Enhancing Extremely Low‐Frequency Signal‐to‐Noise Ratio of Diamond Magnetometry via Nonlinear Response

Extremely low‐frequency (below 10 Hz) current‐induced magnetic field detection has significant applications in high‐voltage DC systems, lithium‐ion battery diagnostics, and industrial process monitoring. Nitrogen‐vacancy (NV) ensembles magnetometry typically employs flux concentrators to enhance magnetic detection sensitivity, but this enhancement comes at the cost of introducing more low‐frequency magnetic noise, such as the thermal magnetization noise of ferromagnetic materials, directly limiting their potential at low frequencies. Here, the enhancement of the signal‐to‐noise ratio (SNR) in extremely low‐frequency magnetic field detection within NV magnetometry, achieved via nonlinear response, is experimentally demonstrated. The approach enables the extension of the magnetic field detection bandwidth to the Hz range while simultaneously enhancing magnetic field sensitivity by using a magnetic flux concentrator. The magnetic field from the coil current, enhanced by the flux concentrator, drives NV ensembles into the nonlinear response region of the differential spectrum of the optically detected magnetic resonance (ODMR). Within this regime, nonlinear effects generate signal‐frequency mixing and 1/f noise suppression. By pre‐modulating the target signal at the driving frequency, its recovery through frequency mixing retains 1/f noise suppression, consequently enhancing SNR. For the 0.5 Hz signal, experimental results demonstrate up to a 2.6‐fold enhancement in SNR. This approach offers a new strategy for utilizing NV ensembles in extremely low‐frequency magnetic field detection.