Magnetic Field Tunable Ferromagnetic Shape Memory Alloy-Based SAW Resonator

Abstract

In this letter, a novel highly magnetostrictive ferromagnetic shape memory alloy (FSMA) interdigital transducers (IDTs) have been fabricated over AlN piezoelectric layer in the surface acoustic wave (SAW) resonator. The influence of an external magnetic field on the fabricated SAW device has been investigated in terms of device parameters such as resonance frequency ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f}_{R}$ </tex-math></inline-formula> ), frequency shift <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol \Delta {f}_{R}/{f}_{R}$ </tex-math></inline-formula> (%) and electro-mechanical coupling coefficient ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${K}^{{2}}$ </tex-math></inline-formula> ). A resonance frequency of ~2.258 GHz was observed at 0 Oe and a frequency shift ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol \Delta {f}_{R}) \sim {89}$ </tex-math></inline-formula> MHz occurs at 1350 Oe. The present device shows high tunability ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol \Delta {f}_{R}/{f}_{R}) \sim {3.9}$ </tex-math></inline-formula> %. It could be ascribed to the strain-induced giant <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol \Delta \text{E}$ </tex-math></inline-formula> -effect in the highly magnetostrictive Ni-Mn-In layer at room temperature. The experimental results have been fitted with a modified Butterworth-Van Dyke (mBVD) circuit model to extract the equivalent circuit parameters in the absence and presence of the magnetic field. A maximum electromechanical coupling coefficient was calculated to be ~0.58, which decreases to 0.35 with the magnetic field. The fabricated SAW resonator displays a high sensitivity of ~0.66 Hz/nT. Such magnetostrictive FSMA IDTs based tunable SAW resonator could be a potential candidate for room temperature remote magnetic field sensors and multifunctional MEMS devices.