An international team of scientists recently introduced a prototype of a logic gate based on spin waves, as reported in Applied Physics Letters. Ever-shrinking complementary metal oxide semiconductor (CMOS)-based devices are approaching the scale at which their performance is limited by heating effects. Components based on spin waves, fundamental excitations in magnetic materials, show promise for spintronic and magnonic devices that complement CMOS technologies and could enable nanoelectronics beyond the CMOS limit.

When a magnetic material is placed in a microwave magnetic field, the precession of magnetic moments results in wave-like excitations called spin waves. These spin waves have frequencies in the GHz range and their wavelengths can easily be tuned to the nanometer scale. Like light and water waves, spin waves demonstrate interference. These properties suggest that spin waves could form the basis of efficient majority logic gates that have excellent scaling potential.

Majority logic gates are commonly used in logic circuits that perform addition, as well as in other systems to safeguard against errors. In a majority logic gate, the output value corresponds to the value of the majority of inputs. For example, a majority gate with inputs corresponding to 0, 1, and 0 would have an output value corresponding to 0. The inputs for traditional majority gates are in the form of current or applied voltage. The research team has now created a three-input majority gate whose inputs were phases of the spin wave signals shifted by either 0 or π. The majority output was determined by the interference of the input waves.

“We demonstrated that it is possible to employ spin waves in information processing,” says Tobias Fischer, lead author of the article and a student under Burkard Hillebrands at the Technical University of Kaiserslautern in Germany. Researchers had previously explored the functionality and performance of spin-wave-based majority gates with numerical simulations. Turning the spin wave majority gate concept into reality required developing a working structure and a corresponding system for investigating its operational characteristics, Fischer says.

In the prototype, a microwave generator sends current through strips of copper that run underneath a waveguide structure. The waveguide is built from yttrium-iron-garnet (YIG), a ferrimagnetic material across which spin waves can travel macroscopic distances. The current induces a magnetic field and generates spin waves in the waveguide. The spin waves are individually phase shifted and then combined, resulting in a wave that is the superposition of the three inputs. The resulting wave generates an electrical signal in an outgoing copper strip that can be read by an oscilloscope.

Using an oscilloscope with a high sampling rate, the team mapped the output signals for all possible spin-wave phase inputs. The results show that the phase of the output signal depends on the majority of the input phases. Tests also indicate that the prototype works well for a wide range of frequencies.

In order to assess the prototype’s potential for subnanosecond signal processing, the researchers studied how quickly the input states can be switched while the output states remain distinguishable. Their results show that switching time is correlated to the length of time it takes the spin waves to travel through the structure. By miniaturizing the device, it should be possible to achieve switching times below 1 ns.

“This is an excellent demonstration of a spintronic majority gate,” according to Andrei Slavin, an expert on spin waves from Oakland University who was not affiliated with this research. “At this point it is not clear how efficient this design will be when scaled down to nanosizes, but the idea and the realization is novel and interesting,” Slavin says.

Creating a majority gate for use in nanoelectronics is largely a materials challenge. Not only does it require a waveguide with large saturation magnetization and low damping of spin waves, Fischer says, the material should ideally be CMOS-compatible and easy to structure with conventional technologies. Scientists have recently made progress on miniaturizing YIG structures and controlling the phase of spin waves, potentially bringing this concept closer to application. The team is also exploring whether efficient waveguides can be made of other materials, such as Heusler compounds.  

Read the abstract in Applied Physics Letters.