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Sunset at the North Campus © Roland Baege​/​TU Dortmund
Paper in Physical Review B Journal

Merging Waves in a Magnet

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Group of people with masks in a laboratory © Martina Hengesbach​/​TU Dortmund
Dmytro Yaremkevych, Serhii Kuktaruk, Prof. (apl.) Dmitri Yakovlev, Felix Godejohann, Dr. Alexey Scherbakov and Prof. Manfred Bayer have published their research results in the renowned journal Physical Review B.
In a world of continuously increasing data volumes, we need new efficient tools for data processing. In a magnetic material, the vibrations of the crystal lattice and the oscillations of the magnetic moments represent two types of excitations that could be efficient information carriers for conventional, quantum and neural networks. Led by Dr. Alexey Scherbakov (TU Dortmund University), an international group of scientists from five different research centers has demonstrated that these two excitations become even more attractive when strongly coupled to each other

In our everyday life, we receive, process, and send huge volumes of data. Conventional electronics, which uses electric currents to handle these tasks, has seen tremendous progress, as exemplified by our mobile phones. However, nowadays it is facing serious challenges due to limitations in further miniaturization and requirements for energy efficiency. As a result, researchers throughout the world are searching for alternative ways to transfer and handle data, especially in quantum computing and neural networks, which would be game-changing paradigms in data processing.

In a magnet, the vibrations of atoms in a crystal lattice and the oscillations of the magnetic moments around a magnetic field are two prospective candidates as information carriers at nanoscale in energy-efficient devices. Both can transport information and show a behavior similar to water waves. These waves are called acoustic waves and spin waves, respectively. As their propagation does not require the transport of electric charge, they do not suffer from electrical resistivity that would cause losses and a heating up of the processor – like in conventional electronics.

Data processing can be radically faster, smaller and more energy efficient

These waves oscillate at frequencies of up to 100 GHz, which is much higher than the clock frequencies of just a few GHz in state-of-the-art processors, while their wavelengths are well below 1 micrometer. This means that future devices for wave-based data processing can be radically faster, smaller and more energy efficient. However, to leverage these advantages reliable tools for handling data need to be developed. Here, we can benefit from the fact that acoustic waves can trigger spin waves and, vice versa, spin waves can be the source of acoustic waves, for example. This requires, however, a coupling between them which is as strong as possible.

In their present work, the international team of scientists from Germany, Russia, Ukraine and the United Kingdom has for the first time succeeded in achieving the strong coupling regime between an acoustic and a spin wave with identical frequencies in an extended structure, like on a chip. To achieve this goal, they followed the interconversion between the two excitations with a time resolution far below one billionth of a second. During interconversion, a new excitation occurs which is simultaneously acoustic and spin.

To demonstrate this strong coupling, the surface of a metal ferromagnet was suitably patterned as a grating with a period of a nanometer, a billionth of a meter. This pattern effects the spatial shaping of both acoustic and spin waves. Their spatial matching at the resonance condition, when the frequencies of acoustic and spin waves also coincide, leads to their strong coupling. These results will open up the way for long-distance propagation of the spin wave through coupling with the acoustic wave, as required for high-frequency data encoding and transfer.  

Graphic
An ultrashort laser pulse excites vibrations of the atoms and precessional movement of their magnetic moments in a metallic magnetic grating. If the frequencies of these two waves coincide, they form a hybridized wave in which the dynamical magnetic excitation becomes “protected”. The inset shows the image obtained through scanning electron microscopy of the gratin under observation.

“We have sought to detect this strong coupling since our first work on ultrafast magneto-acoustics 10 years ago. Bringing together the expertise of all the international cooperation partners involved was essential for this success: Perfect magnetic samples from Nottingham, on which the experiments in Dortmund were performed, with the observations explained through top-notch theoretical support from St. Petersburg and Kiev,” says Dr. Alexey Scherbakov. Professor Manfred Bayer, who is also a member of the team, adds: “We should also mention the contribution of our colleagues from Raith GmbH, a world-leading manufacturer of nanopatterning systems from Dortmund, who produced nanograting patterns of extremely high quality for us.”

Original publication:

F. Godejohann, A. V. Scherbakov, S. M. Kukhtaruk, et al.: Magnon polaron formed by selectively coupled coherent magnon and phonon modes of a surface patterned ferromagnet. Physical Review  102, 144438 (2020)

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