Stabilization of isolated Fe atoms on a 1-nm-thick MgO/Fe(001) insulating surface via critical tunneling for a robust quantum spin platform

Atomically flat insulating films are essential for decoupling adsorbates from metallic substrates and preserving the intrinsic electronic and spin properties of single-atom quantum bits (qubits). Here, we show that the widely used MgO/Fe(001) spintronic system provides a robust platform for single-atom qubits. Using scanning tunneling microscopy (STM) at 4.6 K under ultrahigh vacuum, we investigate ∼1-nm-thick MgO films exhibiting high crystalline quality, a well-defined band gap, and a low defect density. Single Fe atoms deposited on this surface form a double-barrier tunneling junction between vacuum and MgO, with Fe(001) and the STM tip acting as electrodes. In contrast to the commonly studied bilayer MgO films on noble-metal substrates, the increased thickness of MgO on Fe(001) renders STM imaging highly challenging, as the MgO thickness is comparable to the tip–sample separation and imaging near the band gap readily induces adatom displacement. We identify a narrow critical tunneling regime that enables stable imaging, revealing that Fe adatoms are strongly stabilized on the MgO surface. Density functional theory calculations show that crystal and ligand fields lift the orbital degeneracy of Fe, leading to cationic Fe atoms with an effective spin S ≈ 3/2 and strong bonding to surface oxygen atoms. The stability limit of individual Fe atoms on the surface of an insulating MgO film (∼1 nm thick) grown on a Fe(001)-p(1 × 1)O substrate was investigated under UHV conditions at 5 K to evaluate the protective capability of the qubit and the accessibility of tunneling through the double-barrier system.