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Nanocubes as interpreters

SEM image of nobel metal nanotubes. @ CNRS - CINaM

Spintronics is considered a seminal research field in physics. It promises faster electronic components, more delicate sensors, and new approaches for quantum computing. Nevertheless, fundamental questions remain to be answered. For example: How can spintronic components be easily and selectively manipulated with light? Starting in April, the Helmholtz-Zentrum Dresden Rossendorf (HZDR) and the Interdisciplinary Center of Nanoscience of Marseille (CINaM) will be looking for answers to these questions in their joint research project Nano-PLASMAG. The focus is on nanometer-sized, regularly shaped cubes of pure gold. They are intended to serve as "interpreters" and couple light as effectively as possible to nanomagnets in order to influence their state. The project is being funded by the German Research Foundation (DFG) and the French National Research Agency (ANR) with half a million euros over three years.

Like conventional electronics, spintronics, or spin electronics, is based on electrons as information carriers. However, it uses not only their electrical charge but also another particle property – the spin, i.e., the quantum-mechanical intrinsic rotation. The advantage: In contrast to conventional electronics, the computing process does not require transporting electrical charges, which is inevitably associated with losses due to the heat generated in the material. Instead, the spin excitations are only passed from one electron to another, similar to a relay race. In principle, this allows information to travel more efficiently and with minimal losses as magnetic excitation races through the material as a spin wave.

Research has long been concerned with effectively generating and precisely manipulating such magnetic excitation states to provide the prerequisites for constructing usable components. Most approaches are based on triggering spin waves via current and magnetic pulses or short, powerful laser flashes. In contrast, little research has been conducted into another variant – generating magnetic excitations with weak light, such as emitted by an LED. "Using light to influence components has a long history in electronics. Just think of solar cells or light sensors," explains HZDR researcher Dr. Aleksandra Lindner. "With spintronics, we are still at the beginning."

Perfect gold structures

Gold nanocrystals (golden cubes) on surface of thin ferromagnetic (FM) heterostructures. Magnetization dynamics in FM is excited by microwaves (black half-circles) or by THz pulses (magenta arrow). Absorption of white light by gold nanocrystals and in turn excitation of surface plasmon resonance in AuNC affects magnetization dynamics in underlying FM layer. Spins with precession altered by AuNCs`light absorption are depicted with turquoise color, spins with unchanged precession with red. @ HZDR/A. Lindner

The problem: Magnetic excitations in spintronic materials can only be influenced very inefficiently by light. An intermediate instance is therefore needed to convert the light into further excitation states, which can ultimately "speak" to the magnetic system. In the Nano-PLASMAG project, nanometer-sized cubes of gold are to take on this interpreting function. "Our project partners from France, the team led by Olivier Margeat from the University of Aix-Marseille, are able to produce such cubes in a specific and precise manner using chemical processes," enthuses Lindner. "This truly is an art." Under the microscope, the tiny cubes resemble ordinary sugar cubes, with one looking just like the other.

The gold cubes are placed on a thin magnetic layer, the actual spintronic material. When light falls onto them, the nanocubes enter a state of quantum resonance in which the conduction electrons within oscillate collectively, thereby absorbing the light particularly effectively. The size of the cubes determines which color is "swallowed" – be it red, blue, or green.

This effective light absorption can have two consequences: First, the cubes heat up due to the friction of the electron oscillations, and second, the oscillation of the charged particles generates a time-dependent electromagnetic field. "We hope this will allow us to precisely manipulate existing excitation states in the thin magnetic layer, and perhaps magnetic excitations can even be generated directly," says Lindner.

To verify their hypotheses, the experts are planning various experiments over the next three years, particularly at the HZDR facility TELBE at the ELBE-Center for High-Power Radiation Sources. Powered by a particle accelerator, TELBE generates intense terahertz pulses – terahertz is the frequency range between microwaves and infrared radiation. "It is already known that terahertz pulses can cause spin waves in a magnetic layer. This is exactly what we intend to do in our experiments so we can investigate whether the light-irradiated gold cubes have any effect on the spin waves," explains HZDR physicist Dr. Ruslan Salikhov. Among other things, the experts are trying to determine which arrangements and sizes of nanocubes produce the best results.

"Energy harvesting" as a perspective

Further experiments will show whether excitations in the magnetic layer can be generated using weak laser flashes on the nanocubes. "This could possibly result in spin waves with terahertz frequencies," speculates Salikhov. "Currently, spintronics typically operates in the gigahertz range. Terahertz frequencies are significantly higher and would facilitate the use of much faster components in the future." Another perspective application could be "energy harvesting" – the process of obtaining small amounts of energy from the environment.

In addition to the gold cube method, Aleksandra Lindner's team is currently exploring another approach: In the German-Polish MAGPHOT project, so-called chiral organic molecules are being investigated for their suitability as spin wave manipulators. Mobile electrons can be generated very effectively in such molecules through light absorption. The chiral structure of the molecules filters the spin of the charged particles so that, ideally, the molecules only allow electrons with an identical "sense of rotation" to pass through. These spin-filtered electrons can then interact directly with the thin magnetic layer on which the chiral organic molecules are deposited and influence them. The question of which approach is more promising – chiral molecules or gold cubes – will hopefully be answered through the experiments within a few years.

The three-year Nano-PLASMAG research project is funded by the German Research Foundation (DFG) and its French counterpart, the French National Research Agency (ANR). The project launches on April 1, 2024, with funding of around half a million euros. In addition to the HZDR, the Interdisciplinary Center of Nanoscience of Marseille (CINaM) – a research institution affiliated with the French National Center for Scientific Research (CNRS), and the Aix-Marseille University (AMU) – is also involved.

The MAGPHOT research project is a Polish-German collaboration funded by the DFG and the National Science Center Poland (NCN) with a total of half a million euros. The HZDR's partner institution is the Adam Mickiewicz University (AMU) in Poznań, Poland. Helmholtz-Zentrum Dresden Rossendorf (HZDR)


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