Tunable magnetic coupling between two spin 1/2 molecules

The overlap of electronic orbitals from atoms or molecules often leads to specific spin configurations, governed by exchange coupling, to minimize the system's energy. Understanding the relationship between orbital symmetry and exchange coupling is crucial for the development of quantum materials with unique magnetic properties. With a molecular decorated tip, a scanning tunneling microscope can precisely control the interaction between the frontier orbitals of the tip molecule and the target molecule, enabling detailed studies of this phenomenon. Here, we resorted to a spin-1/2 molecule, Co(cyclopentadienyl)₂, whose spin excitation permits a direct measurement of the strength for exchange interaction. By tuning the relative configuration of these spin-1/2 molecules, we demonstrate the ability to switch between antiferromagnetic (AFM) and ferromagnetic (FM) coupling. Furthermore, we map the contour of exchange interactions, revealing angstrom-scale regions where the wavefunctions of the two molecules can be in either magnetic couping.