  Magnetic microscopy of malarial hemozoin nanocrystals was performed using
optically detected magnetic resonance imaging of near-surface diamond
nitrogen-vacancy centers. Hemozoin crystals were extracted from
$Plasmodium$-$falciparum$-infected human blood cells and studied alongside
synthetic hemozoin crystals. The stray magnetic fields produced by individual
crystals were imaged at room temperature as a function of applied field up to
350 mT. More than 100 nanocrystals were analyzed, revealing the distribution of
their magnetic properties. Most crystals ($96\%$) exhibit a linear dependence
of stray field magnitude on applied field, confirming hemozoin's paramagnetic
nature. A volume magnetic susceptibility $\chi=3.4\times10^{-4}$ is inferred
using a magnetostatic model informed by correlated scanning electron microscopy
measurements of crystal dimensions. A small fraction of nanoparticles (4/82 for
$Plasmodium$-produced and 1/41 for synthetic) exhibit a saturation behavior
consistent with superparamagnetism. Translation of this platform to the study
of living malaria-infected cells may shed new light on hemozoin formation
dynamics and their interaction with antimalarial drugs.
