Ferromagnetic Ge(Mn) nanostructures

We present structural, magnetic and transport properties of Mn-doped Ge layers grown by molecular beam epitaxy (MBE) at low substrate temperatures T_S. Atomic force microscopy (AFM), reflection high energy electron diffraction (RHEED) and transmission electron microscopy (TEM) analysis of structures grown at T_S = 70 ^{{ring operator}} C and 120 ^{{ring operator}} C reveal defect-free epitaxy of Ge(Mn) on Ge(0 0 1) substrates. Despite the low T_S we observe the formation of round shaped clusters with a diameter of 15-20 nm which are incoherent with the Ge matrix in TEM analysis for a Mn concentration x of 3.4%. SQUID measurements reveal ferromagnetism and a T_C of around 300 ^{{ring operator}} C for the layers, reminiscent of the intermetallic compound Mn₅ Ge₃. Transport measurements, however, indicate that Mn is incorporated into the Ge matrix between the Mn₅ Ge₃ clusters as well.