A magnet the thickness of a few sheets of paper floats in mid-air with essentially no friction. It can now detect magnetic signals a billion times weaker than Earth’s own field. Researchers from Peking University and Germany’s Johannes Gutenberg University Mainz built the device. They say it could widen the search for dark matter while also opening new possibilities for monitoring brain activity.
The instrument, called the Levitated Magnet Magnetometer, or LeMaMa, works something like a compass whose needle hovers freely rather than resting on a pivot. Tracking that needle’s tiny deflections lets researchers measure magnetic fields down to the femtotesla level, a quadrillionth of a tesla. Their findings were published in the journal Science on August 6.
Balancing an egg
At the heart of LeMaMa sits a tiny sensing magnet just 0.4 millimeters, or less than 0.02 inches, thick. The team suspended a second magnet above this “needle” to provide an upward force that counteracts gravity. A single magnet alone, though, cannot hold that needle steady on its own.
Ji Wei, an assistant professor at Peking University’s School of Physics and the study’s corresponding author, compared the challenge to balancing an egg on a smooth table. It might stay upright for an instant, he told Guangming Daily, but the slightest disturbance knocks it over. To solve that, the team placed specially designed diamagnetic materials underneath the sensing magnet. Those materials push against it rather than attracting it, functioning like an invisible hand supporting it from below.
With both forces working together, the tiny sensing magnet floats stably in mid-air, becoming a compass without any pivot or suspension thread. An external magnetic field causes the sensor to swing slightly. Measuring that deflection lets the team infer the strength and variation of the field outside.
Cutting the noise
Reaching that level of sensitivity meant cutting environmental noise as much as possible. The sensing magnet sits inside a vacuum chamber roughly the size of a lunchbox, with the entire setup resting on a vibration-isolation stage.
Femtotesla-level measurements have traditionally relied on one of two approaches. Superconducting quantum interference device, or SQUID, magnetometers offer excellent sensitivity but must run at cryogenic temperatures near absolute zero, requiring bulky and expensive liquid-helium cooling. Spin-exchange relaxation-free, or SERF, atomic magnetometers reach similar sensitivity. They need heated atomic vapor and near-zero magnetic field conditions inside heavy shielding, though, which forces samples into a dedicated shielded room and limits close-range, high-resolution detection.
LeMaMa needs none of that. It works at room temperature, requires no magnetic shielding, and is small enough to move outside a specialized lab entirely.
Already hunting axions
In lab tests, LeMaMa picked up tiny magnetic fluctuations at the femtotesla level. It did so even against background noise from Earth’s own magnetic field, which is billions of times stronger. Ji told Guangming Daily that the team has already applied LeMaMa to detect axion dark matter. That work improved sensitivity over previous best results by multiple orders of magnitude within a specific mass range.
Because LeMaMa’s core sensor measures just a few hundred micrometers across, according to Peking University’s own website, the device combines high sensitivity with genuine compactness. In biomedicine, the technology could measure neural magnetic signals for brain research and neurological diagnostics. In geophysical exploration, its small size suits high-precision magnetic mapping and mineral resource surveying.