Magnetometry based on a levitated ferromagnetic particle

A “magnetometer” based on a magnetized ferromagnetic micro-particle levitated over a superconductor is demonstrated. The particle is placed inside a well in the superconductor; when the superconductor is cooled below the critical temperature with liquid helium, the particle is levitated as a consequence of the Meissner effect. The levitated particle is isolated from molecular collisions at the cryogenic temperatures and partially shielded from external magnetic fields because the superconductor used for levitation acts as magnetic shield which is free of Johnson noise, with a shielding factor better than $5\times 10^6$. When external magnetic field is applied to the system, the particle only feels a small residual field determined by the geometry of the experiment. The motion of the particle is recorded with a camera. The residual magnetic field is determined by measuring the frequencies of the particle’s motion. Such a “magnetometer” may be useful for measurement of, for example, exotic spin-dependent interactions of electrons.