  Density-functional theory based global geometry optimization is employed to
systematically scrutinize the possibility of multi-doping of hydrogenated Si
clusters in order to achieve high spin states beyond the septet limit of a
single-atom dopant. While our unbiased configurational search reveals that the
previously suggested Si18H12 double hexagonal prism structure is generally too
small to accommodate two dopants in magnetized state, the larger Si24H24 cage
turns out to be suitable for such applications. For dimer dopants M2+ = Cr2+,
Mn2+ and CrMn+, the structural integrity of the host cage is conserved in the
ground-state structure of corresponding M2+@Si24H24 aggregates, as is the
unusually high spin state of the guest dopant, which in case of Cr2+ already
exceeds the single-atom dopant septet limit by almost a factor of two.
Moreover, the possibility of further increasing the cluster spin moment by
encapsulating an even larger number of dopants into a suitably sized
hydrogenated Si cage is illustrated for the example of a (CrMn+)2@Si28H28
aggregate with a total number of 18 unpaired electrons. These results strongly
suggest multi-doping of Si clusters as a viable route to novel cluster-based
materials for magneto-optic applications.
