Magnetic and electric characterization of different Ni systems comprising cylindrical nanowires

Abstract

Cylindrical Ni nanowires (NWs) were synthesized by templateassisted electrodeposition using porous alumina membranes made through a two-step anodization process. Different morphologies of the NWs were obtained by varying the templates and electrodeposition parameters. In this chapter we review the results on the magnetic and electric properties of different systems made up of the so-obtained Ni NWs. The magnetic anisotropy as a function of temperature was studied in a set of three NWs arrays with different diameters and surface topographies. We found that coercivity HC increases with temperature in arrays of NWs with rough surfaces and small diameters (≤ 65 nm). For thicker wires (200 nm in diameter) and relatively smooth surfaces, HC decreases as temperature rises. The experimentally determined coercive fields were compared with results of micromagnetic calculations and the value of the effective magnetic anisotropy was estimated from the hysteresis loops measured at different temperatures, in good agreement. We show that the observed temperature dependence of HC does not arise from an intrinsic property of pure Ni alone, but also from the NWs arrays microsctructure. An inhomogeneous Ni nanostructure (nano-brush) composed of a NWs array and a top layer attached to it was synthesized, having each component the same Ni mass. The magnetic properties were investigated, comparing the results of the whole structure with those obtained for the separate parts, a single layer and a NWs array alone. We show that it is possible to control the nano-brush magnetic behavior through the coupling of the layer and the NWs array. The nano-brush magnetic properties are dominated by the layer, except when the applied field is parallel to the NWs axis, in which case the wires shape anisotropy plays a determinant role. Magnetoelectric measurements were performed on a single NW with diameter of 110 nm, released from the alumina template. For different applied magnetic fields, a non-linear behavior of current vs. voltage was measured, suggesting the presence of conductor/semiconductor interfaces. There is a threshold value between 100 mT and 500 mT for the applied field, above which the system’s breakdown voltage is reduced. A simplified model of two Schottky barriers is proposed in order to account for the results, which agrees with the formation of a thin NiO layer on the NW surface.