PicoTesla Magnetic Sensors based on Magnetic Tunneling Junctions with Double-staged Flux Concentrators

Highly sensitive magnetic tunneling junction (MTJ) sensors have shown great potential as the best miniaturized, low-cost, solid-state magnetic sensors, strongly needed in different applications. We have fabricated high performance MTJ sensors and pushed the magnetic sensing limit to a high level, mainly by incorporating double-staged magnetic flux concentrators (MFC), one on the MTJ chip level and the other on a macroscopic level, to amplify the external field. After multiple optimizations on MFC fabrication conditions, it has increased the sensitivity of the MTJ sensor by a large factor of 517 to 775.4%/Oe in terms of magnetoresistance response. The sensor coercivity is only 0.12 Oe which is desired for sensing application, as we will show that the low-field detectability of magnetic sensor have direct relations with its coercivity. By integrating such MTJ sensors from wafers to integrated circuits, our sensor prototype has achieved detectable field limit of 30pT per root hertz at 10 kHz. We will also present the noise spectrum and the sensitivity spectrum of it and discuss its detectabilities based on these information.