  Magnetic imaging based on nitrogen-vacancy (NV) centers in diamond has
emerged as a powerful tool for probing magnetic phenomena in fields ranging
from biology to physics. A key strength of NV sensing is its local-probe
nature, enabling high-resolution spatial images of magnetic stray fields
emanating from a sample. However, this local character can also form a drawback
for analysing the global properties of a system, such as a phase transition
temperature. Here, we address this challenge by using statistical analyses of
magnetic-field maps to characterize the first-order temperature-driven
metamagnetic phase transition from the antiferromagnetic to the ferromagnetic
state in FeRh. After imaging the phase transition and identifying the regimes
of nucleation, growth, and coalescence of ferromagnetic domains, we
statistically characterize the spatial magnetic-field maps to extract the
transition temperature and thermal hysteresis width. By analysing the spatial
correlations of the maps and their dependence on an external magnetic field, we
investigate the magnetocrystalline anisotropy and detect a reorientation of
domain walls across the phase transition. The employed statistical approach can
be extended to the study of other magnetic phenomena with NV magnetometry or
other sensing techniques.
