We observe spin rotations caused by atomic collisions in a non-equilibrium
Bose-condensed gas of 87Rb. Reflection from a pseudomagnetic barrier
creates counterflow in which forward- and backward-propagating matter waves
have partly transverse spin directions. Even though inter-atomic interaction
strengths are state-independent, the indistinguishability of parallel spins
leads to spin dynamics. A local magnetodynamic model, which captures the
salient features of the observed spin textures, highlights an essential
connection between four-wave mixing and collisional spin rotation. The observed
phenomenon has previously been thought to exist only in nondegenerate gases;
our observations and model clarify the nature of these effective-magnetic spin
rotations.Comment: 13 pages, 7 figure