Researchers Manage to Control Very Short-Wavelength Spin Waves

In the last few years, electronic data processing has been progressing only in one direction: The industry has miniaturized its components to the nanometer range. But this process is, presently, reaching its physical boundaries.

Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) are, therefore, investigating spin waves or so-called magnons—a potential alternative for conveying information in more compact microchips. Partnering with global partners, they have fruitfully produced and controlled very short-wavelength spin waves. The physicists accomplished this feat by harnessing a natural magnetic occurrence, about which they describe in the journal Nature Nanotechnology (DOI: 10.1038/s41565-019-0383-4).

For quite a while, there has been one consistent rule of thumb in the realm of information technology: The number of transistors on a microprocessor doubles about every two years. The resulting performance enhancement delivered the digital prospects that are now taken for granted, from high-speed internet to the smartphone. But with the conductors on the chip becoming more and more minute, issues are arising, as Dr. Sebastian Wintz from HZDR’s Institute of Ion Beam Physics and Materials Research explains: “The electrons that flow through our modern microprocessors heat up the chip due to electrical resistance. Beyond a certain point, the chips simply fail because the heat can no longer escape.” This also prevents an additional increase in the speed of the components.

This is why the physicist, who is also presently working at the Paul Scherrer Institute (PSI) in Switzerland, foresees a different future for information carriers. Rather than electrical currents, Wintz and his colleagues are exploiting a particular property of electrons called “spin”. The minute particles act as if they were continuously rotating around their own axis, thus forming a magnetic moment. In some magnetic materials, like nickel or iron, the spins are usually parallel to each other. If the orientation of these spins is altered in one place, that disruption moves to the adjacent particles, activating a spin wave which can be used to encode and dispense information. “In this scenario, the electrons remain where they are,” says Wintz, describing their benefit. “They hardly generate any heat, which means that spin-based components might require far less energy.”

How can we control the wave?

Thus far, however, there have been two key challenges thwarting the use of spin waves: The wavelengths that can be produced are not sufficiently short for the nanometer-sized structures on the chips, and there is no way of regulating the waves. Sebastian Wintz and his colleagues have currently been able to discover solutions to both issues.

Unlike the artificially made antennas that are commonly employed to excite the waves, we now use one that is naturally formed inside the material. To this end, we fabricated micro-elements comprising two ferromagnetic disks that are coupled antiferromagnetically via a Ruthenium spacer. Furthermore, we chose the material of the disks so that the spins prefer to align along a particular axis in space, which results in the desired magnetic pattern.

Dr. Volker Sluka, Study’s First Author, HZDR

Within the two layers, this forms areas of varied magnetization, divided by what is known as a domain wall. The layers were then exposed by the researchers to magnetic fields alternating with a frequency of one gigahertz or higher. With an X-ray microscope from the Max Planck Institute for Intelligent Systems Stuttgart, which is operated at the Helmholtz-Zentrum Berlin, they were able to notice that spin waves with parallel wavefronts move along the direction perpendicular to the domain wall.

In previous experiments, the ripples of the wave looked like the ones you get when a pebble hits a water surface. This is not optimal, because the oscillation decays quickly as the wave spreads in all directions. To stay in the same analogy, the waves now look as if they were produced by a long rod moving back and forth in the water.

Dr. Volker Sluka, Study’s First Author, HZDR

As the X-ray images have revealed, these spin waves can travel a number of micrometers at wavelengths of just about 100 nm, without any great loss of signal—an essential prerequisite for using them in contemporary information technology. Furthermore, the physicists have found a promising way to manipulate this new information carrier when they fix the stimulation frequency below half a gigahertz. The spin waves, thus, stayed trapped in the domain wall: “In this scenario, the waves were even able to run in a curve,” says Volker Sluka, adding: “Nevertheless we were still able to detect the signals.” With their findings, the scientists have laid crucial foundations for the additional development of spin wave-based circuits.

Ultimately, this might enable a totally novel design of microprocessors, Sebastian Wintz predicts: “Using magnetic fields, we can move domain walls relatively easily. That means that chips that work with spin waves don’t necessarily need a predefined architecture, but they can later be changed and adapted to fulfill new tasks.”