Suppresion of ferromagnetism in Zn$_x$Cr$_{1-x}$Te: A DFT study

The possibility to control the charge and spin degrees of freedom independenlty in diluted magnetic semiconductos (DMS) provides an opportunity in the realization of spintronic devices. Therefore, there is a great desire to understand the physical parameters that control $T_{\rm C}$ in DMS. In this talk, we will report a density functional theory study of Zn$_x$Cr$_{1-x}$Te and Ga$_x$Cr$_{1-x}$As, which show interesting carrier dependence of the $T_{\rm C}$. We found that the stability of ferromagnetism (FM), which is calculated with respect the anti-ferromagnetic (AFM) phase, can be controlled by $p$- and $n$-type co-doping. For randomly distributed Cr, the maximum FM stability is found to be at $\approx1.25$ holes per magnetic ion. Furthermore, we studied the formation of magnetic Cr precipitates by analysing the binding energy of two Cr atoms in the host semiconductors. Our results indicate that a simple Coulomb binding picture cannot provide a consistent explanation for the formation of Cr-rich aggregates. Instead, we show that the variation of Cr-Cr binding energy can be discribed quite well using the band coupling model [G. M. Dalpian, S.-H. Wei, X. G. Gong, A. J. R. da Silva, and A. Fazzio, Solid State Commun. {\bf 138}, 353 (2006)].