Spin waves—the elementary excitations of magnetic materials—are prime candidate signal carriers for low-dissipation information processing. Being able to image coherent spin-wave transport is crucial for developing interference-based spin-wave devices. We introduce magnetic resonance imaging of the microwave magnetic stray fields that are generated by spin waves as a new approach for imaging coherent spin-wave transport. We realize this approach using a dense layer of electronic sensor spins in a diamond chip, which combines the ability to detect small magnetic fields with a sensitivity to their polarization. Focusing on a thin-film magnetic insulator, we quantify spin-wave amplitudes, visualize spin-wave dispersion and interference, and demonstrate time-domain measurements of spin-wave packets. We theoretically explain the observed anisotropic spin-wave patterns in terms of chiral spin-wave excitation and stray-field coupling to the sensor spins. Our results pave the way for probing spin waves in atomically thin magnets, even when embedded between opaque materials.

We present a theory for quantum impurity relaxometry of magnons in thin films, exhibiting quantitative agreement with recent experiments without needing arbitrary scale factors used in theoretical models thus far. Our theory reveals that chiral coupling between prototypical spin>1/2 quantum impurities and magnons plays a central role in determining impurity relaxation, which is further corroborated by our experiments on nickel films interfaced with nitrogen-vacancy centers. Along with advancing magnonics and understanding decoherence in hybrid quantum platforms with magnets, the ability of a quantum impurity spin to sense chiral magnetic noise presents an opportunity to probe chiral phenomena in condensed matter.