The role of magnetic anisotropy in the Kondo effect

TL;DR

This paper reveals how magnetic anisotropy influences the Kondo effect in larger-spin magnetic atoms, showing it is crucial for the emergence and behavior of Kondo resonances and their magnetic field dependence.

Contribution

It demonstrates the decisive role of magnetic anisotropy in Kondo screening for larger-spin atoms using scanning tunnelling microscopy.

Findings

Kondo resonance appears only when anisotropy creates degenerate ground states.

Resonance splitting in magnetic field depends on anisotropy direction.

The splitting rates match the energies of unscreened spin states.

Abstract

In the Kondo effect, a localized magnetic moment is screened by forming a correlated electron system with the surrounding conduction electrons of a non-magnetic host. Spin S=1/2 Kondo systems have been investigated extensively in theory and experiments, but magnetic atoms often have a larger spin. Larger spins are subject to the influence of magnetocrystalline anisotropy, which describes the dependence of the magnetic moment's energy on the orientation of the spin relative to its surrounding atomic environment. Here we demonstrate the decisive role of magnetic anisotropy in the physics of Kondo screening. A scanning tunnelling microscope is used to simultaneously determine the magnitude of the spin, the magnetic anisotropy and the Kondo properties of individual magnetic atoms on a surface. We find that a Kondo resonance emerges for large-spin atoms only when the magnetic anisotropy creates degenerate ground-state levels that are connected by the spin flip of a screening electron. The magnetic anisotropy also determines how the Kondo resonance evolves in a magnetic field: the resonance peak splits at rates that are strongly direction dependent. These rates are well described by the energies of the underlying unscreened spin states.