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Skyrmion-Topological insulator and Weyl semimetal technology

Periodic Reporting for period 2 - SKYTOP (Skyrmion-Topological insulator and Weyl semimetal technology)

Reporting period: 2020-05-01 to 2021-10-31

SKYTOP-Skyrmion-topological insulator and Weyl semimetal technology
SKYTOP develops a technology based on topological quantum matter that can have an impact on information processing and storage. The project gathers expertise from Greece, France, Germany, Italy and Belgium to face challenges in materials growth, device modelling and evaluation as well as scale-up of the technology for future volume production.

Magnetic materials show unusual swirling configurations of their spin (Skyrmions) characterized by non-trivial topology, while a class of topological materials known as topological insulators (TI) and Weyl semimetals (WSM) show unexpected transport properties due to unusual twisting of their electronic band structure. In SKYTOP we aim to combine these different classes of topological matter and demonstrate that they can lead to new energy efficient ways of manipulating and storing bits of information. It will be a big challenge in this project to show that possible compatibility issues can be overcome and that the new topological materials combinations can be synergetic and fully functional.

The overall objectives of the project are:
• Explore synergies between two classes of topological materials: Skyrmions (topology in real space) and topological Insulators (TI) and Weyl semimetals (topology in reciprocal space). The ultimate goal is to enable low power all-electric skyrmion manipulation for magnetic and spintronic devices.
• Open an exploitation route for Weyls (and TIs) for practical applications. The Spin Hall effect is a key property for Weyls (and TIs) that could lead to important applications if combined with magnetic materials such as Skyrmionic materials. Large area growth by a variety of synthetic thin film growth techniques (MBE, MOCVD, sputtering) as well as a growth scale-up activity are key for the development of manufacturable topological devices in the future.
• Mobilize the emerging community on topological matter. An aggressive outreach activity is envisaged to increase awareness in a wider scientific and technological society about the opportunities offered by topological materials.

Expected impacts on society
Our new skyrmionic magnetic devices, enhanced by TI and WSM materials, offer ultra-low power consumption, and enable the design of next generation neuromorphic devices and circuits with expected impact on artificial intelligence (AI) of the future. AI is expected to penetrate every part of our daily lives at home and work, improving services and security and enhancing the well-being of citizens by enabling for example autonomous driving and health monitoring and prevention.
During the first 18 months (1st reporting period), SKYTOP consortium harnessed the necessary synergies among its members and developed suitable protocols for materials properties validation. The effort focused on the development of growth of good quality topological films on large area substrates ensuring that they are suitable for integration with magnetic skyrmionic materials at a later stage. In addition, in this first project period, the consortium set as a target to unveil the interesting properties of topological thin film materials made by synthetic methods on different substrates and verify that they have similar and equally good properties compared to bulk materials grown by equilibrium methods.

During the last 18 months (2nd reporting period), SKYTOP partners set as a main objective to bring materials from different classes together in the same heterostructure (e.g. magnetic/TI, magnetic/Weyl) to make progress towards all electrical skyrmion manipulation which is our ultimate goal. More specifically, the main target in this 2nd reporting period was to demonstrate that:
- materials from different classes can “live” together without adversely affecting the physical properties of one another. More specifically, the target was to find out whether magnetic materials can leave in tact the topological surface states of TIs and Weyl semimetals.
-The topological materials (TIs and Weyls) have a beneficial effect on charge-spin interconversion so that they can be used efficiently in spintronics devices.

In this second period of the project, there have been 3 major achievements
(1) The main project concept is proved: The topological surface states mediate efficiently the charge to spin conversion
(2) Current induced magnetization reversal in a potential skyrmion host 2D Ferromagnet/TI has been obtained
(3) Skyrmion synapse has been realized and imaged at room temperature
In first period, SKYTOP members have demonstrated for the first time at room temperature skyrmions, only 40 nm in size, which "live" in a synthetic antiferromagnetic medium. This discovery, published in Nature Materials is very important since it signifies the formation of skyrmions in stray field-free environment, opening the route for the scaling of skyrmions down to the 10 nm scale, which is extremely important for applications.

In the second period, the consortium partners, using a large number of techniques, found that the topological surface states (TSS) play a significant role in charge to spin interconversion mechanism. The TSS revealed their presence in terms of slower dynamics of spin to charge conversion in high resolution THz experiments and as a large field like torque in second harmonic spin torque experiments. Both are beyond the state of the art results enriching our knowledge in the field.

In addition, current induced magnetization reversal in the new 2D van der Waals CrTe2/Bi2Te3 heterostructure is observed for the first time. This achievement is the first decisive step towards the targeted electrically controlled Skyrmion/TI devices given that indirect evidence of skyrmions in this heterostructure has been obtained within SKYTOP.

Finally, Skyrmions in a conventional heavy metal/magnetic multilayer stack have been successfully generated and moved by current pulses of variable intensity and width demonstrating a skyrmion artificial synapse which is planned to be part of a larger artificial neuron network in hardware. The skyrmion movement was imaged by Kerr microscopy at room temperature and its synaptic plasticity was quantified by means of the Hall voltage showing linear synapse potentiation as required.

The plans for next (final) period is to face the following grand challenges:
-demonstrate the skyrmion synapse in association with artificial neuron at room temperature using topological materials (TI or Weyl) for charge to spin conversion
-demonstrate at low temperature for the first time a synapse based on a 2D ferromagnet/TI system
-Demonstrate 2D ferromagnet/TI spintronic devices at room temperature.

The project is expected to make significant contributions by the end date, first in the scientific domain unveiling the physical properties of new skyrmion materials, especially those classified as 2D ferromagnets, second by scaling up the technology to large area Si substrates with the aim to commercialize new functional devices made of composite skyrmion /Sb2Te3 topological insulator layers, third by fabricating skyrmion synapses and neurons to realize skyrmion artificial neural networks (ANN) in hardware. It is envisaged that our work on skyrmion ANNs will impact neuromorphic computing with great benefits in processing big inhomogeneous data with much improved energy efficiency.
Skyrmions in antiferromagnetic layers (left).Surface states in a topological insulator (right)
Skyrmion-based synapse