Topological matter possesses a characteristic that is robust against perturbation as long as the disturbance is not too violent. This topological protection makes such materials candidates for fault-tolerant application in spintronics or quantum computation. Over the past years, researchers in physics, chemistry and materials science have been predicting, synthesizing and exploring new types of topological matter. Zhang et al. now combine two topological properties in a single sample. They create a skyrmionium, a combination of two skyrmions with opposite winding number, in a ferromagnetic film by coupling it to a magnetic topological insulator (TI).
The researchers grow a film of Cr-doped Sb2Te3 by molecular beam epitaxy. This bulk insulator possesses a topological surface state that is fully spin-polarized at low temperatures. On top, they deposit a wedge of Ni80Fe20 as ferromagnetic layer with varying height. By means of X-ray photoemission electron microscopy, Zhang and co-workers show how a skyrmionium can develop in the NiFe layer when cooling the sample below the transition temperature for magnetic order in the TI. The observed magnetic texture is absent both for the ferromagnetic film being coupled to a non-magnetic TI and for the magnetic TI alone. Hence, the appearance of a skyrmionium is directly linked to the coupling between the NiFe film and the spin-polarized topological surface state. The combination of two prominent classes of topological matter is a first step towards a set of complex samples with combined topological properties.