Motivated by recent experimental progress in 2D magnetism, we
theoretically study spin transport in 2D easy-plane magnets at finite
temperatures across the Berezinskii-Kosterlitz-Thouless (BKT) phase
transition, by developing a duality mapping to the 2+1D electromagnetism
with the full account of spin’s finite lifetime. In particular, we find
that the non-conservation of spin gives rise to a distinct signature
across the BKT transition, with the spin current decaying with distance
power-law (exponentially) below (above) the transition; this is
detectable in the proposed experiment with
NiPS_33
and CrCl_33.