A Non-Perturbative, Low-Noise Surface Coating for Sensitive Force-Gradient Detection of Electron Spin Resonance in Thin Films

The sensitivity of magnetic resonance force microscopy (MRFM) is limited by surface noise. Coating a thin-film polymer sample with metal has been shown to decrease, by orders of magnitude, sample-related force noise and frequency noise in MRFM experiments. Using both MRFM and inductively detected measurements of electron-spin resonance, we show that thermally evaporating a 12 nm gold layer on a 40 nm nitroxide-doped polystyrene film inactivates the nitroxide spin labels to a depth of 20 nm, making single-spin measurements difficult or impossible. We introduce a laminated sample protocol in which the gold layer is first evaporated on a sacrificial polymer. The sample is deposited on the room-temperature gold layer, removed using solvent lift-off, and placed manually on a coplanar waveguide. Electron spin resonance (ESR) of such a laminated sample was detected via MRFM at cryogenic temperatures using a high-compliance cantilever with an integrated 100-nm-scale cobalt tip. A 20-fold increase of spin signal was observed relative to a thin-film sample prepared instead with an evaporated metal coating. The observed signal is still somewhat smaller than expected, and we discuss sources of loss.

Automated summary

The sensitivity of MRFM is limited by surface noise, but coating a thin-film polymer sample with metal can significantly reduce sample-related noise. In this study, a 12 nm gold layer was evaporated on a 40 nm nitroxide-doped polystyrene film, resulting in the inactivation of nitroxide spin labels to a depth of 20 nm. A laminated sample protocol was introduced, and a 20-fold increase in spin signal was observed using MRFM with a high-compliance cantilever. The observed signal is smaller than expected, and potential sources of loss are discussed.