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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. ^{[1] [2]}

Topological charge

The topological charge can be defined as follows. ^[3]

${\displaystyle Q=\int {\vec {m}}({\vec {r}})\cdot (\partial _{x}{\vec {m}}({\vec {r}})\times \partial _{y}{\vec {m}}({\vec {r}}))dr^{2}/4\pi }$

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. ^{[4] [5]} 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 ^{[6] [7]} (similar to the Hall effect), magnetic skyrmioniums are expected to move parallel to electrical-drive currents. ^[8] 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. ^{[9] [10] [11]} Besides theoretical predictions concerning the existence of skyrmioniums such as in the 2D Janus mono layer CrGe(Se,Te)₃, ^[12] a lot of research concentrated on their manipulation by electrical currents, ^{[13] [14] [15]} spin currents ^[16] or spin waves. ^{[17] [18]} 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. ^[19]

Potential applications

Magnetic excitations such as skyrmions or skyrmioniums are potential building blocks of next generation spintronic devices, which enable for instance neuromorphic computing. ^{[20] [21]}

References