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Abstract

Spin waves in magnetic insulators are low-damping signal carriers that can enable a new generation of spintronic devices. The excitation, control, and detection of spin waves by metal electrodes is crucial for interfacing these devices to electrical circuits. As such, it is important to understand metal-induced damping of spin-wave transport, but characterizing this process requires access to the underlying magnetic films. Here it is shown that electronic sensor spins in diamond enable imaging of spin waves that propagate underneath metals in magnetic insulators. This capability is then used to reveal a 100-fold metal-induced increase in spin-wave damping. The damping enhancement is attributed to spin-wave-induced electrical currents as well as, above a certain frequency, three-magnon scattering processes. This interpretation is supported by deriving expressions for the current-induced damping and the three-magnon threshold from the Landau–Lifshitz–Gilbert equation that agree well with the observations. The detection of buried scattering centers further highlights the technique's power for assessing spintronic device quality. These results open new avenues for studying metal – spin-wave interactions and provide access to interfacial processes such as spin-wave injection via the spin-Hall effect.