Radio-frequency magnetic fields for electron spin resonance in atomic force microscopy

Electron spin resonance (ESR) probes transitions between spin states of unpaired electrons induced by radio-frequency (RF) electromagnetic fields. In conventional ESR, the detected signal represents the ensemble-averaged behavior of many spins. The integration of ESR into advanced microscopic techniques, like scanning tunneling microscopy (STM), has taken ESR to new heights, allowing for coherent single-spin control with atomic-scale spatial resolution.

At the same time, atomic force microscopy (AFM) has emerged as an alternative to STM in this context. AFM is compatible with a wide range of systems and substrates while extending spin coherence times due to its less invasive spin-sensing method. The generation of an RF magnetic field for driving spin resonance, however, requires a different strategy, which Spachtholz et al. worked to develop.

“The implementation of RF fields in scanning probe microscopes has been explored in several contexts, in particular for ESR-STM,” said author Raffael Spachtholz. “In this approach, an RF electric field is generated in the tip-sample junction and converted into an RF magnetic field. This indirect field-conversion mechanism is not feasible for AFM measurements with a tip that is not spin polarized.”

The researchers’ scanning-probe setup overcomes this limitation.

Inspired by RF magnetic field generation methods used for alternating-current STM and magnetic resonance force microscopy, their setup includes an RF circuit comprised of a flexible polyimide printed circuit board terminating in a single loop coil. This directly generates an RF magnetic field in the AFM junction. Moreover, a microstrip sample was designed to locally enhance the magnetic field and to add an in-plane RF magnetic field component.

The resulting RF setup enables spin transitions in single molecules on insulating surfaces.

“These ESR-AFM measurements minimally perturb the molecule under study, in part because the RF excitation does not rely on a spin-polarized tip,” said author Lisanne Sellies. “By combining the atomic-scale spatial resolution of AFM with the nanoelectronvolt energy resolution of ESR, this approach represents a leap forward for quantum sensing and the probing of quantum logic at the atomic scale.”

Source: “Implementation of radio-frequency magnetic fields for electron spin resonance in a low-temperature atomic force microscope,” by Raffael Spachtholz, Lisanne Sellies, Franziska Bruckmann, Philipp Scheuerer, and Jascha Repp, Review of Scientific Instruments (2026). The article can be accessed at https://doi.org/10.1063/5.0327676 .