Time domain reflectometry for improved Surface Acoustic Wave magnetic field sensor sensitivity

This paper shows the use of time domain reflec-tometry (TDR) applied to Surface Acoustic Waves (SAW) based magnetic field sensors to improve sensitivity of these devices. The basic operating principle of such sensors is the use of a magnetostrictive material along the SAW path to induce a velocity shift when biased with a magnetic field. This velocity shift is related to the interaction of the SAW with the magnetostrictive material through DE-effect. By looking at the multiple echoes occurring after the main acoustic signal (direct transit) in a delay line configuration, it is possible to extend the interaction path, resulting in an increase of the phase shift as the acoustic wave is travelling back and forth through the magnetostrictive material. The velocity shift shape obtained under a bias magnetic field is well explained using an equivalent piezomagnetic model developed in a previous work to assess elastic stiffness constant dependency of the magnetostrictive thin film with the magnetic field. It is shown that it can improve the sensitivity of the magnetic sensor by one order of magnitude.