Reaching the magnetic anisotropy limit of a 3 <i>d</i> metal atom

TLDR

Designing systems with large magnetic anisotropy is essential for nanoscopic magnets, yet the anisotropy energy per atom in single‑molecule magnets and ferromagnetic films remains 1–2 orders of magnitude below the theoretical limit set by atomic spin–orbit interaction. The record anisotropy arises from a strong axial ligand field at the O adsorption site on MgO(100), producing out‑of‑plane uniaxial anisotropy while preserving Co’s gas‑phase orbital moment, as confirmed by X‑ray magnetic circular dichroism. Co atoms coordinated to the O site of MgO(100) exhibit a record‑high zero‑field splitting of 58 meV and slow magnetization relaxation, demonstrating the maximum magnetic anisotropy achievable for a 3d transition‑metal atom.

Abstract

Designing systems with large magnetic anisotropy is critical to realize nanoscopic magnets. Thus far, the magnetic anisotropy energy per atom in single-molecule magnets and ferromagnetic films remains typically one to two orders of magnitude below the theoretical limit imposed by the atomic spin-orbit interaction. We realized the maximum magnetic anisotropy for a 3d transition metal atom by coordinating a single Co atom to the O site of an MgO(100) surface. Scanning tunneling spectroscopy reveals a record-high zero-field splitting of 58 millielectron volts as well as slow relaxation of the Co atom's magnetization. This striking behavior originates from the dominating axial ligand field at the O adsorption site, which leads to out-of-plane uniaxial anisotropy while preserving the gas-phase orbital moment of Co, as observed with x-ray magnetic circular dichroism.