Ferromagnetic Ge1-xMx (M = Mn, Fe, and Co) nanowires

We synthesized Ge and Ge1-xMx (M = Mn, Co, and Fe, x <= 0.4) nanowires using the thermal vapor transport method. All of the nanowires consisted of single-crystalline Ge nanocrystals grown uniformly with the [ 111] direction. High-resolution X-ray diffraction showed no metal cluster formation for any of the Ge1-xMx nanowires, and the reduction of the lattice constant for the Mn-substituted nanowires. X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements revealed that the Mn2+ ions preferentially occupy the tetrahedral sites, substituting for Ge. The magnetic moment of the Mn ions reaches a maximum for x = 0.1, which is much larger than that of the Fe or Co ions. The magnetic hysteresis confirms the room-temperature ferromagnetism of the Ge1-xMnx nanowires, which is also maximized at x = 0.1. We suggest that the Mn ions produce dopant-acceptor hybridization with host defects in the p-type Ge nanowires. The Mn substitution is more effective to form significant ferromagnetic Ge nanowires, compared to the Fe or Co substitution.