A Magnet Smaller Than a Grain of Rice, Floating in a Vacuum, Now Detects Magnetic Fields a Billion Times Fainter Than Earth's

A team from Peking University and Johannes Gutenberg University Mainz has built a magnetic field detector out of a permanent magnet smaller than a grain of rice, floating unsupported inside a vacuum chamber, and reported in Science on Friday that it reaches a sensitivity of 32 femtoteslas per square root hertz at room temperature.

A femtotesla is a quadrillionth of a tesla. Earth's magnetic field is roughly 50 microteslas, about a billion times stronger than what this instrument can resolve. The magnetic fields produced by electrical activity in the human brain sit in the femtotesla-to-picotesla range, which is why measuring them has historically required SQUID magnetometers cooled with liquid helium and operated inside magnetically shielded rooms — infrastructure that costs a great deal and does not move.

The device the researchers call LeMaMa, for Levitated Magnet Magnetometer, dispenses with the cryogenics. A tiny magnetic disk is suspended in vacuum by a stack of magnets above it pulling upward against gravity, with a graphite plate below providing stabilizing repulsion — graphite is diamagnetic, meaning it pushes back against a magnetic field rather than being drawn into it. When an external magnetic field arrives, it tilts the floating disk very slightly. A laser bounced off the disk's surface shifts position as it tilts, and a photodetector converts that shift into an electrical signal. The magnet is the sensor; the laser only reads out how far it moved.

The performance gain came from attacking a specific noise source. Any conductive material at room temperature generates fluctuating magnetic fields through the random thermal motion of its own electrons — Johnson noise — and in a sensor this delicate, the parts holding the magnet up can drown out what it is trying to measure. By using a lower plate made of epoxy-bound graphite, the team cut that thermal magnetic noise by more than 70%, which is what pushed the detectable field strength down into the range they report.

The work was led by first author Wei Ji and colleagues. The researchers describe the instrument as "suitable for a wide range of applications, including biological sensing, chemical sensing, and fundamental physics research."

That last category is not a throwaway. Ultra-sensitive magnetometers are among the instruments physicists use to hunt for exotic particles — axion-like particles and other proposed dark matter candidates would, in some models, produce faint oscillating magnetic signals that a sensitive enough detector could pick up. Getting that sensitivity out of a room-temperature tabletop device rather than a helium-cooled installation changes who can run the experiment, and how many can run at once.