A low-power, magnetic sensor about the size of a grain of rice that can detect magnetic field changes as small as 50 picoteslas – a million times weaker than the Earth’s magnetic field – has been demonstrated by researchers at the US-based National Institute of Standards and Technology (NIST).
The device can be powered with batteries and is about 100 times smaller than current atom-based sensors with similar sensitivities which typically weigh several kilograms.
The sensor itself works by detecting minute changes in the energy levels of electrons in the presence of a magnetic field.
A tiny sample of the element rubidium is heated within a sealed, transparent cell to form a rubidium vapour. Light from a semiconductor laser is transmitted through the atomic vapour. In the presence of a magnetic field, the amount of laser light that is absorbed by the atoms changes and this is then detected by a photocell.
The key advantages of the new sensor, says Peter Schwindt, one of the NIST developers, are its accuracy and sensitivity given its small size. So called “fluxgate” magnetometers achieve equivalent or better sensitivity but are much less accurate and much larger. They also detect only the portion of a magnetic field pointing along the sensor, while the atomic magnetometers detect the total field strength, a desirable capability for many magnetic imaging and search applications.
Superconducting quantum interference devices (SQUIDs) are more sensitive, but must be cryogenically cooled, making them substantially larger, power hungry and more expensive. “Magnetoresistive” devices like those used in heads that read computer hard drives are small and cheap, but are typically less sensitive and less accurate.