Controlling the Exchange Field of On-Surface Magnetic Molecules and Atoms via Direct-Current Voltages

Recent advances in scanning tunneling microscopy have enabled quantum-coherent control of single surface spins via all-electric electron spin resonance (ESR). Such control requires magnetoelectric coupling, since spin resonance is a magnetic effect. We show that a magnetic tip induces a bias-dependent exchange field on a localized Anderson impurity via virtual particle exchange with the magnetic lead. This field differs from the Heisenberg exchange and can be tuned, reversed, or suppressed by the bias voltage. Our model reproduces bias-controlled resonance shifts for individual electron spin <i>S</i> = 1/2 titanium atoms and iron(II) phthalocyanine molecules. Their distinct bias responses provide unambiguous spectroscopic identification of different magnetic species through their characteristic electrochemical parameters. The exchange field is the magnetoelectric mechanism behind all-electric ESR and enables spin control for atomic-scale spin-based quantum technologies via electric fields.