Ferromagnetic Shape Memory Alloy Integrated Highly Flexible SAW Delay Line Magnetic Sensor

In this article, the surface acoustic wave (SAW) device with delay line was fabricated over highly flexible aluminum nitride (AlN)/Ni–Mn–In/Kapton magnetoelectric heterostructure for flexible MEMS application. The fabricated device resonates at
$\sim 1.40$
GHz. Resonance frequency (
${f}_{R}{)}$
tuning of the fabricated device has also been investigated with the externally applied magnetic field and the tunability
$(\Delta {f}_{R}/{f}_{R})$
of
$\sim 9$
% was observed. The modified Butterworth–Van Dyke (mBVD) circuit model was utilized for fitting the experimentally measured scattering
${S}_{{11}}$
parameters with and without external magnetic field and the corresponding equivalent circuit parameters have been extracted. The quality factor
$({Q})$
, electromechanical coupling coefficient
$({K}^{{2}})$
, and figure-of-merit (FOM) were calculated in the absence and presence of an external magnetic field. A significant sensitivity for the SAW device was found to be of
$\sim 0.94$
Hz/nT at room temperature. The alteration in
${f}_{R}$
can be attributed to the
$\Delta {E}$
-effect based on the magnetostrictive strain transfer mechanism in the AlN/Ni–Mn–In heterostructure. The SAW device fabricated over highly flexible Kapton substrate shows excellent flexibility in terms of
$\sim 200$
bending cycles without any degradation in the resonance frequency. Such novel ferromagnetic shape memory alloy (FSMA, i.e., Ni–Mn–In) integrated SAW delay line-based flexible piezoresonator displays full potential toward futuristic flexible and wearable electronics applications.