Saravanan Sengottuvel, Omkar Dhungel, Mariusz Mrózek, Arne Wickenbrock, Dmitry Budker, Wojciech Gawlik, and Adam M. Wojciechowski (Phys Rev. Applied 23(3), 4001 (2025))
Magnetometry using nitrogen-vacancy (N-V) color centers in diamond predominantly relies on microwave spectroscopy. However, microwaves may hinder certain studies involving biological systems or thin conductive samples. This work demonstrates a wide-field, microwave-free imaging magnetometer utilizing N-V centers in nanodiamonds by exploiting the cross-relaxation feature near zero magnetic fields under ambient conditions without applying microwaves. For this purpose, we measure the center shift, contrast, and linewidth of zero-field cross relaxation in 140-nm nanodiamonds dropcast on a current-carrying conductive pattern while scanning a background magnetic field, achieving a sensitivity of 4.5μT/Hz. Our work allows for applying the N-V zero-field feature in nanodiamonds for magnetic-field sensing in the zero- and low-field regimes and highlights the potential for microwave-free all-optical wide-field magnetometry based on nanodiamonds.