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Co nanowire arrays have been fabricated into anodic aluminum oxide templates at 20C by dc electrodeposition.It is shown that Co nanowires fabricated at lower and higher growth voltages have a hexagonal close packing (hcp) structure with preferred [100] orientation along the nanowire axis and a face-centered cubic (fcc) structure with preferred [220] orientation,respectively.With increasing growth voltage,the room-temperature coercivity along the nanowire axis is enhanced gradually.For fcc Co nanowires,the coercivity increases monotonically with increasing temperature while for hcp Co nanowires,a minimal coercivity is obtained along both parallel-to-axis and perpendicular-to-axis orientations with the temperature rising from 50K to 390K.The abnormal temperature dependence of the coercivity can be attributed to the competition between the shape anisotropy and magnetocrystalline anisotropy as a function of temperature.