Researchers at the University of Augsburg and the University of Vienna have discovered co-existing magnetic skyrmions and antiskyrmions of arbitrary topological charge at room temperature in magnetic Co/Ni multilayer thin films.

The measurements were carried out using the transmission electron microscope
at the Augsburg Institute of Physics. The screen on the right shows spin textures.
© University of Augsburg

Their findings have been published in the journal Nature Physics and open up the possibility for a new paradigm in skyrmionics research. The discovery of novel spin objects with arbitrary topological charge promises to contribute to advances in fundamental and applied research, particularly through their application in information storage devices.

Magnetic skyrmions are localised, stable topological magnetic spin textures resembling a tornado like whirl in a magnetic material. They can be very small, with diameters in the nanometre range, and behave as particles that can be moved, created, and annihilated, which makes them suitable for ‘abacus’-type applications in information storage and logic devices.

In their recently published article in Nature Physics entitled Dipolar skyrmions and antiskyrmions of arbitrary topological charge at room temperature, a group of researchers at the University of Augsburg led by professor Manfred Albrecht demonstrated that these spin objects can only be found in a distinct phase pocket in the stability diagram where the quality factor Q has a value of about 1, which is given by the ratio between the uniaxial magnetic anisotropy and the magnetic shape anisotropy. Thanks to extensive simulations carried out by Sabri Koraltan and colleagues from the simulation group at the University of Vienna, led by professor Dieter Suess, and supported by doctor Nikolai Kiselev, Forschungszentrum Jülich, researchers were also able to identify the exact reasons why spin objects can be found in the stability diagram, their underlying formation process, as well as the necessary material properties that can now also be applied to other material systems.

Magnetic skyrmions and antiskyrmions of arbitrary topological charge
imaged experimentally (right) with Lorentz Transmission Electron Microscopy,
and reproduced in micromagnetic simulations (left).
3D spin arrangement of the 2D
topological spin-objects by stereographic projection.
© University of Augsburg

Outstanding scientific advance in the field of skyrmions and topological spin objects
“We are very enthusiastic about the exciting insights gained by the discovery of these spin objects, which can be easily fabricated at room temperature. This is an outstanding scientific advance in the field of skyrmions and topological spin objects,” says Albrecht.

These nanoscale skyrmionic spin textures provide extra degrees of freedom and can be embedded in thin-film devices enabling different applications ranging from unconventional computing to new storage concepts.

A further very essential aspect of spin objects is that a spin-polarised current induces their motion. When a charge current passes through a conducting magnetic material, the polarised electron spin will exert a torque on the magnetization known as the spin-transfer-torque. This torque can set the higher order skyrmions in motion.

“Using micromagnetic simulations we could demonstrate the efficient control of the motion of these extraordinary spin objects, which opens up further opportunities for skyrmionic devices,” says Koraltan, doctoral candidate, computational group, University of Vienna.

Lorentz transmission electron microscopy at the University of Augsburg was used extensively in the study, which is currently being expanded to visualise the current-induced motion of these multiple charge spin objects.

“To what extent our predictions about their motion characteristics can be confirmed experimentally will be very exciting to research in the near future,” explains Mariam Hassan, postdoctoral researcher, University of Augsburg.

Article: Dipolar skyrmions and antiskyrmions of arbitrary topological charge at room temperature

Nature Physics has published an article written by Mariam Hassan, Institute of Physics, University of Augsburg, Augsburg, Germany, and ISM – CNR, nM2-Lab, Monterotondo Scalo, Roma, Italy, Sabri Koraltan, Physics of Functional Materials, Faculty of Physics, University of Vienna, Vienna, Austria, Vienna Doctoral School in Physics, University of Vienna, Vienna, Austria, and Research Platform MMM Mathematics – Magnetism – Materials, University of Vienna, Vienna, Austria, Aladin Ullrich, Institute of Physics, University of Augsburg, Augsburg, Germany, Florian Bruckner, Physics of Functional Materials, Faculty of Physics, University of Vienna, Vienna, Austria, Rostyslav O. Serha, Vienna Doctoral School in Physics, University of Vienna, Vienna, Austria, and Nanomagnetism and Magnonics, Faculty of Physics, University of Vienna, Vienna, Austria, Khrystyna V. Levchenko, Nanomagnetism and Magnonics, Faculty of Physics, University of Vienna, Vienna, Austria, Gaspare Varvaro, ISM – CNR, nM2-Lab, Monterotondo Scalo, Roma, Italy, Nikolai S. Kiselev, Peter Grünberg Institute and Institute for Advanced Simulation, Forschungszentrum Jülich and JARA, Jülich, Germany, Michael Heigl, Institute of Physics, University of Augsburg, Augsburg, Germany, Claas Abert, Physics of Functional Materials, Faculty of Physics, University of Vienna, Vienna, Austria, and Research Platform MMM Mathematics – Magnetism – Materials, University of Vienna, Vienna, Austria, Dieter Suess, Physics of Functional Materials, Faculty of Physics, University of Vienna, Vienna, Austria, and Research Platform MMM Mathematics – Magnetism – Materials, University of Vienna, Vienna, Austria, and Manfred Albrecht, Institute of Physics, University of Augsburg, Augsburg, Germany.

Abstract: “Magnetic skyrmions are localized, stable topological magnetic textures that can move and interact with each other like ordinary particles when an external stimulus is applied. The efficient control of the motion of spin textures using spin-polarized currents opened an opportunity for skyrmionic devices such as racetrack memory and neuromorphic or reservoir computing. The coexistence of skyrmions with high topological charge in the same system promises further possibilities for efficient technological applications. In this work, we directly observe dipolar skyrmions and antiskyrmions with arbitrary topological charge in Co/Ni multilayers at room temperature. We explore the dipolar-stabilized spin objects with topological charges of up to 10 and characterize their nucleation process, their energy dependence on the topological charge and the effect of the material parameters on their stability. Furthermore, our micromagnetic simulations demonstrate spin-transfer-induced motion of these spin objects, which is important for their potential device application.“