Real-Space Observation of Spin-Induced Lattice Distortion of O₂ Monolayers Revealed by Atomic Force Microscopy

The monolayer formed by the adsorption of oxygen (O₂) molecules onto the substrate constitutes a two-dimensional spin system. Real-space imaging of the magnetic structures poses a challenge due to weak molecular interactions and the insulating character of the system. Here, we achieved noninvasive atomic-scale observation of monolayers of O₂ molecules weakly adsorbed on Ag(111) substrates using atomic force microscopy. This technique enables the real-space observation of spin-induced lattice distortions. Density functional theory calculations, incorporating dispersion interactions, and Monte Carlo simulation, reveal that the spin reduction of O₂ molecules on the substrate quantitatively explains the lattice shape both in the interior and at the boundaries of the domain. The precise measurement of lattice distortion enables the determination of the local magnetic structures.