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Multilayered Core/Shell Nanowires Displaying Two Distinct Magnetic Switching Events

Adv. Mater. 2010, 22, 2435–2439 2010 WILEY-VCH Verlag G The size-dependent properties of pseudo-one-dimensional nano-objects have been abundantly documented for single-phase rods or wires. Elongated nanostructures that coaxially combine several phases of distinct physical properties could generate additional effects—spintronic, multiferroic, magnetoplasmonic, to name a few. Such core/shell wires have already been prepared and investigated: they combine two materials of a common class (epitaxial semiconductors to enhance confinement), they utilize a purely structural core as substrate for a functional tube, or they derive from a post-synthetic chemical reaction at a wire surface. To date, however, multiphase one-dimensional nanostructures incorporating several functionalities in a single object are rare. Indeed, a general preparative method is still missing to generate core/shell wires with the following four advantageous characteristics: (i) ability to combine two chemically and physically very different materials; (ii) possibility to introduce an inert layer (insulator, diffusion barrier, spacer); (iii) tunability of each geometric parameter (core radius, shell thickness, and separation between them); (iv) scalability. We propose the combination of atomic layer deposition (ALD) and electrodeposition in an ordered nanoporous template as one such preparative strategy. The template defines the order of the material and the diameter of the wires, ALD is used to conformally deposit one or several shells (including the inert layers), whereas electrodeposition furnishes the core. In this manner, the materials of core and shell can be chosen independently of each other and the thickness of every individual layer is accurately tunable. We demonstrate this method by synthesizing ordered nanostructures embedded in an alumina matrix and consisting of a nickel core and an iron oxide shell separated by a silica spacer layer. This combination of two coaxial nanomagnets is of interest for increasing data storage densities: it could either shield each object and decouple it from its neighbors, or store more than one bit of information per object. However, the occurrence of several distinct switching events in coaxially arranged magnetic phases has not been evidenced experimentally to date (except in larger microwires of diameter >10mm). This situation contrasts with the variety and practical importance of the effects described in multilayered magnetic films (in particular GMR and TMR). We start with porous alumina membranes prepared electrochemically as templates featuring hexagonally arranged pores of diameter (150 15) nm and length 20mm. The subsequent preparative steps are displayed in Figure 1. First, a thin acid-resistant SiO2 layer is coated onto the inner pore walls (Fig. 1a). After dissolution of the aluminum substrate, a long H3PO4 etch is performed to ensure complete dissolution of the barrier layer (Fig. 1b), without risk of pore widening. Then reactive ion etching, RIE (Fig. 1c), removes the exposed SiO2 tips and achieves a clean opening of the pores for the electrodeposition step. In the ALD processes that follow for Fe2O3 and SiO2 (Fig. 1d), near-perfect conformality of the tubes is ensured by long purge times, which prevent any undesired CVD side-reaction. The Ni cores are electrodeposited inside the multilayer nanotube array after definition of a gold electrode on one side of the sample (Fig. 1e). Here, the inner silica tube also serves as electrical insulator. Finally, gold is removed and Fe2O3 is converted to Fe3O4 by dihydrogen (Fig. 1f). [22] The thicknesses of the Fe3O4 shell and the silica spacer can be varied at will: this in turn defines the diameter of the Ni core. The quality and versatility of this preparative strategy are revealed from the microscopic investigations of the samples. Scanning electron microscopy (SEM) images taken in top view (Fig. 2a–d) evidence the hexagonal order of the tubes and the homogeneity of their diameters after various steps of elaboration. A clean opening is obtained after RIE (Fig. 2a) and remains after the ALD steps (Fig. 2b). Thus, electrodeposition results in complete filling of the pores (Fig. 2c,d). The uniform SEM contrast observed from the side (Fig. 2d) proves the homogeneous Ni deposition inside the nanotubes of the whole sample and along their whole length. Isolated tubes and core/shell