Strategies for pTesla Field Detection Using Magnetoresistive Sensors With a Soft Pinned Sensing Layer

The detection of low-intensity and low-frequency signals requires magnetic sensors with enhanced sensitivity, low noise levels, and improved field detection at low operating frequencies. Two strategies to improve the detectivity levels of devices based on tunnel magnetoresistive sensors are demonstrated: 1) a large number of sensors connected in series or 2) an individual sensor with a large sensing area and integrated magnetic flux guides. State-of-the-art MgO-based magnetic tunnel junctions with a soft pinned sensing layer were used in this paper. For 952 sensors in series a sensitivity of 29.3%/mT and a detection level of 455 pT/Hz^1/2 were obtained at 100 Hz, whereas the integration of magnetic flux guides in a single sensor yielded a sensitivity of 138.3%/mT and a detection level of 576 pT/Hz^1/2 at the same frequency. These two strategies imply a large device footprint, being suitable when a high spatial resolution is not an application requirement.