@@ -20,7 +20,12 @@ Another position measurement uses the magnetic field instead of the electric fie
For more sensitive, low field devices magnetoresistive sensing elements are often used. <ahref='http://www.physics.nyu.edu/kentlab/Lectures/Pappas_Tutorial_APSMM2008.pdf'>David Pappas of NIST compares</a> noise floors of these technologies, noting Hall effect sensors register around 300,000 $`pT/\sqrt{Hz}`$, while magnetoresistive show roughly 200 $`pT/\sqrt{Hz}`$. Honeywell sells a variety of very sensitive magnetoresistive magnetometers, and NVE Corporation sells sensors based on giant magnetoresistance for very low field measurement. Despite these favorable properties, sensors with these technologies are usually more expensive and less dense than those using the Hall effect.
A similar description holds for fluxgate technology, which can achieve the lowest noise of room temperature magnetic field sensing (10 $`pT/\sqrt{Hz}`$), according to Pappas. The sensors achieving this specification are often of cubic centimeter volume, and cost thousands of dollars (c.f. Barrington). Some have been miniaturized, like Texas Instruments' DRV425, but this sensor only reports 1500 $`pT/\sqrt{Hz}`$.
<ahref='https://link.springer.com/content/pdf/10.1023%2FB%3AJMSE.0000011350.93694.91.pdf'>Giant magnetoinductance</a> sensors are based on changes in skin depth of a signal due to magnetic field, and show low-noise, low-cost sensing (c.f. https://www.scirp.org/journal/PaperInformation.aspx?PaperID=36471). Interesting prototypes have been built, but nothing is commercially available.
Finally, <ahref='http://aip.scitation.org/doi/pdf/10.1063/1.2836410'>magneto-electric sensors</a> are a nascent technology with exciting prospects (100 $`pT/\sqrt{Hz}`$ according to Pappas). In these sensors, a magnetostrictive material (usually a metglas) is combined with a piezoelectric material (usually lead zirconate titonate) into stacked layers. The magnetic field produces strain in the stack, and the piezoelectric materials produce a corresponding voltage that can be read. There is a <a href='https://www.nature.com/nmat/journal/v7/n2/pdf/nmat2106.pdf
'>great Nature Materials paper</a> demonstrating that a common SMD 1206 ceramic capacitor can be used as a 1 cent magnetic field sensor based on this physical phenomenon. Again, this technology is promising, but not yet commercially available.
