Cheap flexible sensor detects magnetic fields down to 20nT

February 07, 2019 // By Julien Happich
Cheap flexible sensor detects magnetic fields down to 20nT
A team of researchers from Germany and Argentina have devised an ultra-thin planar Hall effect (PHE) sensor capable of detecting very faint magnetic fields even when flexed or wrapped around a wire.

In a paper titled “Highly compliant planar Hall effect sensor with sub 200 nT sensitivity” published in Nature’s npj Flexible Electronics journal, the researchers describe a very simple fabrication process, whereby assymetrical Permalloy (Py) Hall crosses just 20nm thick are patterned on 6µm-thick PET foils before being magnetized through multiple magnetic field sweeps (between ±3 mT).

Fabrication steps of the planar Hall effect (PHE) sensor.

The Hall cross described in the paper, occupying an area of about 400x400µm2, has a high aspect ratio of 10:1. This induces a preferred magnetization axis of the Py structure by shape anisotropy, further facilitated by one of the stripes being elliptically-shaped instead of rectangular. Four electrical contacts complete the device, sensitive mostly to the fields applied parallel to its elliptical axis.

SEM photos of the flexible sensor, flat and bent,
showing the elliptical stripe.

Testing the flexible PHE sensor under various bending radii, the researchers report a resistance variation of only 0.3% after more than 150 bending cycles, and a maximum sensitivity of 0.86 V/T in the linear range of ±50 µT, making these sensors sensitive to magnetic fields down to 20nT, on par with state of the art values reported for rigid sensors. Even when bent to 1mm, the sensors still exhibited a high sensitivity of 0.63 V/T, the paper reports.

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