  We study the critical energy dissipation in a spin-1 antiferromagnetic
Bose-Einstein condensate by an oscillating magnetic obstacle, which is formed
by focusing a near-resonant laser beam. When the magnetic obstacle is weak not
to saturate local spin polarization, a sudden onset of spin-wave excitations is
observed as the oscillation frequency increases, demonstrating the spin
superfluid behavior of the system. For a strong obstacle that causes density
perturbations via local spin saturation, half-quantum vortices (HQVs) are
created by the moving magnetic obstacle, which shows that the critical
dissipative dynamics evolves from spin-wave emission to HQV shedding. Critical
HQV shedding is further investigated using a pulsed linear motion of the
obstacle, and we identify two critical velocities to create HQVs with different
core magnetization.
