The discovery of the first 2D material that acts as a magnetic topological insulator in the absence of an external magnetic field has been claimed by physicists in South Korea and the US. The material is chromium triiodide and its magnetic properties were characterized by analysing spin oscillations that were induced by neutron scattering.
When atoms inside a 2D material are arranged in certain patterns, their constituent electrons can display a fascinating range of behaviours not usually seen in everyday materials. Within the 2D honeycomb lattice of graphene, for example, so-called “Dirac electrons” can move at relativistic speed and behave much like photons with zero mass.
Some materials can also have interesting properties related to topology. In 2D, topological insulators are a class of materials in which electrons flow freely along the edges of a sheet but cannot flow along the surface. This effect is dependent upon the spin of the electrons and as a result, 2D topological insulators are of great interest to physicists developing spintronic devices in which information is stored and processed using the spin states of electrons.
Particle-like collective oscillations
Another route to spintronics is to store and transport information using magnons, which are particle-like collective oscillations of the spin magnetic moments of a material. Physicists have predicted that some 2D magnets could be 2D magnetic topological insulators. In such materials, magnons could travel along the edges of a sheet, much like electrons in a conventional 2D topological insulator. What is more, these oscillations are expected to be photon-like “Dirac magnons” that could propagate for long times without dissipating energy as heat. This could make them very useful for creating practical spintronic devices.
To look for evidence of topological Dirac magnons, Lebing Chen at at Rice University and colleagues fabricated precisely-aligned sheets of chromium triiodide, which is a magnetic compound that has a honeycomb lattice structure. The samples were then studied at the Spallation Neutron Source at Oak Ridge National Laboratory using inelastic neutron scattering. Neutrons have magnetic moments and this means that they can create magnons when they scatter from a chromium triiodide sheet. By measuring the energy lost by neutrons during the scattering process, the team was able to work-out the properties of magnons in chromium triiodide.
No field required
The team found evidence for topological Dirac magnons, even in the absence of an applied magnetic field. This is unlike previous studies of a different 2D material, which observed similar effects but only in the presence of an external magnetic field.
Chen’s team believe that the effect in chromium triiodide could be caused by the interaction between the spins of moving electrons and the magnetic field created by the relative motion of positively-charged ions in the 2D material – an effect called spin-orbit coupling.
Through future research, Chen and colleagues hope to continue to explore the potential for the materials to be used in the rapidly-advancing field of spintronics.
The research is described in Physical Review X.
