron content

Magnetic field sensors based on acoustic resonators and the ΔE effect in ferromagnetic thin films enable compact size, high sensitivity, wide bandwidth, chip-scale field detection. The sensing performance of the device strongly depends on the soft magnetic and magnetostrictive properties of ferromagnetic films. Here, we systematically investigate the relationship between the magnetic field response of ΔE effect-based delay line surface acoustic wave (SAW) devices and the boron (B) doping concentration in Fe-Ga-B magnetostrictive thin films. Structural characterizations reveal a transition from a polycrystalline to a fully amorphous phase at a B content of 9.69 at. %, accompanied by a sharp reduction in both coercivity and in-plane anisotropy field. This transition leads to a step-like improvement in sensor performance. By optimizing the B composition, we identify a doping range that significantly enhances the performance of SAW devices, with a maximum phase shift of 60.81° at a B content of 12.33 at. % and a peak phase sensitivity of 5.95°/Oe at a B content of 18.84 at. %, providing a magnetic material design guideline for advancing magnetoelectric acoustic devices.