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 National Institute
of Standards and Technology (NIST). Described in the Dec.
27 issue of Applied Physics Letters,* 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 (about 6 pounds).
The new
magnetic sensor is based on the principles of a NIST
chip-scale atomic clock, announced in August 2004. Expected
applications for a commercialized version of the new sensor
could include hand-held devices for sensing unexploded ordnance,
precision navigation, geophysical mapping to locate minerals
or oil, and medical instruments.
Like
the NIST chip-scale clock, the new magnetic sensor can be
fabricated and assembled on semiconductor wafers using existing
techniques for making microelectronics and microelectromechanical
systems (MEMS). This offers the potential for low-cost mass
production of sensors about the size of a computer chip. When
packaged with associated electronics, the researchers believe
the mini magnetometer will measure about 1 cubic centimeter
or about the size of a sugar cube.
Magnetic
fields are produced by the motion of electrons either in the
form of an electrical current or in certain metals such as
iron, cobalt and nickel. The NIST miniature magnetometer is
sensitive enough to detect a concealed rifle about 12 meters
(40 feet) away or a six-inch-diameter steel pipeline up to
35 meters (120 feet) underground.
The sensor 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 vapor. Light from a semiconductor laser
is transmitted through the atomic vapor. In the presence of
a magnetic field, the amount of laser light that is absorbed
by the atoms changes and this is detected by a photocell.
Larger magnetic fields produce proportionally bigger changes
in the atomic energy levels and change the absorption by the
atom.
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. A separate NIST research group has developed
a new
magnetoresistive magnetic sensor.
The research
was funded by the U.S. Defense Advanced Research Projects
Agency (DARPA-MTO).
* P.
Schwindt, S. Knappe, V. Shah, L. Hollberg, J. Kitching, L.
Liew, J. Moreland. "Chip-scale atomic magnetometer."
Applied Physics Letters. 27 Dec. 2004
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