Abstract
Magnetic tunnel junctions (MTJs) have larger magnetoresistance ratio and higher magnetic sensitivity than other magnetoresistive structures, whereas their 1/f noise is also larger. To reduce the 1/f noise and improve the low frequency detection capability of MTJ sensors, some magnetic flux modulation schemes based on MEMS flux concentrators have been put forward, and several prototypes with gap-changed modulation (GCM) were fabricated by Army Research Laboratory (ARL). Presently, the problem of low magnetic modulation efficiency in GCM is still an obstacle for MTJ sensors with a resolution of pico-Tesla. In this paper, a novel magnetic flux modulation called vertical motion modulation (VMM) is proposed. In VMM, a modulation film driven by a MEMS actuator vibrates vertically above the air gap of the flux concentrator and the MTJs in the air gap detect an AC magnetic field with a DC bias. Theoretically the modulation efficiency of VMM can exceed 30% when the air gap varies from 30μm to 70μm. A preliminary prototype sensor with VMM was fabricated and its modulation efficiency achieves 17.4%. It is expected that the modulation efficiency of MTJ magnetic sensors with VMM can surpass 30% through modifying the sizes and improving the magnetic permeability of modulation films.
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