Non-stoichiometric Mn doped ZnO clusters: First principles calculations*

It has been reported that cage like hollow clusters of (ZnO)$_n$ with $n$ = 12 \& 34 are stable and hence {\it magic}. Doping Mn impurity in ZnO (ZnO:Mn) clusters is a well studied problem. In most of the studies, single Mn doping has been achieved by substituting it on a surface Zn site, leading to a stoichiometric configuration of Zn$_{n-1}$MnO$_n$ and a large magnetic moment of 5 $\mu_B$. However, we show that using first principles methods, Mn doping would lead to O rich, non-stoichiometric clusters with significantly reduced magnetic moment. Specifically, we show that clusters of configuration Zn$_{12}$MnO$_{15}$ and Zn$_{34}$MnO$_{37}$, obtained when Mn is substituted in (ZnO)$_n$ ($n$ = 13 \& 35) cages, become magic. The magnetic moments in these clusters is reduced to 1 $\mu_B$. These clusters can also be considered as a composite structure where a MnO$_x$ ($x$ = 1 to 4) molecule is attached to ZnO$_n$ ($n$ = 12 \& 34) cages from outside.\footnote{S. P. Nanavati et al. Phys. Rev. B (in press, 2012).} We believe that these results would have important implications for the understanding of magnetism in ZnO:Mn nanostructures as well as thin films, for which recent experiments suggest mixed and higher oxidation states of Mn, {\it viz.,} Mn$^{+3}$ and Mn$^{+4}$.