Our cantilevers have a low intrinsic damping rate: this enables us to measure with low noise, since damping is proportionate to noise. However, when the magnet is close to the surface of a sample we want to study, we measure a much higher effective damping rate. This is caused by magnetic interaction with flipping electron spins at the surface. These spin flips could be suppressed by application of a strong static magnetic field. Additionally, a strong static magnetic field would increase the polarization and the signal from the nuclear spins that we do MRI on.

Application of a magnetic field seems simple, but the challenge is that the flux noise must be low enough to not raise the noise floor of our SQUID magnetometer detection, which is on the order of a millionth of a flux quantum through a 10x10 micron loop. The only way to achieve that is to apply the field only locally, inside the smallest superconducting shielding around the experiment, and to use a Persistent Current Switch[1] to eliminate external noise sources by trapping the current in a superconducting loop.