We use spin-density-functional theory to study recently reported hysteretic magnetoresistance ρ_xx spikes in Mn-based 2D electron gases [Jaroszyński et al. Phys. Rev. Lett. (2002)]. We find hysteresis loops in our calculated Landau fan diagrams and total energies signaling quantum-Hall-ferromagnet phase transitions. Spin-dependent exchange-correlation effects are crucial to stabilize the relevant magnetic phases arising from distinct symmetry-broken excited- and ground-state solutions of the

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... n-Sham equations. Besides hysteretic spikes in ρ_xx, we predict hysteretic dips in the Hall resistance ρ_xy. Our theory, without domain walls, satisfactorily explains the recent data.