Electrical-to-Spin-Wave Transducers for Spin-Wave Computing Devices

Spin waves are currently being investigated as an emerging alternative to electrical signals for high-speed computing. This work contributes new designs of electrical-to-spin-wave transducers for the realization of computing devices based on the interference and diffraction of spin waves in a magnetic thin film.

These devices need a material in which spin waves will be excited. All computing is then done via the diffraction and interference of the spin waves that travel in the material. Yttrium iron garnet (YIG) is chosen as the spin-wave medium in this work. Because of its very low Gilbert damping parameter, spin waves in YIG are able to travel millimeters before dissipating.

A significant barrier to the adoption of spin-wave devices is the inability to grow good quality YIG on silicon substrates. To this end, this work presents several studies on the quality of YIG thin films grown on silicon dioxide via radio frequency magnetron sputtering. Rapid thermal annealing as the post deposition anneal was explored for different thicknesses, and a surprisingly narrow temperature window was found to produce the highest saturation magnetization for ultra-thin films. Transmission electron microscopy imaging visually shows a correlation between the saturation magnetization and the structure of the film, and shows a change in structure when annealing above the temperature window.

Future applications of spin-wave devices that this work is focused on require spin-wave transducers that operate across a relatively wide bandwidth. Coplanar-waveguides (CPW) would typically be unsuitable for this application due to their comb-like frequency response, which depends on the dimensions of the CPW. Micromagnetic simulations were performed using Mumax3 with fields extracted from HFSS to explore the use of the edge of a YIG film to excite spin waves. The simulations show that this produces spin waves in a relatively broad frequency range, but at the expense of peak transducer efficiency.

Finally, this work explores using the edge of a YIG film to excite spin waves with sub-micrometer wavelengths. Because more wavelengths will fit in a smaller area, this would enable further shrinking of spin-wave devices. Not only does this increase the density of these devices, but it also allows for the use of poorer quality YIG, which could lead to the integration of spin-wave devices on silicon substrates.

Typically, the shortest wavelength that a CPW placed on top of a magnetic film can excite depends on the dimensions of the CPW. The possibility of using the edge of a YIG film was explored using Mumax3 simulations, which show that the loss in transducer efficiency is too great for this to be a viable solution. However, this work builds on those designs and presents a novel solution that uses a Supermalloy film adjacent to the YIG film. Relatively large CPWs are used to launch spin waves having long wavelengths in the Supermalloy; these waves travel into the YIG where they are converted into short-wavelength spin waves. Micromagnetic simulations show that this design greatly increases the amplitude of the launched spin waves for the same power delivered to the transducer. This design can also be used in reverse to convert short-wavelength spin waves to long-wavelength spin waves, which would allow for the use of CPWs as outputs for short-wavelength spin-wave devices.