Tuning the magnetic anisotropy of single U atoms on MgO/Ag(001)

Searching for single-atom systems with large magnetic anisotropy energies and tunable magnetic states is of vital importance for both fundamental research of magnetism at the atomic scale and realization of future spin-based quantum computation or information storage schemes. Single $5f$ electron based actinide atoms are potential candidates for inducing large magnetic anisotropy energies (MAEs), yet they have been much less studied as compared with $3d$ or $4f$ single-atom systems. Here we present the adsorptive, electronic, and magnetic properties of a single $5f$ electron based uranium atom on two-monolayer MgO/Ag(001) by combining scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory. Our results reveal that single U atoms spontaneously adsorb at the hollow sites of the MgO/Ag(001) surface and they can be controllably switched between the hollow and the O-top sites of MgO/Ag(001) via STM atom manipulation. Most importantly, single U atoms at the O-top sites reveal complex tunneling spectral features, including a symmetric dip at the Fermi energy, which is the manifestation of the existence of a relatively large $5f$-driven magnetic anisotropy energy, whereas single U atoms at the hollow sites exhibit a two-lobe subatomic structure stemming from the valence electron orbitals of U itself and show no signs related with magnetic anisotropy. This work proves that single $5f$ electron based U atoms can possess a considerable uniaxial magnetic anisotropy via adsorbing at the appropriate sites on the carefully chosen supporting surface, and their magnetic states can be tuned by atom manipulation techniques.