  Two dimensional (2D) semiconductors present unique opportunities to
intertwine optical and magnetic functionalities and to tune these performances
through defects and dopants. In 2D semiconductors, the properties of excitons
often vary due to the underlying magnetic order, but the modification of their
magnetic properties by excitonic processes remains difficult to achieve. Here,
we integrate exciton pumping into a quantum sensing protocol on
nitrogen-vacancy centers in diamond to image the optically-induced transient
stray fields in few-layer, antiferromagnetic CrCl$_3$. We discover that exciton
recombination enhances the in-plane magnetization of the CrCl$_3$ layers, with
an outsized effect in the surface monolayers and when the surface is
unencapsulated. Concomitantly, time-resolved photoluminescence measurements
reveal that a defect-assisted Auger recombination dominates for atomically thin
CrCl$_3$ with tightly localized, nearly forbidden excitons and amplified
surface-to-volume ratio. These experimental signatures establish the
magnetically enhanced state to result from an exciton-activated electron
transfer between surface impurities and the spin-polarized conduction band.
Density functional theory calculations identify that the mediating impurity
state could originate from oxygen adsorbates with spins antiparallel to the
magnetization of their host layer. Our work unambiguously demonstrates the
tunability of single intrinsic magnetic layers via defect engineering and opens
a pathway to dynamically reconfigure these effects in devices through optical
control.
