Direct\nObservation of Fully Spin-Polarized Tunnel\nCurrent Between Quantum Spins Using a Single Molecule Sensor

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