Scanned-probe detection of electron spin resonance from a nitroxide spin probe

We have developed an approach that extends the applicability of ultrasensitive force gradient detection of magnetic resonance to samples with spin-lattice relaxation times (T₁) as short as a single cantilever period. To demonstrate the generality of the approach, which relies on detecting either cantilever frequency or phase, we used it to detect electron spin resonance from a T₁ = 1 ms nitroxide spin probe (cf. Figure) in a thin film at 4.2 K and 0.6 T. Using a custom fabricated cantilever with a 4 μm diameter nickel tip, we achieve a magnetic resonance sensitivity of 410 Bohr magnetons in a 1 Hz bandwidth. Our theory for this quantitatively predicts both the lineshape and the magnitude of the observed cantilever frequency shift as a function of field and cantilever-sample separation. Good agreement was found between nitroxide T₁’s measured mechanically and inductively, indicating that the cantilever magnet is not an appreciable source of spin-lattice relaxation here. We suggest that the new approach has a number of advantages that make it well suited to push magnetic resonance detection and imaging of nitroxide spin labels in an individual macromolecule to single-spin sensitivity. In the figure below, we show the schematic of the scanned-probe electron spin resonance experiment. A microstripline half-wave resonator delivers a transverse magnetic field, B₁, oscillating at 17.7 GHz. In the center of the resonator, the microwave field oscillates along the x direction. A longitudinal Zeeman field of magnitude B₀ ˜ 0:6 T is applied along the z axis. The high-compliance cantilever has its long axis along y and oscillates in the x direction. The cantilever’s 4 μm-diameter nickel tip was affixed by hand. The sample is a 230 nm-thick film of 40 mM TEMPAMINE in perdeuterated polystyrene, coated with 20 nm of gold. The sample film was spin-coated onto a 250 μm-thick quartz wafer. For clarity, sample and substrate are not drawn to scale.