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High sensitivity magnetic sensors will have a significant impact on security, industry, and quality of life. Many of these applications require sensitivities better than 1 nT/Hz1/2 as well as low cost, small size, low maintenance, and low power consumption. There is a need to develop novel sensors applicable to the diverse detection and discrimination problems of munitions and explosives of concern (MEC) contaminated sites.
The objective of this project was to investigate a low-cost, practical magnetic sensor system suitable for high sensitivity magnetic field mapping, based on solid-state magnetic tunneling junction (MTJ) devices with a sensitivity in the picotesla (pT, 10-12 tesla) range at room temperature.
The key features of this design are (1) decreasing the noise by the use of a 64 element bridge, (2) reducing the magnetic noise by annealing of MTJ in high magnetic field and a hydrogen environment, and (3) increasing signal by using a external flux-to-field magnetic flux concentrators (MFC).
The researchers developed a low power, compact, magnetoresistive sensor that combines a 64 element MTJ bridge and a set of low noise magnetic flux concentrators. Sensitivity in the range of a few picotesla has been achieved. Magnetic field sensitivities of the prototype magnetic sensor are about 1 pT/Hz1/2 at 1 kHz, 5 pT/Hz1/2 at 10 Hz, and 50 pT/Hz1/2 at 1 Hz. The magnetic sensor only dissipates 15 mW of power while operating.
In the future, further improvements of MTJ sensors for operation in the femtotesla (fT, 10-15 tesla) range at room temperature are possible. It can be achieved by using (1) new magnesium oxide (MgO) based tunneling junctions, (2) a better design of magnetic flux concentrator, and (3) optimized stack deposition conditions and annealing procedures to eliminate defects giving rise to 1/f noise. It has been shown that proper annealing reduces magnetic noise by about an order of magnitude in MgO based tunneling junctions.
A highly sensitive magnetic sensor can be applicable to the diverse detection and discrimination problems of sites contaminated with MEC or unexploded ordnance.