Berlin 2005 – wissenschaftliches Programm

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MA: Magnetismus

MA 33: Messmethoden, Anisotropie, Spinelektronik/Spininjektion

MA 33.5: Vortrag

Mittwoch, 9. März 2005, 12:00–12:15, TU EMH225

Percolation limits the Curie temperature of Dilute Magnetic Semiconductors — •Peter H. Dederichs¹, Kazunori Sato^1,2, Brahim Belhadji¹, and Hiroshi Katayama-Yoshida² — ¹Institut für Festkörperforschung, Forschungszentrum Jülich, D-52425 Jülich — ²ISIR, Physics Department, University of Osaka, Japan

We present detailed ab-initio calculations for the electronic and magnetic properties of dilute magnetic semiconductors (DMS), using the KKR-CPA method. The exchange coupling constants J_ij between two magnetic impurities are calculated by embedding the impurities in the CPA medium and using the Lichtenstein formula. Two types of DMS can be distinguished: (i) DMS with the d states deep in the valence band, like e.g. (Ga,Mn)As and (Ga,Mn)Sb. Here the exchange interaction is dominated by p-d (“kinetic”) exchange and is long ranged. (ii) DMS with impurity d-bands in the gap, like e.g. (Ga,Mn)N and (Zn,Cr)Te. In this case the exchange interaction is dominated by double exchange and is short ranged. Using Monte Carlo methods and Binder’s cumulant expression we calculate the Curie temperatures for a series of magnetic semiconductors, with the J_ij calculated in LDA and LDA+U. We find that for short ranged interactions, in particular for (Ga,Mn)N, the Curie temperature is strongly reduced from the very high values obtained in the mean field approximation. The physical reason is that, for small concentrations of Mn, percolation cannot be achieved by the strong nearest neighbour interaction. Even for (Ga,Mn)As, where the interaction is longer ranged, the mean field values are reduced by a factor of two. Thus percolation is a serious obstacle to high Curie temperatures in DMS.