Skip to main content

Understanding the speed limits of magnetism

Periodic Reporting for period 2 - MAGNETIC-SPEED-LIMIT (Understanding the speed limits of magnetism)

Periodo di rendicontazione: 2018-08-01 al 2020-01-31

"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 the responsible energy use is a growing concern for society, an unexpected 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 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 early 2017, we started the procurement of table-top laser system which was delivered and finally installed at Stockholm University in august 2017. Up to now, three postdocs and two Ph.D. students have been hired with the ERC grant. One very recently, and he will finish right before the end of the project together with the two Ph.D. students.

- 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.

- 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 (MML) are the highlights, as well as four contributed talk at the 2020 annual Magnetism and Magnetic Materials conference. A total of 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.

- In terms of publications, we have five papers (four original articles, one in Physical Review Letters and one in Optics Express, and one invited review in Physics Reports) already published. Three other manuscripts are in preprint forms on arXiv and submitted, one of them is under review in Nature Physics, another one is associated with an open source code that we expect to become very useful to the community. We are currently finishing up three manuscript: two with data from the laboratory in Stockholm (with one potential candidate in Physical Review Letters) and one from the first experiment at the European XFEL. We have already recorded, and partly analysed, data for at least four papers from the activities in Stockholm, and two papers from the activities in Venice and at Elettra in Trieste with potential for high-impact publications. With the scheduled European XFEL experiments (two at the moment) and at FERMI, and investigations in the labs in Stockholm and Venice, a reasonable estimate of 20 good publications is expected from the project. The general attitude, which I ask all the researchers in the group, is to write papers that are complete and aim at good journals, even if this may take a bit longer to publish.
"We have reached three milestone results.

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 nutations ferromagnetic systems. The full understanding of this phenomenon will continue until the end of the project.

Regarding research objective #2, we are now understanding how ultrafast magnetism happens in model systems. Surprisingly, a systematic study in systems which are easier to model, has not yet appeared in 20 years of research. We have important results which we are compiling in a manuscript together with a leading theorist in the field, which will shade light on an important aspect of ultrafast magnetism, namely the coupling to the lattice. In addition, new data from experiments at the FERMI free electron laser in Trieste have lead to some surprising evidence on how heat is transported at the nanometer scale in metallic magnetic materials. We are currently analysing the data.

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, we are now analysing the large amount of data recorded (400 TB). We have also been instrumental in allowing follow-up ultrafast x-ray magnetic scattering experiments where we were also strongly involved, and which also happens to be very successful. At least two more experiments will happen by the end of the project, where we will also have a leading role and a realistic chance of realising the first ultrafast movie of spin dynamics with nanometer resolution.

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 and found in literature. The newly bought THz-imaging and spectroscopy system in Venice is also state-of-the-art, and we expect interesting data to be recorded with that instrument as well."