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We study the interaction of surface acoustic waves (SAWs) with spin waves (SWs) in a Co 40 Fe 40 B 20 /Au/Ni 81 Fe 19 system composed of two ferromagnetic layers separated by a nonmagnetic Au spacer layer. Because of interlayer magnetic dipolar coupling between the two ferromagnetic layers, a symmetric and an antisymmetric SW mode form, which both show a highly nondegenerate dispersion relation for oppositely propagating SWs. Due to magnetoacoustic SAW-SW interaction, we observe highly nonreciprocal SAW transmission in the piezoelectric-ferromagnetic hybrid device. We experimentally and theoretically characterize the magnetoacoustic wave propagation as a function of frequency, wave vector, and external magnetic field magnitude and orientation. Additionally, we demonstrate that the nonreciprocal SW dispersion of a coupled magnetic bilayer is highly tuneable and not limited to ultrathin magnetic films, in contrast to the nonreciprocity induced by the interfacial Dzyaloshinskii-Moriya interaction. Therefore, magnetoacoustic coupling in ferromagnetic multilayers provides a promising route towards building efficient acoustic isolators.

We report ultra-low intrinsic magnetic damping in Co 25 Fe 75 heterostructures, reaching the low 10 −4 regime at room temperature. By using a broadband ferromagnetic resonance technique in out-of-plane geometry, we extracted the dynamic magnetic properties of several Co 25 Fe 75 -based heterostructures with varying ferromagnetic layer thickness. By measuring radiative damping and spin pumping effects, we found the intrinsic damping of a 26 nm thick sample to be α 0 3.18 × 10 −4 . Furthermore, using Brillouin light scattering microscopy we measured spin-wave propagation lengths of up to (21 ± 1) µm in a 26 nm thick Co 25 Fe 75 heterostructure at room temperature, which is in excellent agreement with the measured damping.Itinerant ferromagnets (FM) are advantageous for spintronic and magnonic devices. They benefit from, e.g., large magnetoresistive effects and current-induced spinorbit torques 1 . In many magneto-resistive technologies (e.g., anisotropic magnetoresistance, giant magnetoresistance, tunnel magnetoresistance) electronic conductivity is indispensable. Moreover, due to high saturation magnetization in metallic FMs, spin-wave (SW) group velocities are in general significantly higher than in insulating ferrimagnets 2-5 . High saturation magnetizations in general ease detection. Nevertheless, itinerant FMs typically have considerable magnetic damping 6,7 . This is unfavorable for many applications. For example, low damping is crucial for oscillators based on spin transfer torques and spin orbit torques as well as for achieving large spin-wave propagation lengths (SWPL) 8-10 . The need for thin film materials with low magnetic damping has triggered the interest in the insulating ferrimagnet yttrium-iron garnet (Y 3 Fe 5 O 12 , YIG) 11-13 . Although for YIG, very small total (Gilbert) damping parameters in the order of α G ≈ 10 −5 , and large SWPLs of a few tens of micrometers (up to ∼ 25 µm) in thin films (∼ 20 nm) have been reported 5,13,14 , its insulating properties and requirement for crystalline growth are challenges for large scale magnonic applications.Schoen et al. recently observed ultra-low intrinsic magnetic damping in Co 25 Fe 75 (CoFe) metallic thin films (α 0 = (5 ± 1.8) × 10 −4 ) 15 , and Krner et al. reported PLs of 5 µm − 8 µm in CoFe using time resolved scanning magneto-optical Kerr microscopy 4 . This motivated our study on sputter-deposited CoFe-based thin film heterostructures. We use broadband ferromagnetic resonance (BB-FMR) spectroscopy 16 in outa) Electronic of-plane (OOP) geometry and Brillouin light scattering (BLS) microscopy 17 and find intrinsic damping parameters in the lower 10 −4 regime as well as SWPLs of more than 20 µm. The damping is therefore comparable to YIG/heavy metal (HM) heterostructures 18 and the SWPL is comparable to that of state-of-the-art YIG thin films 5,13 . Thin film CoFe is a promising candidate for all-metal magnonic devices, as it combines low magnetic damping with good electrical conductivity and large saturation magnetization, while enabling easy fab...