Figure 1: Nanoscale probing of the magnetic fields generated by a ferromagnetic microdisc.

(a), As a model system to study magnetic excitations, we consider a ferromagnetic microdisc (Ni₈₁Fe₁₉, diameter 6 μm, thickness 30 nm) fabricated on top of a diamond surface. NV centres implanted at ∼50 nm below the surface sense the local magnetic fields B_M generated by the magnetization M. (b) Scanning confocal microscopy image showing a photoluminescence map of NV centres close to the disc. Scale bar, 3 μm. The external static magnetic field B_ext is applied along the axis of target NV centres. (c) Optically detected electron spin resonance (ESR) traces of the m_s=0↔+1 transition of NV_A (close to the disc) and NV_ref (at ∼11 μm from the disc centre), where m_s denotes the spin-projection onto the NV-axis, f is the drive frequency, and γ=2.8 MHz/G. From the ESR frequency of NV_ref we extract the external magnetic field B_ext. From the difference between NV_A and NV_ref we extract the disc stray field at the site of NV_A. The dashed circle indicates power broadening caused by amplification of the drive field by a spin-wave excitation. (d) Projection onto the NV axis B_∥ of the measured disc stray field as a function of external field, showing opposite behaviour at the sites of NV_A and NV_B. The sign of the field is relative to B_ext. Error bars represent±1 standard deviation determined statistically from typically ≳100 repetitions of the same measurement. (e) Calculated spatial profile of the projection of the disc stray field onto the NV axis in a plane 50 nm below the disc. B_ext=700 G. (f) Numerically calculated projection of the disc stray field onto the NV axis as a function of the external field at the sites of NV_A and NV_B, in qualitative good agreement with the measurements in d.