Br-Mediated Spin-State Control in Nickelocene and Cobaltocene

Single-molecule magnets represent promising materials due to their stable magnetic states and long relaxation times. Precise engineering of their quantum properties is of importance to realize advanced electronic devices, such as high-density data storage, quantum computing, and spintronics. Here, we investigate the spin state of nickelocene (NiCp₂) and cobaltocene (CoCp₂) molecules manipulated by Br atoms. With a combination of scanning tunneling microscopy and density functional theory calculations, we reveal that the high electronegativity of Br atoms significantly changes the magnetic properties of both NiCp₂ and CoCp₂. For NiCp₂, the spin-state transition from its intrinsic S = 1 to S = 1/2 occurs when the Br atoms underlying the molecule consist of more than five atoms. The spin state is further shifted to S = 0 by approaching a Br-terminated tip toward the molecule. In contrast, a strong hybridization between CoCp₂ and Br atoms leads to a complete quenching of its spin moment. This strategy for tuning molecular spin states provides a promising route toward the scalable design of molecular spintronic devices.