  We present a detailed study of the magnetic properties of unique cluster
assembled solids namely Mn doped Ge46 and Ba8Ge46 clathrates using density
functional theory. We find that ferromagnetic (FM) ground states may be
realized in both the compounds when doped with Mn. In Mn2Ge44, ferromagnetism
is driven by hybridization induced negative exchange splitting, a generic
mechanism operating in many diluted magnetic semiconductors. However, for
Mn-doped Ba8Ge46 clathrates incorporation of conduction electrons via Ba
encapsulation results in RKKY-like magnetic interactions between the Mn ions.
We show that our results are consistent with the major experimental
observations for this system.
