  Precision magnetometry is fundamental to the development of novel magnetic
materials and devices. Recently, the nitrogen-vacancy (NV) center in diamond
has emerged as a promising probe for static magnetism in 2D van der Waals
materials, capable of quantitative imaging with nanoscale spatial resolution.
However, the dynamic character of magnetism, crucial for understanding the
magnetic phase transition and achieving technological applications, has rarely
been experimentally accessible in single 2D crystals. Here, we coherently
control the NV center's spin precession to achieve ultra-sensitive,
quantitative ac susceptometry of a 2D ferromagnet. Combining dc hysteresis with
ac susceptibility measurements varying temperature, field, and frequency, we
illuminate the formation, mobility, and consolidation of magnetic domain walls
in few-layer CrBr3. We show that domain wall mobility is enhanced in ultrathin
CrBr3, with minimal decrease for excitation frequencies exceeding hundreds of
kilohertz, and is influenced by the domain morphology and local pinning of the
flake. Our technique extends NV magnetometry to the multi-functional ac and dc
magnetic characterization of wide-ranging spintronic materials at the
nanoscale.
