The preparation and magnetic properties of ordered Co/Al₂O ₃ nanowire arrays

Highly ordered membrane of porous anodic aluminum oxide (AAO) was prepared by electrochemical method. By ac electrodepositing using a sulphate bath, Co nanowires in the AAO membranes were reduced and grown and the Co/Al ₂O₃ nanowire arrays were prepared. The aluminum substrate and the surplux Co in the surface of arrays were detached by the pulse electrochemical detaching process. This can improve the perpendicular magnetic anisotropy of the samples. Microstructures of the membranes and the nanowire arrays were investigated by using of Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-ray diffraction Analysis (XRD). Magnetic properties of Co nanowire arrays were measured by using of Vibration Sample Magnetometer (VSM). The effect of some factors, i.e., diameter of pores, interpore distance, length-diameter ratio, and pH values and temperature of the electrolyte, on nanostructure and magnetic properties of the nanowire arrays were also investigated. The results shows that the coercivity (Hc) and remnant magnetization ratio(Mr/Ms) reduce with the increasing of the diameter of the holes on the condition of the constant interpore distance and the length-diameter ratio. Co nanowires prefer to grow along the direction of (100) and (002). Applying the external magnetic field parallel to the direction of nanowire, the hysteresis loops show good perpendicular magnetic anisotropy with high remnant magnetization ratio (0.6-0.85) and coercivity (80 000·-m^-1 ∼150 400A·m^-1), which are greater than that of the plane anisotropic magnetization of continuous films.