Spin waves could replace electrical currents in chips

A research team led by Germany’s Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has been exploring the use of spin waves as an alternative for transporting information in compact microchips, in order to keep up with the data demands of the future. Their work has been published in the journal Nature Nanotechnology.

For a long time, there has been one reliable rule of thumb in the world of information technology: the number of transistors on a microprocessor doubles approximately every two years. The resulting performance boost brought us the digital opportunities we now take for granted, from high-speed internet to the smartphone. But as the conductors on the chip become ever-more minute, we are starting to face problems.

“The electrons that flow through our modern microprocessors heat up the chip due to electrical resistance,” said Dr Sebastian Wintz, from HZDR’s Institute of Ion Beam Physics and Materials Research. “Beyond a certain point, the chips simply fail because the heat can no longer escape.”

This is why Dr Wintz envisions a different future for information carriers. Instead of electrical currents, he and his colleagues are capitalising on a specific property of electrons called ‘spin’.

“Electrons not only have an electric charge, but also a spin,” said HZDR’s Dr Kilian Lenz. “They rotate around their own axes, like a children’s spinning top, generating a magnetic moment. We want to harness this magnetic property for new technologies.”

In certain magnetic materials, like iron or nickel, these spins are typically parallel to each other. If the orientation of these spins is changed in one place, that disruption travels to the neighbouring particles, triggering a spin wave which can be used to encode and distribute information.

“In this scenario, the electrons remain where they are,” Dr Wintz said. “They hardly generate any heat, which means that spin-based components might require far less energy.”

So far, there have been two fundamental challenges complicating the use of spin waves for transporting information: the wavelengths that can be generated are not short enough for the nanometre-sized structures on the chips, and there is no way of controlling the waves. Dr Wintz and his co-workers have now been able to find solutions to both problems.

“Our key challenge was to create an antenna that was small enough to generate such ultrashort waves,” Dr Wintz said. “Our solution was to use a nanoscale magnetic antenna that naturally self-assembles.”

“Unlike the artificially made antennas that are commonly employed to excite the waves, we now use one that is naturally formed inside the material,” added Dr Volker Sluka, first author on the study. “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.”

Within the two layers, this creates areas of different magnetisation, separated by what is called a domain wall. The scientists then exposed the layers to magnetic fields alternating with a frequency of one gigahertz or higher. Using an X-ray microscope from the Max Planck Institute for Intelligent Systems in Stuttgart, which is operated at the Helmholtz-Zentrum Berlin, they were able to observe that spin waves with parallel wave fronts travel 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,” Dr Sluka said. “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.”

As the X-ray images have shown, these spin waves can travel several micrometres at wavelengths of only about 100 nanometres, without any significant loss of signal — a necessary prerequisite for using them in modern information technology. Moreover, the physicists have discovered a possible way to control this new information carrier when they set the stimulation frequency below half a gigahertz. The spin waves thus remained trapped in the domain wall.

“In this scenario, the waves were even able to run in a curve,” Dr Sluka said. “Nevertheless we were still able to detect the signals.”

With their results, the researchers have laid important foundations for the further development of spin wave-based circuits. In the long run, this might facilitate a completely novel design of microprocessors.

“Using magnetic fields, we can move domain walls relatively easily,” Dr Wintz said. “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 fulfil new tasks.”

Top image caption: A spin wave spreading along a magnetic domain wall. Image credit: HZDR/Juniks.

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