  We show that the interplay between spin-changing collisions and quadratic
Zeeman coupling provides a novel mechanism for the formation of repulsively
bound composites in high-spin fermions, which we illustrate by considering spin
flips in an initially polarized hard-core 1D Mott insulator of spin-3/2
fermions. We show that after the flips the dynamics is characterized by the
creation of two types of exciton-biexciton composites. We analyze the
conditions for the existence of these bound states, and discuss their
intriguing properties. In particular we show that the effective mass and
stability of the composites depends non-trivially on spin-changing collisions,
on the quadratic Zeeman effect and on the initial exciton localization.
Finally, we show that the composites may remain stable against inelastic
collisions, opening the possibility of novel quantum composite phases.
