Figure 4: Dynamical variations in the local magnetic structure observed by single-spin magnetometry.

(a) Measurement schematic: nanodiamond containing a single NV centre is immobilized on the FeGe surface. The local magnetic stray field (dashed red lines) induces Zeeman shifts to the NV centre's electronic spin transitions (m_s=0↔m_s=±1) that are measured using optically detected EPR. Black arrows indicate the helical spin texture of FeGe. (b,c) Optically detected EPR spectrum during cool-down and warm-up, respectively, revealing the paramagnetic-to-helimagnetic phase transition at T_N≈286 K. Colour coding reflects normalized fluorescence intensity. (d,e) EPR signal of two additional NV centres, representing the two other characteristic types of traces that were observed. (f) T₁ measured above, at, and below T_N for an NV centre that showed a trace similar to e. Arrows indicate order of measurement. The scale on the right side provides the magnetic noise spectral density at the EPR frequency (∼2.9 GHz) calculated as S_B=2/(γ²T₁), where γ is the electron gyromagnetic ratio. T₁ values were measured using the protocol of ref. 56.