Spin waves with short wavelengths make magnonic computer components possible
The central processing units (CPUs) we use in our laptops, desktops or even phones are made up of billions of transistors based on complementary metal oxide semiconductor (CMOS) technology. With the need to miniaturize these devices, physical constraints raise concerns about their sustainability. In addition, high power consumption and energy losses are forcing scientists to look for alternative computer architectures.
One of the most promising candidates are magnons, the quanta of spin waves. "Imagine a calm lake. If we drop a stone into the water, the waves spread away from where they originated. Now we replace the lake with a magnetic material and the stone with an antenna. The propagating waves are called spin waves and can be used to transfer energy and information from one point to another with minimal loss," says Sabri Koraltan from the University of Vienna, first author of the study recently published in the journal "Science Advances". Once generated, the spin waves can be used for magnonic functional devices to perform classical and unconventional computational tasks. "To reduce the footprint of magnonic devices, we need to use spin waves with short wavelengths, which are difficult to generate with modern nanoantennas due to their limited efficiency," adds Sebastian Wintz from Helmholtz-Zentrum Berlin and coordinator of the research project. Nano-antennas can only be produced in clean rooms, highly specialized nanofabrication facilities, using advanced lithography techniques.
The researchers from Austria and Germany have found a much simpler solution: The electric current flows directly through a magnetic stack with swirling magnetic patterns. "Our research shows that by using a lateral alternating current geometry in synthetic ferrimagnetic vortex pairs, we can achieve spin wave emission with an efficiency that exceeds conventional methods by several orders of magnitude," says Sabri Koraltan. Synthetic ferrimagnetic systems have opposite magnetization patterns. If the top layer has a clockwise rotating vortex, the bottom layer has a counterclockwise rotation. This allows the magnetization pattern to be efficiently excited by the magnetic fields generated by the alternating currents. "With our high-resolution X-ray microscope ’Maxymus’ at the electron synchrotron BESSY II in Berlin, we were able to directly observe the predicted spin waves even at nanoscale wavelengths and gigahertz frequencies," adds Sebastian Wintz. "By using special materials that can change their magnetization when mechanically stressed, we have also shown that the direction of these spin waves can be dynamically controlled by simply adjusting the strength of the applied current. This can be seen as an important step towards active magnetic devices," adds Sabri Koraltan. "The new generation of our micromagnetic simulation software magnum.np makes it possible to perform simulations on very large spatial scales. These simulations were essential for understanding the main mechanisms behind the efficient and controllable spin wave excitation observed here," says Dieter Süss, Head of the Department of Physics of Functional Materials at the University of Vienna.
The ability to redirect spin waves on demand opens up new possibilities for creating reprogrammable magnon circuits that could lead to more adaptable and energy-efficient computer systems. The findings, published in the journal Science Advances, represent an important advance in the search for new ways to generate magnons for the potential and future development of next-generation magnon-based technologies.
Original publication:
Sabri Koraltan et al. "Steerable current-driven emission of spin waves in magnetic vortex pairs" Science Advances 10.39 (2024)
DOI: 10.1126/sciadv.ado8635
https://www.science.org/doi/10.1126/sciadv.ado8635
Illustration:
Fig. 1: Coherent spin waves excited by alternating currents in a simple magnetic three-layer stack. C: Sabri Koraltan
