Ferromagnetism of self-organized Ge1-xMnx nano-pillars

In the search for high-T-C Si or Ge based ferromagnetic semiconductors, we present here the magnetic and transport properties of Ge1-xMnx nano-pillars. These pillars self-organize during the MBE growth of thin Mn doped (6 %) Ge films as a consequence of the in-plane and out-of-plane diffusion of Mn atoms. Their composition is close to Ge2Mn and their average diameter and spacing are 3 nm and 10 nm respectively. Magnetic measurements evidence a ferromagnetic phase up to 400 K giving rise to anomalous Hall effect at room temperature. Moreover we evidence a large positive magnetoresistance (up to 7000 % at 30 K and 9 T) probably due to the conductivity mismatch between the Mn-rich nano-pillars and the Mn-poor surrounding Ge matrix.