A Macroscopic Magnet Precesses
In 1861, physicist James Clerk Maxwell proposed that a magnet behaves to some extent like a spinning gyroscope [1], but his experiments never managed to demonstrate the effect. Since then, researchers have observed various manifestations of so-called gyromagnetism, mostly in specialized magnetic materials or with spinning magnets, but now a research team has detected signatures of gyroscopic motion corresponding to Maxwell’s original ideas [2]. The team used a microscopic magnetic sphere in a technique that, with improvements, could be employed for ultrasensitive magnetic-field detection, which could be useful for research on biological magnetism.
If you try to tilt a gyroscope spinning around a vertical axis, it will respond by tilting at 90° from the push direction, an effect that leads to precession in response to gravity—such as the slow loop executed by the axis of a spinning top. An electron in a magnetic field behaves like a gyroscope in a gravitational field because the electron has a magnetic moment, which is associated with intrinsic angular momentum, or spin. So you might expect that a material whose microscopic spins align—such as an ordinary ferromagnet—would have a macroscopic angular momentum and behave like a gyroscope.
Indeed, early 20th century experiments revealed that changing the magnetization of a freely suspended magnet changes its rotational motion [3] and that a rotating magnet acquires additional magnetization along its rotation axis [4]. Maxwell’s experiments took a different approach, looking for signs of sideways deflection of an electromagnet that was rotated about an axis perpendicular to its coil’s axis. There was no addition or subtraction of mechanical angular momentum to the intrinsic angular momentum associated with the magnet. His experiments and others failed because the large inertia of the magnet overwhelmed the feeble angular momentum associated with electron motion or electron spin. But smaller magnets have less inertia and could allow the effect to be observed.
Armed with much smaller magnets—spheres 40–60 µm in diameter—Andrea Vinante and Felix Ahrens of the Institute for Photonics and Nanotechnologies in Italy picked up where Maxwell left off. They levitated a magnet by placing it inside a 2.5-mm-diameter hole bored into a piece of lead chilled below its superconducting transition temperature. Superconductors expel magnetic fields, so they can form stable magnetic traps. In this experiment, at equilibrium, the magnet’s magnetic moment aligns with a small, nearly horizontal field present in the trap. Following an excitation by the field, the magnet oscillates around its aligned equilibrium orientation for some 20 seconds.
The magnet’s motion can be excited into either of two perpendicular oscillations, horizontal and vertical, with distinct frequencies. But the magnet’s intrinsic angular momentum should generate precession that turns the two oscillations’ otherwise linear trajectories into narrow ellipses.
To detect these subtle motions, Vinante and Ahrens placed two extremely sensitive magnetometers above the levitated magnet, each sensitive to one of the two oscillation modes. By analyzing the two signals, the team determined the tiny amount of intrinsic angular momentum associated with the sphere’s magnetic moment. They also determined the sphere’s g factor, a number proportional to the ratio of a particle’s magnetic moment to its intrinsic angular momentum. The values determined for several spheres were within about 10% of the team’s estimates based on a simple theory accounting for the magnets’ sizes and composition.
Antonio Pontin, a physicist who specializes in quantum optomechanics at the National Institute of Optics in Italy, points out that previous work has found similar effects in the oscillation of optically levitated nanoparticles driven either by an external force or by thermal noise. But demonstrating gyromagnetism that arises from the total internal spin of a particle “has eluded researchers for a very long time,” he says.
Benjamin Stickler, who specializes in quantum optomechanics at Ulm University in Germany, says that observing spin–rotation coupling in a microscale system like a levitated magnet “not only is impressive from a technical point of view, but might also turn out to be practically relevant.” He cites magnetometry as one possible use. Vinante agrees and says that exploiting gyroscopic properties of levitated nanomagnets 100–1000 times smaller could eventually be employed to measure extremely weak magnetic fields.
–Rachel Berkowitz
Rachel Berkowitz is a Corresponding Editor for Physics Magazine based in Vancouver, Canada.
References
- J. C. Maxwell, “Dynamical Theory of Electromagnetism,” in A Treatise on Electricity and Magnetism (Clarendon Press, Oxford, 1873), sec. 575.
- F. Ahrens and A. Vinante, “Observation of gyroscopic coupling in a nonspinning levitated ferromagnet,” Phys. Rev. Lett. 136, 146703 (2026).
- A. Einstein and W. de Haas, “Experimental proof of the existence of Ampere’s molecular currents,” K. Akad. Wet. Amsterdam, Proc. 18, 696 (1915), https://dwc.knaw.nl/DL/publications/PU00012546.pdf.
- S. J. Barnett, “Magnetization by rotation,” Phys. Rev. 6, 239 (1915).