Abnormal Temperature Dependence of Coercivity in Cobalt Nanowires

Co nanowire arrays have been fabricated into anodic aluminum oxide templates at 20°C 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 50 K to 390 K. 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.