Synthesis, micro-structural and magnetic properties of Mn-doped ZnO nanowires

Single-crystalline Mn-doped ZnO nanowires of different compositions were synthesized by chemical vapor deposition. Their micro-structures were characterized by high-resolution transmission electron microscopy and X-ray diffraction. The results reveal that Mn doping leads to production of planar and point defects, lattice expansion and disorder. These micro-structural features are also reflected by characteristic changes in the Raman spectra of the nanowires, such as the red-shift and asymmetric broadening of the E-2H mode, the appearance of an additional mode, and the anomalous enhancement of the E-1(LO) mode. All the doped nanowires show ferromagnetism with a Curie temperature above 350 K. The small effective magnetic moment of similar to 0.02 mu(B)/Mn suggests the presence of isolated Mn ions and/or antiferromagnetic superexchange interactions. The long-range ferromagnetic interaction is understood in terms of two possible mechanisms, including the percolation of bound magnetic polarons and surface-hole-mediated ferromagnetic coupling.