  Recently, the Berezinskii-Kosterlitz-Thouless transition was found to be
mediated by half-quantum vortices (HQVs) in two-dimensional (2D)
antiferromagnetic Bose gases [Phys. Rev. Lett. 97, 120406 (2006)]. We study the
thermal activation of HQVs in the experimentally relevant trapped quasi-2D
systems and find that the crossover temperature is shifted upwards if skyrmions
are allowed. Above the defect binding temperatures we observe transitions
corresponding to the onset of a coherent condensate and a quasi-condensate and
discuss the absence of a fragmented condensate.
