# Magnetic Nanowires
## 2 Materials and Methods  
Polymer membranes are made using the nuclear track-etch technology in which a film (typically 5-50 \(\upmu\)m thick) of polymer (polycarbonate, PET, polyimide,...) is bombarded by heavy ions of high energy [68]. The bombarded film is then irradiated with UV light to increase the track etching selectivity and finally etched in an adequate solution where tracks are revealed, leading to the formation of pores with a diameter defined by the etching time. Pore size and pore density can be controlled over wide ranges, between 15 nm and a few microns and in the range 10\({}^{5}\)-10\({}^{10}\) cm\({}^{-2}\), respectively. For such randomly distributed pores array, this implies that the mean separation distance between nanomaterials can be tuned between \(\sim\)100 nm up to several tens of microns, with a maximum porosity close to 30%. Porous alumina films formed by anodic oxidation of aluminum in an acidic solution have also been intensively used to fabricate nanostructured materials [69]. These templates contain cylindrical pores of uniform diameter oriented perpendicular to the surface and approximately arranged in a hexagonal array. The interpore distance and pore length increase in proportion to the applied anodization voltage and time, respectively. The pore diameter can be controllably increased by a pore widening treatment. Porous alumina with pore density as large as 10\({}^{12}\) cm\({}^{-2}\) and pore diameters ranging from approximately 5 nm up to \(>\)300 nm can be produced. Typical porosities are in the range 10-65%. It has also been shown that a two-step anodization process can lead to highly ordered two-dimensional pores arrays with a hexagonal pattern over large areas [70]. Figure 2a,b show the Scanning Electron Microscopy (SEM) images of the surface of the two types of porous templates. Electrochemical deposition of a material within the pores of polymer and alumina membranes is done by coating one face of the membrane with a sufficiently thick metal film to cover the pores and serve as a cathode for electrodeposition [5,10]. The nanoporous membrane is then placed in a home-made Teflon-cell and exposed to the electrolyte (see Figure 2c,d). A standard three-electrode setup is employed to reduce ions in solution at the cathode and make the nanowires. When necessary, the pH values of the as-prepared electrolytes were adjusted to the desired pH by addition of a diluted HCl solution or by addition of NaOH. Different types of parallel NW arrays made from pure  
Figure 1: Illustration of the versatility of the template-assisted synthesis of nanowires. metals, alloys of controlled composition and multilayered nanowires were successfully grown by electrodeposition at room temperature (RT) [5; 10; 23]. Typical SEM images of metal nanowires obtained via the simple potentiostatic mode after dissolution of the host nanoporous media are shown in Figure 3a,b. Note that, after the template is filled with magnetic wires, the term packing factor is used instead of the membrane porosity, since interest is focused on the properties of nanowire networks. The technique mostly used for the fabrication of multilayered nanowires (see Figure 3c) consists in a pulse-plating method in which two metals are deposited from a single solution (in which the nobler element is kept in dilute concentration) by switching between the deposition potentials of the two constituents. Following a procedure described elsewhere [5; 10], the deposition rates of  
Figure 3: (**a,b**) SEM images of metal nanowires; (**c**) schematic of Co/Cu nanowires embedded in a polymer template; (**d,e**) TEM image of Ni80 Fe20/Cu and Co/Cu multilayered nanowires; (**f**) SEM image of Ni nanotubes. Figure 2: SEM images of (**a**) track-etch polycarbonate membrane and (**b**) porous alumina; (**c**) photograph and (**d**) schematic of the electrodeposition cell used to grow nanowire arrays. each metals were determined from the pore filling time.