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Magnetic Skyrmionium
In magnetic systems, excitations can be found that are characterized by the orientation of the local magnetic moments of atomic cores. A magnetic skyrmionium is a ring-shaped topological spin texture and is closely related to the magnetic skyrmion.

Topological Charge
The topological charge can be defined as follows.

$$Q=\int \vec{m}(\vec{r})\cdot (\partial_x \vec{m}(\vec{r}) \times \partial_y \vec{m}(\vec{r})) dr^2/4\pi$$

With this definition, the topological charge of a skyrmion can be calculated to be ±1. A magnetic skyrmionium is a topological quasi particle that is composed of a superposition of two magnetic skyrmions of opposite topological charge adding up to zero total topological charge. On this basis one can view the core of a skyrmionium as a skyrmion (yellow central disk in figure) with opposite charge compared to a bigger skyrmion (green disk) in which it is situated.

Different to magnetic skyrmions, that experience a transverse deflection under current driven motion known as the skyrmion Hall effect (similar to the Hall effect), magnetic skyrmioniums are expected to move parallel to electrical-drive currents. The current-driven motion of magnetic excitations is one example of the direct link between topological charge and a physical observable.

Theoretical Predictions
Skyrmioniums have been the subject of numerous theoretical investigations. Besides theoretical predictions concerning the existence of skyrmioniums such as in the 2D Janus mono layer CrGe(Se,Te)3, a lot of research concentrated on their manipulation by electrical currents  , spin currents or spin waves. So far, there is only little experimental evidence for the existence of magnetic skyrmioniums. One example is the observation of skyrmionium in a NiFe-CrSb2Te3 hetero-structure.

Potential Applications
Magnetic excitations such as skyrmions or skyrmioniums are potential building blocks of of next generation spintronic devices, which enable for instance neuromorphic computing.