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Understanding the speed limits of magnetism

Periodic Reporting for period 3 - MAGNETIC-SPEED-LIMIT (Understanding the speed limits of magnetism)

Reporting period: 2020-02-01 to 2021-07-31

Magnetism is one of the first phenomenon known by civilisation, yet one of the most poorly understood. While the origin of magnetism in materials is in atomic interactions of characteristic ultrafast time scales (millionth of a billionth of a second), it has been long believed that magnetism could only be manipulated at nanosecond rates, exploiting external magnetic fields. However, in the past decade researchers have been able to observe ultrafast magnetic dynamics at these intrinsic ultrafast time scales without the need for magnetic fields, rather using ultrafast laser, thus revolutionising the view on the speed limits of magnetism. Despite many achievements in ultrafast magnetism, the understanding of the fundamental physics of ultrafast magnetism is still only partial, hampered by the lack of experimental techniques suited to fully explore these phenomena.

As the environment and a responsible energy use are growing concerns for society, an often forgotten source of energy consumption is the large data volumes used across the internet. Large data centres, where information is still stored in the form of tiny magnetic bits, are wasting most of their energy in form of heat, and compression and transmission of the data requires much computing power to deliver content on demand. Addressing how to store and manipulate information not only at higher rate (ideally in the terahertz regime, 1000 times faster than existing technology) but also in a much more energy efficient way, is crucial if our society has to continue to prosper based on technological advances.

The overall objective of the project is hence to better understand the puzzling phenomenon of ultrafast magnetism using new experimental tools (intense terahertz radiation and x-ray free electron lasers), which could be used to design the future of data storage. In the specific, we outlined three main research objectives:
1. To unveil the process of magnetization dynamics controlled by strong terahertz radiation in magnetic metals (those commonly used to store magnetic information), which will add up an important piece to the puzzle of ultrafast magnetism.
2. To understand the details of how ultrafast magnetism is linked to the crystal structure of the materials investigated.
3. To establish how spins move in space and time using x-ray imaging to create the first "movie" of ultrafast magnetism with femtosecond and nanometer resolution at the newly built x-ray free electron lasers.
- Immediately after the signing of the contract, we started the procurement of table-top laser system which was delivered and finally installed at Stockholm University in august 2017. In 2019, a complementary table-top laser system was installed at the Ca' Foscari University of Venice. Up to now, three postdocs and two Ph.D. students have been hired with the ERC grant. The experimental activities have been delayed for a few months due to the COVID emergency.

- My group and I become early and key users of two major European facilities: the European XFEL in Hamburg, Germany, and the TELBE facility in Dresden, Germany. We have performed pioneering and successful experiments there, and more were scheduled in April/May 2020 but were postponed due to the CoViD19 emergency. In addition, thanks to the inclusion of the University of Venice and the collaboration with the Elettra Sincrotrone Trieste, the project benefitted of a strong collaboration with the research at the FERMI free electron laser too.

- We have been presenting results from the research description in the action at major international conferences in the magnetism community. Invited talks at the Ultrafast Magnetism Conference, at two Gordon Research Conferences, at the symposium Metallic Multilayers are the highlights, and four contributed talk at the 2020 annual Magnetism and Magnetic Materials conference. More than 10 invited talks at peer-reviewed conferences and thematic workshops on magnetism, terahertz or x-ray science were given to research related to this project. Because of the COVID emergency, many of those conferences have been canceled or moved to hybrid format. Within these limitations, we have been presenting our results at several online events, reaching out the community in a satisfactory way.

- In terms of publications, the research related to the project has now published 17 papers (15 original articles, in Nature Physics, Physical Review Letters, Optics Express, Nano Letters, Applied Physics Letters, and one resulted in an open source code that we expect to become very useful to the community; and two invited reviews). Three other manuscripts are in preprint forms on arXiv and under review, one is an invited submission to a special topic in Ultrafast Magnetism on Applied Physics Letters. We are currently completing nine additional manuscripts: four with data from the laboratory in Stockholm, three from the first experiments at the European XFEL and two from experiments at FERMI. Most of them with potential for high-impact publications. We now expect to surpass the original estimate of 20 high-level publications from the project.
We have reached several milestones, which are grouped following the research objectives of the project.

Regarding research objective #1, we now understand much better terahertz-driven magnetisation dynamics in different metallic systems; most importantly, we have found strong experimental evidence for the discovery of spin nutation ferromagnetic systems, a signature of magnetic inertia. The full understanding of this phenomenon will continue until the end of the project resulting in at least two more publications.

Regarding research objective #2, we have understood how ultrafast magnetism happens in model systems with a well defined crystalline structure. Surprisingly, a systematic study in these systems, had not yet appeared in 20 years of research, until our first published work. We proven a fundamental aspect of ultrafast magnetism, namely the dependence of femtosecond spin dynamics on the lattice structure. In another work currently under review, we observed that even the overall change in ordering of the material structure affects the spin and electronic transport.

Regarding research objective #3, we have been chosen by the international x-ray community to lead the first ultrafast x-ray imaging experiment at the European XFEL. We have successfully set up the experiment, analyzed the large amount of data recorded (400 TB) and a manuscript is almost ready. We have also been instrumental in allowing two follow-up ultrafast x-ray magnetic scattering experiments where we were also strongly involved, and we are currently writing two manuscripts up based on those data. The final imaging experiments that we were planning has been delayed by more than a year due to the pandemic. It is unclear whether we will be able to perform it before the end of the grant, or right after that.

The terahertz source set up from scratch at Stockholm University is state-of-the-art and the sensitivity our setup in detecting small magneto-optical signals is beyond what is typically reported in literature. The newly bought THz-imaging and spectroscopy system in Venice is also state-of-the-art. The planned experiments there are more complex than originally expected. We are currently testing new strategies, which have the potential to greatly improve the sensitivity of terahertz measurements in general.
Spinning tops made of magnetic materials