Passive Wireless Magnetic Sensor

SAW-based magnetic sensors enable the remote interrogation of local magnetic fields. Designs utilizing a magnetostrictive delay line or a magnetic IDT have been proposed, showing a large sensing range with a rather low sensitivity. Another concept, employing a SAW device loaded with a GMI element, has been shown to provide higher sensitivity. However, the GMI element is a microwire structure, which only operates up to a few tens of MHz. In combination with SAW transponders, this low frequency range greatly limits the practicability, since the antenna size will be in the meter range. Multilayer thin film GMI elements, on the other hand, operate up to some GHz. Moreover, exploiting standard thin film technology ensures low fabrication costs and simple integration with other devices. In this work, we explore an integrated SAW-GMI sensor designed for 430 MHz. Multilayer thin film GMI sensor and SAW device are fabricated on the same substrate with GMI element serving as a magnetically dependent impedance load. The optimization of the device is studied through the matching of the load impedance. The result shows a superior magnetic sensitivity comparing to the previous published result. The amplitude of the reflected signal has a linear range from 0.15 mT to 0.4 mT (Fig. 1 (a)). A 1.25uT sensing resolution is obtained (Fig. 1(a) inset). With a proper matching component connected in serial with the GMI sensor, a total magnitude change of 7.8 dB is observed for S11 (Fig. 1 (b)). A corresponding sensitivity of 1.6 dB/Oe are obtained over a linear region of 2.5 Oe.