Design of a Coplanar-Waveguide-Based Microwave-to-Spin-Wave Transducer

Abstract

Spin waves (SWs) are emerging as a potential alternative to electrical signals for computing and signal processing. Because SWs have relatively short wavelengths at microwave frequencies, they have the potential for applications in high-speed chip-scale electronics [1]. This work seeks to design and fabricate a spin-wave-based real-time spectrum analyzer that exploits SW interference in analogy to an optical spectrometer [2]. Here, microwave electrical signals are converted into SWs. All processing is then done by the diffraction and interference of SWs as they propagate in a magnetic thin film. After the interference pattern forms, SWs are then converted back into electrical signals.This type of device requires SWs having an isotropic dispersion relation, which, in turn, requires an out-of-plane DC bias field. These spin waves are forward-volume spin waves (FVSW). The focus of the work presented here is to explore the design space of a microwave-to-spin-wave transducer for FVSWs, where the microwave signal is introduced by a coplanar waveguide (CPW).The SW excitation spectrum of an external RF magnetic field acting on a magnetic thin film is the result of the interference between SWs launched from all points of the film that are excited by the magnetic field. For a magnetic field from a CPW, this interference results in a comb-like spatial-frequency response, which is equivalent to the Fourier transform (FT) of the spatial distribution of the magnetic field that acts on the magnetic film [3]. However, because the device described above requires a relatively constant transfer function over the full bandwidth, i.e., without nulls, the comb-like excitation spectrum from a CPW would not be suitable for our applications.Launching spin waves from the edge of the film can be used to improve the spin-wave excitation spectrum for a couple of reasons. First, it has been shown that the edge of a film can be an efficient broadband source of spin-waves [4]. And second, because only part of the CPW’s field acts on the magnetic film, the FT of only that part describes the excitation spectrum; the FT of the spatial distribution is more delta-function-like and therefore is more broadband than the previous case.To design the edge-launched SW transducer, micromagnetic simulations were done in Mumax3 [5] using the magnetic field from CPWs simulated in Ansys HFSS. Our simulations (Fig. 1) show the frequency response of this simulated structure with the edge 2 μm away from the CPW. These simulations verify that the transducer’s excitation spectrum is improved, i.e., made more uniform, by placing the CPW beside the edge of the film rather than on top of the film. This improved bandwidth comes at the cost of transducer efficiency, since only the field to the side of the CPW acts on the magnetic film.Because the magnetic field decreases rapidly with distance from the CPW, the edge of the film should be as close to the CPW as possible to increase coupling between the CPW and the film. Representative waveforms given in Fig. 2 show that because the small out-of-plane component of the RF magnetic field is parallel to the large DC-bias magnetic field, it contributes very little to the excitation of a SW compared to the in-plane components of the field. The results of the simulations show that for the same magnetic field, raising the bottom of the CPW increases the in-plane component of the field and increases the amplitude of the SW.Yet another configuration would be to place the CPW on the magnetic film and create an edge-launcher by etching a gap in the film to the side of the CPW. Excessively large fields can produce unstable spin-waves with additional unwanted undesirable frequency components [6]. With a gap in the magnetic material, the internal magnetic field crosses the gap at a reduced field strength, removing the unwanted structure and resulting in SWs at the desired wavelength, as well as a smoother frequency response (Fig. 1.).The structure of the previous paragraph was simulated with the properties of an YIG thin film having different thicknesses (30 nm and 243 nm). It is found that increasing the distance between the gap and the CPW increases the amplitude of the resulting SW with diminishing returns, as shown in Fig. 2.With a gap that is 2 μm wide and 6 μm away from the CPW, we find that for the thinner film this structure produces a SW with a smaller amplitude than when the material underneath the CPW is nonmagnetic and the edge is flush with CPW. However, with the thicker film (Fig. 2), the structure with the magnetic film under the CPW produces a larger SW than the structure without the magnetic material under the CPW. **