Figure 1.

Detecting coherent spin waves and incoherent magnon densities using NV spins in diamond. (a) A diamond cantilever, with NVs implanted ∼20 nm below the tip surface, is mounted in an AFM setup and used for probing the stray field of spin waves that are excited by a gold stripline. The NV spins are initialized using a green laser and read out via spin-dependent photoluminescence (PL). Insets: scanning electron micrographs of diamond cantilever and tip. (b) Normalized NV photoluminescence vs external field B₀ and microwave drive frequency. The ESR frequencies (f_±) of the NV family that is most aligned with B₀ are labeled. The strong PL response close to the ferromagnetic resonance (FMR) is a result of the process depicted in (d). The FMR is calculated as , where γ = 28 GHz/T, μ₀ = 4π × 10^–7 H/m, M_s = 1.42 × 10⁵ A/m, and B_IP is the in-plane magnetic field component (Supporting Information Note 4). (c) Spatial map of the normalized NV ESR PL showing a coherent spin wave excited unidirectionally (to the right) by applying a microwave current at f_– through the stripline. On the left of the stripline there is no detectable spin-wave signal. The film is magnetized along the stripline direction by a magnetic field B₀ (orange arrow), which is set to a low value (B₀ ≈ 0 mT) in this measurement. At each pixel, the measured PL under microwave driving is normalized to that without microwave driving (PL₀). The image is low-pass filtered to reduce pixel-by-pixel noise. (d) Sketch of the spin-wave dispersion (black line) and its occupation by magnons (color gradient). Without microwave driving (left), only thermally excited magnons are present. Microwave (MW) driving near the FMR frequency (oscillating arrow in right panel) increases the magnon density, which can be detected via the increased stray-field noise at the NV ESR frequencies (f_±).