Colloquium: Atomic spin chains on surfaces

Choi DJ, Lorente N, Wiebe J, von Bergmann K, Otte AF, and Heinrich AJ.
Reviews of Modern Physics 91, 041001 (2019)

An open-shell atom has a spin that can be oriented in the presence of an external magnetic field giving rise to a magnetic moment. When there are several spins, their mutual interaction can orient them, and gives rise to the rich field of magnetism. The simplest systems one can think of are lines of atoms. These one-dimensional systems are called spin chains and they are incredibly difficult and complex. The best theories and intricate calculations have been devoted to unravelling their physics and exploring all possibilities. Experimentally, a lot of attention has been devoted to their construction and analysis.

The progress in local probe techniques has allowed researchers to study matter atom-by-atom. Single atom manipulation has been achieved, allowing positioning them on solid surfaces, and creating new systems. Spin chains are natural systems to study with local probes. This article explores the work recently done, showing the different ingredients at play in the rich physics of this fully quantum objects.

The article explores the physics of spin chains on surfaces, reviewing the theory and addressing the particular concepts that are needed to understand these objects. The article proceeds with a review of experiments following the different substrates where the spin chains are going to be hosted. Contrary to having self-standing spins, the present work on surfaces needs to take into account and assess the effect of the surface. Indeed, the article shows that regarding the type of coupling between the spin chain and the host, the physics can varied very much, from having exotic correlated systems like spinors, to usual magnetic excitations like spin waves, and also Haldane gaps or Majorana bound states on superconducting surfaces.

A chain of open-shell atoms (Mn9) on a CuN surface on top of a Cu substrate. Cu is not magnetic, and the nitrogen on the first layers allows to have a directional Mn structure grown on it. The last atom on the right side of the chain is an Fe atom. The N atoms of the surface are promoted between the Mn atoms to glue the spin chain together. The colors display the orientation of the local magnetic moments (red is up and yellow is down) showing that the spin chain is antiferromagnetically oriented. This system behaves like an object with a spin of ½ despite the fact that each local spin is either 2 (Fe) or 5/2 (Mn) showing how different the outcome can be from the individual constituents of the spin chain.

The article is a vivid account of on-going research that should have an impact in the design of future magnetic systems as well as experiments with the new local probes that are being currently tested such as electron-spin resonance with the scanning tunneling microscope. All this shows the excitement of the field and the very complex physics that can be nowadays studied by looking at single atoms in atomic systems.