Thermal Annealing Impact on the Sensitivity of Piezomagnetic Surface Acoustic Waveguide to Applied Magnetic Field

Magnetostrictive thin films, exhibit significant utility as functionalization materials for Surface acoustic wave sensors designed for magnetic field measurements. This research investigates the influence of annealing under vacuum and magnetic field at various temperatures on the magnetic properties of the FeCo/TbCo₂ thin film and therefore on the sensitivity of shear and Rayleigh acoustic waveguides functionalized with these magnetic layers. The observed effects include a reduction in magnetic anisotropy field and an increase in magnetostriction. XPS and TEM coupled with EDS microanalysis provide insights into the variations in the properties of the nanostructured magnetic film. To safeguard the magnetic film, a thin layer of SiO₂ is deposited on top, serving as a protective shield. The inclusion of this layer amplifies sensitivity to applied magnetic fields for modes with shear polarization while reducing sensitivity for Rayleigh mode. In ST-cut Quartz, the shear waves become waveguided upon the incorporation of magnetic thin films, with further enhancement achieved by introducing a SiO₂ layer. Rayleigh waves are evanescent modes, the penetration depth is in the order of wavelength, and the addition of SiO₂ decrease the wave confinement and therefore the sensitivity. The findings are supported by a theoretical model and experimentally validated results.