Implementation of radio-frequency magnetic fields for electron spin resonance in a low-temperature atomic force microscope

The implementation of electron spin resonance in scanning tunneling microscopy (ESR-STM) represented a milestone in controlling spin systems at atomic scales. To drive spin resonance, a radio-frequency (RF) magnetic field is required. Yet, so far, in ESR-STM, instead of an RF magnetic field an RF electric field has been used for driving, which translates into an effective RF magnetic field. For cases in which such a field-conversion mechanism is not feasible, we developed an implementation of a low-GHz RF magnetic field in a scanning-probe setup. To this end, we utilized a single-loop coil based on a flexible polyimide printed-circuit-board to generate the RF magnetic field. To locally enhance the RF magnetic field in the scanning-probe junction, we used a gold-microstrip on an insulating support as a sample surface. We found that up to 3 GHz, the transmission only moderately depends on frequency and exhibits no sharp resonances. This development enabled the implementation of ESR in atomic force microscopy, as was demonstrated for individual pentacene molecules.