Direct Observation of Fully Spin-Polarized Tunnel Current Between Quantum Spins Using a Single Molecule Sensor

Controlling spin-polarized currents at the nanoscale is of immense importance for high-density magnetic data storage and spin-based logic devices. As electronic devices are miniaturized to the ultimate limit of individual atoms and molecules, electronic transport is strongly influenced by the properties of the individual spin centers and their magnetic interactions. In this work, we demonstrate the precise control and detection of spin-polarized currents through two coupled spin centers at a tunnel junction by controlling their spin-spin interactions. We attach a nickelocene (Nc) molecule to a scanning probe tip and place it over a spin center (either an Fe atom or another Nc molecule) located on a surface. By changing the adsorption orientation of Nc at the tip apex and adjusting the tip-sample distances, we control the wave function overlap between two spin systems, resulting in strong changes in their magnetic exchange coupling, quantum spin states, and spin excitation energies. Coupling the Nc molecule to the surface spin induces exchange-split spin states, enabling the quantitative determination of the spin polarization of tunnel currents. Strongly asymmetric tunneling spectra reveal almost 100% spin-polarized currents through the coupled Nc-Fe spin system. Our findings highlight the potential of these spin systems at the tunnel junction for high-performance spin-based devices engineered at the atomic scale.