| 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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