Detecting and Quantifying Surface Atomic-Layer Magnetism via X-ray-Excited Tunneling

We report the detection of surface atomic-layer magnetism in Ni/Cu(111) films using X-ray-excited tunneling (XET) implemented in synchrotron X-ray scanning tunneling microscopy (SX-STM). Circularly polarized X-rays incident on Ni/Cu(111) heterostructures generate a tunneling current that carries local X-ray magnetic circular dichroism (XMCD) contrast. By simultaneously recording tunneling and total-electron-yield signals, we directly compare surface-sensitive and bulk-averaged magnetism, revealing enhanced magnetic moments confined to the topmost atomic layer, consistent with previous theoretical predictions of surface-moment enhancement. Density-functional calculations provide electronic-structure and charge-transfer context. This work establishes XET as a powerful probe of element-specific surface magnetism, bridging tunneling detection with synchrotron X-ray spectroscopy.