Quantum size effect (QSE) on the electronic structures of nanoscale antiferromagnets with a monolayer (ML) thickness is of fundamental importance in antiferromagnetic (AFM) spintronics. Here, we have carried out systematic studies on the size-dependent unoccupied electronic states of ML Mn nanoislands on Ag(111) by employing scanning tunneling microscopy/spectroscopy (STM/STS) together with density-functional theory (DFT). According to bias-dependent height profiles, a lower apparent height has been found on the larger Mn island within a bias voltage range of $\pm 1.0\mathrm{eV}$, suggesting a smaller Mn–Ag interlayer distance. Additionally, a single broad $\text{d}I\text{/d}U$ peak from small Mn islands gradually evolves into two distinct peaks at approximately $1.3\text{eV}$ (peak 1) and $1.5\text{eV}$ (peak 2) as the island size increases. On top of that, peaks 1 and 2 move about $0.37\pm 0.05\mathrm{eV}$ and $0.30\pm 0.03\mathrm{eV}$ toward lower energy positions when the area size of the ML Mn island increases. Given the projected density of states (PDOS) deduced from orbital-dependent electronic band structures of ML Mn/Ag(111), two unoccupied $\mathrm{d}I/\mathrm{d}U$ peaks originate from the contributions of out-of-plane Mn-$3d$ orbitals. Further PDOS comparison analyses reveal that the Mn–Ag interlayer coupling develops a stronger energy shift in unoccupied states than the Mn–Mn atomic bonding, yielding the two-peak feature in $\mathrm{d}I/\mathrm{d}U$ spectra resolved experimentally.

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• Electronic structure
• Antiferromagnets
• Nanostructures
• Density functional theory
• Scanning tunneling microscopy
• Scanning tunneling spectroscopy