  We report on the observation of half-quantum vortices (HQVs) in the
easy-plane polar phase of an antiferromagnetic spinor Bose-Einstein condensate.
Using in situ magnetization-sensitive imaging, we observe that pairs of HQVs
with opposite core magnetization are generated when singly charged quantum
vortices are injected into the condensate. The dynamics of HQV pair formation
is characterized by measuring the temporal evolutions of the pair separation
distance and the core magnetization, which reveals the short-range nature of
the repulsive interactions between the HQVs. We find that spin fluctuations
arising from thermal population of axial magnon excitations do not
significantly affect the HQV pair formation dynamics. Our results demonstrate
the instability of a singly charged vortex in the antiferromagnetic spinor
condensate.
