Levitation of a permanent magnet within a superconducting radio frequency cavity*

We report on the magnetic levitation of millimeter-sized neodymium permanent magnets within the interior of a superconducting radio frequency (SRF) cavity. To the best of our knowledge, this is the first experimental work on levitating a magnet within an SRF cavity. The cavity is a coaxial quarter-wave microwave resonator made from 6061 aluminum, having a resonance frequency of 10 GHz and a loaded Q of about 2,000. The 3 disc magnets have identical dimensions of 0.5-mm high, 1-mm diameter, and 3.75-mg mass. We performed a sequence of experiments, below the 1.2-K superconducting transition temperature of aluminum, where only the magnetic field strength changes, while the size, shape and mass of the magnet are held constant . For this purpose, the magnets of remanence (maximum magnetic field) 1.22T, 1.32T, 1.44T, 1.47T is used. The coaxial mode's resonance frequency shifts as a function of the levitation height of the magnet, giving an estimate of the magnet’s position, and mechanical motion. Both the levitation height and levitation temperature are found to increase with increasing magnetic field. The measurements are consistent over several heating and cooling cycles.