Waveguides as sources of short-wavelength spin waves for low-energy ICT applications

Spin waves offer intriguing possibilities for transmitting and processing information in future low-power electronics. Most proposed devices, however, require the efficient excitation and detection of spin waves in the sub-micrometer range, that is a rather challenging task. In fact, coplanar and microstrip waveguides have been widely used in the past to excite and detect spin waves with wavelengths of tens of microns in thin films of both metallic ferromagnets and on magnetic insulators, but the scalability of these structures micrometer or sub-micrometer have not been investigated in detail. In this study, we present a combined experimental/computational study of a few possible input structures consisting of either symmetrical or asymmetrical coplanar waveguides on top of CoFe films, with widths going all the way down to 250 nm. The primary goal of this work is to present a case study, aiming to explore the limitations of waveguides in creating short-wavelength spin waves for future nanoelectronic applications. We use micro-focused Brillouin light scattering measurements and micromagnetic simulations to analyze the characteristics of the emitted spin waves, achieving reasonable agreement between experiment and simulations. We find that due to the inherently delocalized field distributions of waveguides, and also to the relatively high resistivity of narrow waveguides, they all show poor efficiency for generating spin waves with wavelength below about 2 μm, corresponding to frequencies above 10 GHz in a moderate external field. This means that the intensity of the generated spin waves for a given input power drops quickly for the frequency/wavelength range which is most relevant for emerging applications. This case study demonstrates many of the inherent inefficiencies and limitations of waveguide-based spin wave generation in this regime. Our work supports the conclusion that one may have to use a different mechanism for spin wave generation, exploiting multiferroic structures, spin-orbit torques or nanopatterned, multi-layered magnetic materials, all being the subject of intense current research.