A laser beam tracks the motions of the tiny magnet suspended in a vacuum chamber as it wobbles in response to magnetic fields.Credit: Wei Ji, Peking University
Scientists have turned a levitating magnet into a device for measuring magnetic fields, known as a magnetometer — one that is sensitive enough to detect the faint signals of the brain’s electrical activity. The technique, which is simpler than existing approaches, is reported today in Science1.
Some of the most sensitive magnetometers use looped superconducting quantum interference devices (SQUIDs), which only work at temperatures close to absolute zero and therefore require complex cryogenics. Another state-of-the-art device, called a spin-exchange relaxation-free (SERF) magnetometer, needs near-perfect shielding from external magnetic interference. Both of these requirements increase the complexity of the device and extend the distance between the sample and the magnetometer.
The levitating magnetometer described in the latest work reduces this distance to a few hundred micrometres. It is also orders of magnitude more sensitive than another type of magnetometer made from diamond.
The device is, at its heart, an exquisitely sensitive compass needle that is less than one millimetre in size. Wei Ji, a physicist at Peking University in Beijing, and his collaborators suspended the sensor magnet between a ‘lifting’ magnet above it and a diamagnet — a type of material that generates its own opposing magnetic field in response to the field of the lifting magnet — underneath it. The diamagnet therefore pushes the sensor magnet up, stabilizing it.
A laser reflects light off the sensor magnet onto a detector, measuring changes in the sensor’s orientation when a sample is brought close to the device. The whole contraption sits inside a vacuum chamber that is about the size of a Tupperware box.
Ji says that the instrument’s accuracy was achieved using an intricate combination of noise reduction techniques. Magnetic coils and damping systems reduce noise from loose magnetic fields and mechanical vibrations. “When you’re probing the femtotesla scale, even a thin film of aluminium foil can introduce 100 femtoteslas of noise,” says Ji. (A femtotesla is ten billion times weaker than Earth’s magnetic field.)
