Unidirectional Magnetic Anisotropy in Dense Vertically-Standing Arrays of Passivated Nickel Nanotubes

Abstract

We report the facile and low-cost preparation as well as detailed characterization of dense arrays of passivated ferromagnetic nickel (Ni) nanotubes (NTs) vertically-supported onto solid Au-coated Si substrates. The proposed fabrication method relies on electrochemical synthesis within the nanopores of a supported anodic aluminum oxide (AAO) template and allows for fine tuning of the NTs ferromagnetic walls just by changing the cathodic reduction potential during the nanostructures’ electrochemical growth. Subsequently, the experimental platform allowed further passivation of the Ni NTs with the formation of ultra-thin antiferromagnetic layers of nickel oxide (NiO). Using adequately adapted magnetic measurements, we afterwards demonstrated that the thickness of the NT walls and of the thin antiferromagneticNiO layer, strongly influences the magnetic behavior of the dense array of exchange-coupled Ni/NiO NTs. The specific magnetic properties of these hybrid ferromagnetic/antiferromagnetic nanosystems were then correlated with the morpho-structural and geometrical parameters of the NTs, as well as ultimately strengthened by additionally-implemented micromagnetic simulations. The effect of the unidirectional anisotropy strongly amplified by the cylindrical geometry of the ferromagnetic/antiferromagnetic interfaces has been investigated with the magnetic field applied both parallel and perpendicular to the NTs axis.

Effect of the Cadmium Telluride Deposition Method on the Covering Degree of Electrodes Based on Copper Nanowire Arrays

“…where φ is the average NW diameter and D is the mean distance between the centers of two adjacent NWs, with both geometrical parameters directly evaluated through SEM analysis. Notably, the estimated porosity value was slightly lower compared with other studies [23][24][25]27] due to the specifically selected parameters engaged in the anodization process, ultimately yielding Cu NWs of slightly reduced density. This further facilitates adequate penetration of the CdTe through the NW array.…”

“…The current curve obtained for entirely anodizing the Al layer (1 µm thick) is presented in Figure 2a. As explained elsewhere [23,24,27], the measured current density drops at the beginning of the process due to the formation of a compact barrier-type Al 2 O 3 on the initial Al surface, leading to a plateau when a competitive equilibrium between field-enhanced Al 2 O 3 dissolution and Al 2 O 3 formation is established before sharply increasing at the end of the process as the nanopores progressively approach the Au electrode. Subsequently, an optimized electrochemical deposition protocol was employed to grow Cu within the AAO nanopores, by using a three-electrode potentiostatic setup using a Voltalab PGZ-100 Potentiostat (Radiometer Analytical Inc., Hegnau, Switzerland).…”

“…In the first stage, the substrate was prepared by in-situ direct-current (DC) sputtering of a multi-layer sequence consisting of Ti/Au/Ti/Al (5/50/5/1000 nm) onto a freshly cleaned p-doped <100> Si substrate (525 µm thickness and 5 mΩ•cm resistivity) (SIEGERT WAFER GmbH, Aachen, Germany) using commercially-available targets (Kurt J. Lesker Limited, Saint Leonards-on-sea, United Kingdom) and a Plassys MP500S magnetron as sputtering equipment (PLASSYS Bestek, Marolles-en-Hurepoix, France) (Figure 1a). The processing plasma parameters have been detailed elsewhere [27]. In this configuration, the thin Au film is the actual electrode, the aluminum (Al) layer is introduced to develop the supported AAO template through anodization, and the ultra-thin titanium (Ti) film is used to enhance the adhesion between successive metallic layers.…”

Multifunctional Magnetic Nanocomposites: Innovative Processing and Applications

“…The observed substantial exchange bias effects of the nanocomposite mixtures were interpreted in terms of the unidirectional anisotropy induced by pinning the long-range interacting net magnetic moments of defect hematite nanoparticles (i.e., without the Morin transition), as mediated by the diluted ferromagnetism of MoO 2 , to those less defect hematite nanoparticles (i.e., supporting the Morin transition). Unidirectional magnetic anisotropy effects were also reported by Claudiu Locovei et al in dense, vertically standing arrays of passivated Ni nanotubes (Ni NTs) [ 13 ]. Ni NTs with different thicknesses of the walls were obtained by electrochemical synthesis.…”