Electroplating and magnetostructural characterization of multisegmented Co54Ni46/Co85Ni15 nanowires from single electrochemical bath in anodic alumina templates

Abstract

Highly hexagonally ordered hard anodic aluminum oxide membranes, which have been modified by a thin cover layer of SiO2 deposited by atomic layer deposition method, were used as templates for the synthesis of electrodeposited magnetic Co-Ni nanowire arrays having diameters of around 180 to 200 nm and made of tens of segments with alternating compositions of Co54Ni46 and Co85Ni15. Each Co-Ni single segment has a mean length of around 290 nm for the Co54Ni46 alloy, whereas the length of the Co85Ni15 segments was around 430 nm. The composition and crystalline structure of each Co-Ni nanowire segment were determined by transmission electron microscopy and selected area electron diffraction techniques. The employed single-bath electrochemical nanowire growth method allows for tuning both the composition and crystalline structure of each individual Co-Ni segment. The room temperature magnetic behavior of the multisegmented Co-Ni nanowire arrays is also studied and correlated with their structural and morphological properties.

Figure 1

SEM bottom view of a typical H-AAO membrane. Employed for the electrochemical synthesis of the multisegmented Co-Ni nanowire arrays displaying the hexagonally ordered pore distribution with 180 ± 20 nm in size and pore interspacing of around 305 nm.

Figure 2

Pulsed electrodeposition potential sequence employed for the synthesis of multisegmented Co-Ni nanowires in H-AAO templates.

Figure 3

Co content (left) and Co-Ni nanowire growth rate (right) dependence on the deposition potential, V_ED.

Figure 4

STEM-HAADF images, variation of Co and Ni contents, and EDS analysis. (a, c) STEM-HAADF images of multisegmented Co-Ni nanowires. (b) Variation of cobalt (red) and nickel (blue) contents along the orange line highlighted in (a) determined via elemental analysis by EDS line scan. (d) EDS analysis measured in the two points marked in the HAADF-STEM image of (c). The presence of Si and O and the absence of Co and Ni can be seen in the EDS spectrum of point 1.

Figure 5

TEM images and SAED patterns. The left image shows TEM images of multisegmented Co-Ni nanowires. The right image shows SAED patterns of the different nanowire segments marked in the left image of the figure. SAED pattern with number (1) can be indexed to the [0001] zone axis of a Co-Ni alloy with a hcp structure. SAED pattern number (2) can be indexed to the [−321] zone axis of a Co-Ni alloy with a fcc structure.

Figure 6

Room temperature hysteresis loops of multisegmented Co₅₄Ni₄₆/Co₈₅Ni₁₅ nanowires. Measured in the parallel and perpendicular directions with respect to the nanowire long axis. The inset shows an enlargement in the low-field region.