Control of single-spin magnetic anisotropy by exchange coupling

Abstract

The properties of quantum systems interacting with their environment, commonly called open quantum systems, can be affected strongly by this interaction. Although this can lead to unwanted consequences, such as causing decoherence in qubits used for quantum computation, it can also be exploited as a probe of the environment. For example, magnetic resonance imaging is based on the dependence of the spin relaxation times of protons in water molecules in a host's tissue. Here we show that the excitation energy of a single spin, which is determined by magnetocrystalline anisotropy and controls its stability and suitability for use in magnetic data-storage devices, can be modified by varying the exchange coupling of the spin to a nearby conductive electrode. Using scanning tunnelling microscopy and spectroscopy, we observe variations up to a factor of two of the spin excitation energies of individual atoms as the strength of the spin's coupling to the surrounding electronic bath changes. These observations, combined with calculations, show that exchange coupling can strongly modify the magnetic anisotropy. This system is thus one of the few open quantum systems in which the energy levels, and not just the excited-state lifetimes, can be renormalized controllably. Furthermore, we demonstrate that the magnetocrystalline anisotropy, a property normally determined by the local structure around a spin, can be tuned electronically. These effects may play a significant role in the development of spintronic devices in which an individual magnetic atom or molecule is coupled to conducting leads.

Interplay between the Kondo effect and the Ruderman–Kittel–Kasuya–Yosida interaction

“…Many phenomena in strongly correlated electron physics, especially for heavy-fermion metals such as unconventional superconductivity or quantum critical behaviour are attributed to the competition of RKKY interaction with Kondo physics 5 . While single-impurity Kondo physics, where the impurities can be treated individually, has been studied for a long time in macroscopic measurements 6 and more recently in nanostructures [7][8][9][10][11][12][13][14][15][16][17][18] a detailed knowledge of the transition towards a systems of interacting impurities is still lacking.…”

A non-perturbative treatment of quantum impurity problems in real lattices

“…Research in engineering and fabrication of magnetic nano-structures has been, and continues to be, a fast growing field [35][36][37][38][39]. Their usefulness can potentially be applied to data storage, spintronics, sensors, magnetic imaging, quantum computing, and more [40].…”

“…Many exciting ideas have emerged since the turn of the century regarding Kondo and STM, such as the long range Kondo signature of a cobalt atom buried beneath a copper (111) surface [44], controlling a single spin's anisotropy through varying substrate coupling [35], and even atomically building spin-lattices [45]. Another popular subject of research is the behavior of magnetic adatoms on the surface of noble metals, whose (111) surface is known to host a surface state [46][47][48].…”

“…The generic form of the RKKY interaction is written as 35) with χ(r i , r j ) being the electronic susceptibility of the conduction electrons. This interaction can be visualized as an electron being excited from a state of energy E < E f to a state with energy E > E f due to a scattering event with a local magnetic impurity at r 1 .…”

“…Naturally, this leads to exciting and promising candidates for single atom memory and quibits [164][165][166][167][168][169][170]. It has also been shown that the anisotropy can be controlled through the coupling of the spin and a conductive electrode [35]. These magnetic nanostructures can also serve as a platform to simulate magnetic quantum matter in the same spirit as cold atomic systems [135,137,171,172].…”