# Magnetic Nanowires
## 3 Results
### Tunable Magnetic Anisotropy of Magnetic Nanowire Arrays
From our previous studies, the electrolytic bath acidity, or pH, was demonstrated to be a key parameter in determining the structure of electrodeposited cobalt nanowires. Indeed, it was found that face-centered cubic (fcc) Co is favored at low pH (typically less than 2.5) while hexagonal close-packed (hcp) Co is formed at pH larger than \(\sim\)3.5 [24; 29]. Moreover, it was shown that an abrupt transition in crystallographic orientation takes place for pH in the range 4.0-6.5, with a preferred in plane orientation at low pH and a reorientation of the c-axis normal to the film at large pH. These structural changes have been attributed to the influence of evolution of hydrogen at the cathode, which are known to depend on the pH [75; 76]. Another relevant parameter of the operative conditions in cobalt electrocrystallization is the current density. Figure 6: (**a**) microwave absorption spectra as a function of the applied field parallel to the wires of an array of Ni nanowires of diameter 100 nm and packing density of 27%. Continuous lines correspond to measurements made with the applied field value indicated by the numbers; (**b–d**) dispersion relations and the corresponding hysteresis loops measured in arrays of Ni nanowires with the field applied parallel (closed symbols) and perpendicular (open symbols) to the wires. The wire diameter and packing density are (**b**) 180 nm, 4%, (**c**) 100 nm, 27%, and (**d**) 115 nm, \(>\)35% (from [19]). Figure 7a shows the variation of the effective field for a 30 nm Co nanowires array as a function of the pH in the range 2.0-6.6 for two different plating current densities, around 50 mA\(\cdot\)cm\({}^{-2}\) and 5 mA\(\cdot\)cm\({}^{-2}\), respectively. These data were extracted from FMR experiments [24; 29]. Let's first consider the results obtained at 50 mA\(\cdot\)cm\({}^{-2}\). For reference, the effective field \(H_{\text{EF}}=2\pi M_{\text{s}}(1-3P)\approx 8\) kOe expected when there is no contribution from the MC anisotropy is indicated by the dotted line in Figure 7a (\(P\approx 3\%\) is for the array samples in this study). This value agrees with that of the low-pH samples, for which only the shape anisotropy and dipolar coupling should be considered due to the lack of texture in the electrodeposited Co microstructure and possible presence of fcc phase. In contrast, for pH values of 3.8-4.0, it was found that the microstructure of the deposited Co nanowires contains a dominant fraction of large hcp grains with the c-axis oriented perpendicular to the wire. A similar result was observed for all the diameters considered in the range 30-120 nm [24; 29]. For such samples, the effective anisotropy field is weaker than for the low-pH samples due to the competition between shape anisotropy and MC anisotropy (which has an upper bound of \(H_{\text{MC}}=6.4\) kOe for bulk hcp Co). Moreover, a re-orientation of the hcp c-axis along the wire axis is also observed for pH values \(\geq 6.0\)[24], resulting in an additive MC contribution to the total anisotropy field. For both low and high plating currents, the overall variation of the effective field with bath acidity is similar, see Figure 7a. However, the transition between the low and high effective fields takes place around different pH values depending on the plating current density. Therefore, for a limited pH range, it is possible to selectively obtain both structural phases (c-axis parallel and perpendicular to the wires) from the same electrolyte simply by changing the plating current (or deposition rate). These results are corroborated by TEM investigations as shown in Figure 7b,c [29]. In the case of low current density, the TEM bright field images and corresponding diffraction patterns clearly show an orientation of the c-axis parallel to the wire axis, while, in the case of high current density, the images clearly show a perpendicular orientation of the c-axis. CoPt alloy nanowires constitute another system whose magnetic anisotropy can be changed drastically by appropriate annealing treatment after electrodeposition.