  We have characterized a film of Ge_0.9Mn_0.1 forming self-organized
nanocolumns perpendicular to the Ge substrate with high resolution scanning
transmission electron microscopy combined with electron energy loss
spectroscopy, and bulk magnetization and positive muon spin rotation and
relaxation (muSR) measurements. The Mn-rich nanocolumns form a triangular
lattice with no detectable Mn atoms in the matrix. They consist of cores
surrounded by shells. The combined analysis of bulk magnetization and muSR data
enables us to characterize the electronic and magnetic properties of both the
cores and shells. The discovered phase separation of the columns between a core
and a shell is probably relevant for other transition-metal doped
semiconductors.
