Tuning the quantum spin states of Co-phthalocyanine on good, semi-, and half-metals

Control of quantum spin states of magnetic molecules on surfaces has been actively pursued for emergent exotic physics and potential technological applications. Co-phthalocyanine (CoPc) is a prototypical molecular magnet with a spin of 1/2, which has been shown to be too fragile when the molecule is adsorbed on good metals but can be preserved on the semimetallic Sb(111) substrate. Here, we use first-principles calculations to reveal that, counterintuitively, the spin of individual CoPc is also quenched on the semimetallic Bi(111) substrate, and the underlying reason is attributed to a significant elevation of the Fermi level close to the Co-${d}_{{z}^{2}}$ orbital by the stronger spin-orbit coupling in bismuth. We further show that the molecular spin can be recovered via intermolecular coupling, accompanied by a topological phase transition from a quantum spin Hall state to a magnetic topological semimetal. Finally, we demonstrate that the spin state of CoPc is enhanced on the half-metallic $\mathrm{Cr}{\mathrm{O}}_{2}$(110) substrate, associated with reverse band alignment and charge transfer. Collectively, our comparative studies establish rich tunability of molecular spins on different metal substrates for future molecular spintronic and quantum information devices.