  We study the critical energy dissipation in an atomic superfluid gas with two
symmetric spin components by an oscillating magnetic obstacle. Above a certain
critical oscillation frequency, spin-wave excitations are generated by the
magnetic obstacle, demonstrating the spin superfluid behavior of the system.
When the obstacle is strong enough to cause density perturbations via local
saturation of spin polarization, half-quantum vortices (HQVs) are created for
higher oscillation frequencies, which reveals the characteristic evolution of
critical dissipative dynamics 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.
