Filtering and imaging of frequency-degenerate spin waves using nanopositioning of a single-spin sensor

TL;DR

This paper demonstrates a nanoscale control method using a single NV sensor to filter and image frequency-degenerate spin waves in magnetic insulators, revealing detailed spin-wave band occupation.

Contribution

It introduces a distance-tunable filtering technique with a single NV sensor to characterize and image spin waves, enabling new insights into magnon behavior.

Findings

Suppression of small-wavelength modes by retracting the NV tip.

Observation of entire iso-frequency contour occupation.

Potential for imaging magnon condensates and coherent modes.

Abstract

Nitrogen-vacancy (NV) magnetometry is a new technique for imaging spin waves in magnetic materials. It detects spin waves by their microwave magnetic stray fields, which decay evanescently on the scale of the spin-wavelength. Here, we use nanoscale control of a single-NV sensor as a wavelength filter to characterize frequency-degenerate spin waves excited by a microstrip in a thin-film magnetic insulator. With the NV-probe in contact with the magnet, we observe an incoherent mixture of thermal and microwave-driven spin waves. By retracting the tip, we progressively suppress the small-wavelength modes until a single coherent mode emerges from the mixture. In-contact scans at low drive power surprisingly show occupation of the entire iso-frequency contour of the two-dimensional spin-wave dispersion despite our one-dimensional microstrip geometry. Our distance-tunable filter sheds light on the spin-wave band occupation under microwave excitation and opens opportunities for imaging magnon condensates and other coherent spin-wave modes.