Prototype for Spin-Wave Majority Logic Gate for Information Processing

The trident-shaped majority gate structure consisting of Yttrium Iron Garnet. The transparent material underneath is a Gallium Gadolinium Substrate. (credit: Fischer/Kewenig/Meyer)

The size of computer electronics is constantly shrinking that the very electrical currents essential for their operations can no longer be employed for logic computations in the same ways as their larger-scale ancestors. A traditional semiconductor-based logic gate known as a majority gate, for example, outputs current to match either the “0” or “1” state that contain as a minimum two of its three input currents (or equivalently, three voltages). But how to construct a logic gate for devices that are too small for classical physics?One new experimental demonstration employs the interference of spin-waves -- synchronous waves of electron spin alignment seen in magnetic systems. The details of the research are published this week in Applied Physics Letters, from AIP Publishing. The spin-wave majority gate prototype, built using Yttrium-Iron-Garnet, was born due to a recent collaborative research center funded by the German Research Foundation, named Spin+X. The research has also been supported by the European Union within the project InSpin and has been directed in collaboration with the Belgian nanotechnology research institute IMEC.

The motto of the research center Spin+X is ‘spin in its collective environment,’ so it basically aims at investigating any type of interaction of spins - with light and matter and electrons and so on. More or less the main picture we are aiming at is to employ spin-waves in information processing. Spin waves are the fundamental excitations of magnetic materials.

Tobias Fischer, Doctoral Student, University of Kaiserslautern

So rather than utilizing classical electric voltages or currents to transmit input data to a logic gate, the Kaiserslautern-based international team uses vibrations in a magnetic material’s collective spin - fundamentally producing nanoscale waves of magnetization that can then interfere to create Boolean calculations.

You have atomic magnetic moments in your magnetic material which interact with each other and due to this interaction, there are wave-like excitations that can propagate in magnetic materials. The particular device we were investigating is based on the interference of these waves. If you use wave excitations instead of currents […] then you can make use of wave interference, and that comes with certain advantages.

Tobias Fischer, Doctoral Student, University of Kaiserslautern

Using wave interference to create the majority gate’s output offers two parameters to apply in regulating information: the wave’s amplitude, and phase. In principle, that makes this theory more efficient also since a majority gate can replace up to 10 transistors in advanced electronic devices.

“The device we were investigating consists of three inputs where we excite waves and they combine,” Fischer said. “Depending on the input phases where you encode the information, that determines the phase of the output signal, hence, defining the logic output state ‘0’ or ‘1’. That is actually information processing and that’s what we want.”

This first device prototype, although physically larger than what Fischer and his colleagues perceive for ultimate large-scale use, unmistakably shows the applicability of spin-wave phenomena for consistent information processing at GHz frequencies.

Since the wavelengths of these spin waves are easily decreased to the nanoscale, so too (though possibly not quite as easily) can be the gate device itself. Doing so may essentially enhance the functionality, minimizing its sensitivity to undesirable field fluctuations. Moreover, nano-scaling will boost spin-wave velocities that will allow for better computing speed.

What we aim for is the miniaturization of the device, and the smaller you make the device, the less sensitive it becomes to these influences. If you look at how many wavelengths fit into this propagation length, the fewer there are, the less influence a change of the wavelength has on the output. So basically downscaling the device would also come with more benefits.

Tobias Fischer, Doctoral Student, University of Kaiserslautern

Additionally, similar to antennae, a single device can be worked at multiple frequencies at the same time. This will allow for parallel computing using the same “core” of   advanced spin-wave processor.

“One of my colleagues in Kaiserslautern is into spin-wave multiplexing and de-multiplexing,” Fischer said. “We are also going in that direction, to use multiple frequencies and that would be a good compliment […] to this majority gate.” 

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