Advances in SQUID-detected Magnetic Resonance Force Microscopy

Magnetic Resonance Force Microscopy (MRFM) is a technique that combines magnetic resonance protocols with an ultrasensitive cantilever to measure the forces exerted by extremely small numbers of spins. The fundamental limit for the sensitivity of MRFM is given by the thermal force noise, so experiments should be performed at the lowest possible temperature. For this reason, we have developed a three stage mechanical low pass filter, which combines good vibration isolation (expected attenuation > 100 dB at 100 Hz) with a high thermal conductance (cooling power 113 μW at 100 mK). The MRFM can be operated at temperatures around 20 mK thanks to a SQUID-based detection scheme and the mechanical generation of the alternating B1 fields required for the magnetic resonance. We use these technical advances to perform MRFM experiments on a thin copper film, where we obtain frequency-shift signals from the Boltzmann polarization of spins in a volume as small as (40 nm)³. We propose an experiment on a sample containing protons where magnetic resonance imaging with a voxel size < (10 nm)³ should be possible.