Nonreciprocal sound for spin wave dynamics

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Surface acoustic waves (SAWs) have made their way into many everyday devices, such as rf-filters and sensors, thanks to the greatly reduced wavelength of SAWs compared to free-space microwaves of the same frequency. Coherent SAWs in the MHz- to GHz-range can be efficiently launched and detected on piezoelectric substrates with periodic metallic gratings. However, SAWs are in general propagating reciprocally. As we have recently demonstrated, resonant magnetoacoustic coupling of reciprocal SAWs with nonreciprocal spin waves (SWs) gives rise to nonreciprocal magnetoacoustic wave transmission in piezoelectric/ferromagnetic hybrid systems, which opens the possibility to build novel nonreciprocal microwave devices. Based on these first results, this joint project aims to explore these nonreciprocal effects in novel material systems in more detail.First, we suggest studying the nonreciprocity of GHz-frequency magnetoacoustic waves in hybrid devices combining piezoelectric substrates with thin film synthetic antiferromagnets and compensated ferrimagnets. Thereby, we will investigate the impact of relative chirality of SAW and SW modes on nonreciprocity and exploit the SAW-SW coupling by internal magnetoacoustic driving fields that can couple to materials with vanishing net magnetic moment. These tailored piezoelectric/magnetic hybrids can be used as a basis for miniaturized, on-chip, nonreciprocal microwave elements, such as isolators. Second, we suggest using SAWs for SW-spectroscopy. To this end, we will exploit magnetoacoustic interactions to characterize SW dispersions and magnetic texture in magnetic thin films with interfacial Dzyaloshinskii-Moriya interaction. Because the wavelength of SAWs can be shorter than optical wavelengths, we expect that our method allows for highly sensitive spectroscopy beyond the resolution limits of conventional magneto-optical techniques. Finally, we will use magneto-optic imaging to study the interaction and interconversion of coherent magnons and phonons. With these spatially- and phase-resolved techniques, we can determine wave vectors of SAWs and SWs and thus address the excitation of SWs by SAWs (and vice versa) in detail. This will allow us to verify and refine our model assumptions for magnon-phonon interactions.