Skyrmions

Researchers at Switzerland EPFL, China's Anhui University, Germany's University of Cologne and University of New Hampshire in the US have developed a technique that can visualize and control the rotation of a handful of spins arranged in a vortex-like texture at the fastest speed ever achieved. The breakthrough can advance spintronics devices like computer memory, logic gates, and high-precision sensors.

"The visualization and deterministic control of very few spins has not yet been achieved at the ultrafast timescales," says Dr. Phoebe Tengdin, a postdoc at EPFL, pointing out the very tight timeframes that this control needs to happen for spintronics to ever make the leap into applications. Now, the team developed a new technique that can visualize and control the rotation of a handful of spins arranged in a vortex-like texture, a kind of spin "nano-whirlpool" called a skyrmion.

A research team from the Technical University of Kaiserslautern (TUK) has been awarded a Consolidator Grant from the European Research Council (ERC) to develop spintronic devices.

Professor Dr. Mathias Weiler, lead of the study, will receive €2 million over the next five years. Scientists are working on spin waves and new spintronic devices that could drastically accelerate the storage, processing, and transmission of information.

A China-Australia collaboration has, for the first time, illustrated that Dzyaloshinskii-Moriya interactions (DMI), an antisymmetric exchange vital for forming various chiral spin textures such as skyrmions, can be induced in a layered material tantalum-sulfide (TaS₂) by intercalating iron atoms, and can further be tuned by gate-induced proton intercalation.

Magnetic-spin interactions that allow spin-manipulation by electrical control allow potential applications in energy-efficient spintronic devices.

Recent studies have suggested that 2D skyrmions could be the genesis of a 3D spin pattern called hopfions, but no one had been able to experimentally prove that magnetic hopfions exist on the nanoscale. Now, a team of researchers co-led by Berkeley Lab reported the first demonstration and observation of 3D hopfions emerging from skyrmions at the nanoscale in a magnetic system.

Artist’s drawing of characteristic 3D spin texture of a magnetic hopfion. Berkeley Lab scientists have created and observed 3D hopfions. Credit: Peter Fischer and Frances Hellman/Berkeley Lab (from Phys.org)

The researchers say that their discovery is a major step forward in realizing high-density, high-speed, low-power, yet ultrastable magnetic memory devices that exploit the intrinsic power of electron spin.

Researchers from Shinshu University, the Chinese University of Hong Kong, the University of Tokyo, Tsinghua University, Kyoto University and Nanyang Technological University have experimentally demonstrated a breakthrough in manipulation of skyrmions using only electric field.

The team, led by Professor Xiaoxi Liu of Shinshu University, designed and fabricated magnetic multilayer films in the form of racetracks where the thickness of the films had a slope. They demonstrated that many skyrmion bubbles can be created and directionally displaced about 10 micrometres by applying a voltage as low as 9 volt in a repeatable manner. They also found that the domain wall displacement and velocity induced by the variation of electric field are proportional to the absolute value of voltage.

Researchers from the EPFL managed to produce controllable and stable skyrmions using laser pulses. The scientists could write and erase skyrmions in less than a few hundred nanoseconds to a few microseconds.

To create the skyrmions, the researchers used iron-germanium alloy, which can offer skyrmions at about 0 degrees Celsius, very closet o room temperature. The ultra-short laser pulses create an ultra-fast temperature jump, and the super-cooling effect at the end of the jump restricts the place in which skyrmions exist - to places in which they do not exist normally.

Researchers from Japan and China have discovered the exotic dynamics of frustrated magnetic skyrmions - which are different from that of magnetic skyrmions in common ferromagnetic materials. Magnetic skyrmions are very interesting for several spintronic applications, including magnetic memory and logic computing devices.

In conventional ferromagnetic materials, the helicity (degree of freedom) of a skyrmion cannot be effectively controlled, but the researchers found that in frustrated magnetic materials it is possible to control the skyrmion helicity by utilizing the helicity locking-unlocking transition of the material. The researcher further conclude that one can use frustrated skyrmions as a binary memory utilizing two stable Bloch-type states, where the helicity can be switched by applying current.

Researchers from Singapore's A*STAR and NTU developed a tunable room-temperature platform that can be used to control and detect magnetic skyrmions. This platform is actually a thin film made from multi-layer stacks of Ir/Fe/Co/Pt.

In this material, the magnetic interactions governing skyrmion properties can be controlled by varying the ferromagnetic layer composition. The skyrmions exhibit a smooth crossover between isolated (metastable) and disordered lattice configurations across samples, while their size and density can be tuned by factors of two and ten, respectively.

Researchers from MIT and the Johannes Gutenberg University Mainz (JGU) achieved the billion-fold reproducible motion of skyrmions (special spin structures) between different positions. The researchers say that this kind of process is needed to produce magnetic shift registers - and so this is a critical step towards skyrmions applications in spintronics devices.

Using specially design thin film structures (asymmetric multilayer devices) that exhibit broken inversion symmetry that stabilize the skyrmions. In such structures, skyrmions have a unique stability - which makes them compelling for such spintronic devices. The researchers say that those skyrmions, that can be shifted by electrical currents and move relatively undisturbed through the track, are very promising to make racetrack devices.

The Johannes Gutenberg University Mainz (JGU), with funding from the German Research Foundation (DFG), is setting up an Emmy Noether independent junior research group to study spintronics.

Specifically, the TWIST (Topological Whirls in SpinTronics) work group will study skyrmions - magnetic "particles" or nodes within a magnetic texture. Skyrmions are more stable than other magnetic structures and react particularly readily to spin currents - which makes them interesting for spintronics applications.