Regensburg 2007 – wissenschaftliches Programm
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MA: Fachverband Magnetismus
MA 10: Spinelectronics/Spininjection in Heterostructures
MA 10.3: Vortrag
Montag, 26. März 2007, 15:45–16:00, H23
Exchange Interactions in Dilute Magnetic Semiconductors: Local Environment Effects — •Peter Dederichs1, Kazunori Sato2, and Hiroshi Katayama-Yoshida2 — 1IFF, Forschungszentrum Jülich, 52425 Jülich — 2ISIR, Osaka University, Ibaraki, Osaka 567-0047, Japan
The ferromagnetism in dilute magnetic semiconductors like (Ga, Mn) As is due to the exchange interactions Jij between neighboring Mn impurities. The disorder in these systems is usually described by the powerful coherent potential approximation (CPA). Thus, the exchange interactions Jij are also usually calculated by applying Lichtenstein's formula and using the CPA Green function Gij between the sites i and j. Here we investigate the validity of this approach, by embedding a whole cluster of impurities around the atoms i and j into the CPA medium, and by explicitly calculating the resulting exchange interactions as a function of the disordered environment. We find large fluctuations of the Jij values depending on the positions of neighboring third and fourth impurities. Upon ensemble averaging significant deviations from the standard Lichtenstein-CPA formula are obtained, in particular for small distances i-j and small concentrations. However good agreement with the ensemble average is obtained, if we replace the Green functions by the one for the impurity pair (i, j) properly embedded in the CPA medium. We discuss the reason for the failor of the standard approach in the dilute limit.