Measurements of Landau-level crossings and extended states in magnetic two-dimensional electron gases

We present longitudinal and Hall magnetoresistance measurements of a "magnetic" two-dimensional electron gas formed in modulation-doped Zn_1-x-yCd_xMn_ySe quantum wells. The electron spin splitting is temperature and magnetic field dependent, resulting in striking features as Landau levels of opposite spin cross near the Fermi level. Magnetization/measurements on the same sample probe the total density of states and Fermi energy, allowing us to fit the transport data using a model involving extended states centered at each Landau level and two-channel conduction for spin-up and spin-down electrons. A mapping of the extended states over the whole quantum Hall effect regime shows no floating of extended states within experimental resolution as Landau levels cross near the Fermi level.