Probing Short-Wavelength Magnon Scattering with Scanning NV Magnetometry

Abstract

Magnetic order in thin films, heterostructures, and two-dimensional materials often varies on sub-micron length scales, yet few experimental probes combine nanoscale spatial resolution with the spectral selectivity needed to characterize it. This thesis develops scanning nitrogen-vacancy (NV) magnetometry as a platform for imaging short-wavelength magnon scattering, using propagating spin waves as non-invasive probes of mesoscopic magnetic texture in yttrium iron garnet (YIG) thin films. The central experimental advance is a sub-micron magnon scattering platform. By integrating magnetic coplanar waveguides directly onto YIG, I overcome the Fourier-limited excitation efficiency of conventional striplines and generate coherent magnons with wavelengths tunable from approximately 980 nm down to 550 nm. Phase-resolved scattering measurements from patterned permalloy and cobalt targets reveal interference signatures that depend strongly on target size, shape, and probe wavelength—features that cannot be explained by simple edge diffraction. Micromagnetic simulations show that the targets imprint spatially structured domain-closure textures into the underlying YIG, creating effective scattering potentials whose characteristic length scale matches the probe wavelength. These results establish that the platform is sensitive to internal magnetic structure rather than gross target geometry. To push beyond the approximately 500 nm bandwidth limit of resonant NV detection, I implement off-resonant sensing via the AC Zeeman effect. Operating at high bias fields places the NV spin resonance below the magnon ferromagnetic resonance—a counter-intuitive strategy that suppresses thermal magnon noise by positioning the sensor in a spectral gap with zero magnon density of states. The resulting recovery of long spin coherence times enables phase-resolved imaging of magnons with wavelengths as short as 313 nm, the shortest spin waves imaged in real space by scanning NV magnetometry to date. Together, these advances establish a scalable, non-invasive approach for probing nanoscale magnetic order in emerging low-dimensional and two-dimensional magnetic systems.