Effects of Spin Polarization on Electron Transport in Modulation Doped Cd1−xMnxTe/Cd1−yMgyTe:I Heterostructures

We examine and identify magnetoresistance mechanisms in 2D system containing a sizable concentration of magnetic ions. We argue that some of these mechanisms can serve as a tool to measure spin polarization. Lack of spin degeneracy and enhanced localization make it possible to detect an additional QHE plateau associated with extended states floating-up in vanishing magnetic field. In diluted magnetic semiconductors (DMS), owing to the strong s-d exchange between the effective mass carriers and the localized Mn spins, a high degree of spin-polarization of the carrier liquid can be achieved in moderately strong magnetic fields [1]. This offers a tool to find out how properties of quantum structures evolve as a function of spin polarization. Our work extends previous transport studies of II-VI DMS low-dimensional systems [2, 3, 4, 5, 6, 7] in two major directions. First, one of the challenging problems that face spintronics [8] is how to read the local spin configuration. We have combine millikelvin studies of low-field magnetoconductance with theoretical calculations in order to single out possible effects of spin-polarization on electron transport in high electron mobility modulation-doped heterostructures of Cd1−xMnxTe/Cd1−yMgyTe:I. Based on our results, we argue that the temperature dependence of conductivity can serve as a tool to detect and quantify the degree of carrier polarization generated, for instance, by spin injection. Second, it has been already demonstrated that the large spin-splitting in Cd1−xMnxTe makes it possible to test quantitatively scaling theory of the quantum Hall effect (QHE) over a wide range of the filling factors ν [7]. Importantly, such a system offers also a novel environment to examine the transition between electron liquid and quantum Hall states. Scaling theory of localization predicts that only localized states exist at zero magnetic field in two dimensional (2D) systems. Thus, according to Khmielnitskii [9] and Laughlin [10], extended states associated with Landau levels should float up and cross the Fermi energy when B → 0. Hence, the resulting global phase diagram of the QHE [11] allows only for a direct transition between the insulating phase and the QHE liquid state with the filing factor ν = 1. If the spin degeneracy is not removed, the diagram predicts the transition involving the ν = 2 QHE state. However, a number of recent experiments call into question this picture. In particular, a zero-field metallic state is observed [12], the floating scenario is questioned [13], and transition from the insulating phase into QHE states with ν ≥ 3