Size effects and origin of easy-axis in nickel nanowire arrays

High quality compact Ni nanowire (NW) arrays with aspect ratios (wire length/diameter) varying between 70-171 for a wire length of similar to 6 mu m, and between 3-400 for a constant wire diameter of 60 nm were successfully grown by direct current electrodeposition into free standing porous alumina templates having a lattice constant, i.e., interpore distance, of 105 nm. The NWs have been investigated using a combination of scanning-and transmission-electron microscopies, selected-area electron diffraction, x-ray diffraction analysis, ferromagnetic resonance (FMR), and vibration sample magnetometer techniques at room temperature. Microscopic and diffraction results show that the wires are uniform and mostly single-crystalline, being 220-oriented along the growth direction. Magnetic properties of samples are heavily dependent on the wire length as well as the diameter or packing factor, P (the volume fraction of wires/template). The FMR spectra and the field orientation dependence of the resonance field values were fitted using the imaginary part of magnetic susceptibility and a dispersion relation of magnetization, including the Bloch-Bloembergen type damping term. Combined with hysteresis (M-H) results, these indicate that the preferred (easy) axis of magnetization is parallel to the NW-axis for P < 33%. On the other hand, strong interwire exchange interactions are determined to supply a magnetization state with preferred axis perpendicular to the wires for samples with P > 33%. Comparison with relatively thick (110 nm diameter) and loosely packed (P similar to 10%) Ni NW arrays suggest that the magnetization reversal mechanism could be that of curling type. (C) 2011 American Institute of Physics. [doi:10.1063/1.3531565]