Novel sample preparation and detection method for magnetic resonance force imaging of electron spins

Magnetic resonance force microscopy (MRFM) is a technique that opens up the possibility of extremely sensitive detection and imaging of electron spins, with sensitivity capable of detecting a single radical. In our experiment, we detect a change in magnetization of nitroxide radical probes as a shift in the frequency of a magnet-tipped cantilever. We discuss recent developments that have allowed us to overcome difficulties with this measurement. One difficulty is frequency noise from sample dielectric fluctuations interacting with charges on the cantilever tip. We have found that we can reduce this noise by applying a 10 nm metal coating over the sample. However, deposition of this metallic coating produces a “dead layer” in which nitroxide labels are no longer active. We show a new sample preparation technique which applies this coating without exposing sample radicals during metal deposition. Another difficulty is that the motion of the cantilever magnet causes a change in the resonance frequency of spins during the microwave pulses, which leads to image blurring and reduction in sensitivity. We show ways to “pause” cantilever motion during microwave application to prevent blurring and improve sensitivity.