Co-rich cobalt platinum nanowire arrays: Effects of annealing

The effects of annealing on the crystal structure and magnetic properties of Co-rich cobalt platinum nanowire arrays embedded in anodic aluminium oxide membranes have been investigated. For this purpose, a rapid thermal annealing to temperatures of 300 degreesC to 800 degreesC has been used. Transmission electron microscopy and scanning electron microscopy show that the nanowires have a mean diameter of 14 nm and an estimated wire density of 7.8x10(10) cm(-2). From x-ray diffraction patterns, we find that the nanowires are hcp and possess a preferred texture in which the c axis of the grains tends to lie along the major axis of the wire. Vibrating sample magnetometry measurements indicate that the easy axis is along the nanowire axis direction. Hysteresis loops, saturation magnetization, squareness ratio (M-r/M-s), and coercivity (perpendicular and parallel to the nanowire axis) have all been investigated as a function of the annealing temperature (T-A). Coercivity parallel to the wire axis first increases with T-A, attains a maximum at 600 degreesC (which is 150% of the as-deposited sample), and then decreases. By contrast there is relatively little change in the coercivity measured perpendicular to the wires. The saturation magnetization for the as-deposited sample is 1360 emu/cc and remains almost constant for annealing temperatures up to 500 degreesC: for T-A>500 degreesC it decreases significantly. The maximum (M-r/M-s) ratio attained in this study is 0.99, the highest value reported thus far for cobalt platinum alloy nanowires. The data suggest that these materials are potential candidates for high-density magnetic recording media. (C) 2005 American Institute of Physics.