  We report a new manifestation of first-order magnetic transitions in
two-dimensional electron systems. This phenomenon occurs in aluminum arsenide
quantum wells with sufficiently low carrier densities and appears as a set of
hysteretic spikes in the resistance of a sample placed in crossed parallel and
perpendicular magnetic fields, each spike occurring at the transition between
states with different partial magnetizations. Our experiments thus indicate
that the presence of magnetic domains at the transition starkly increases
dissipation, an effect also suspected in other ferromagnetic materials.
Analysis of the positions of the transition spikes allows us to deduce the
change in exchange-correlation energy across the magnetic transition, which in
turn will help improve our understanding of metallic ferromagnetism.
