Magnetism at the Nanoscale: Nanoparticles, Nanowires, Nanotubes and their Ordered Arrays

Magnetic nanomaterials are attracting considerable interest due to their unique properties and potential applications, ranging from magnetic storage and recording devices, sensors and biomedical chips, to contrast agents in ressonance imaging and drug carriers. There are various challenges associated with the fabrication of highly ordered large area magnetic nanostructures and the accurate understanding of their magnetic behavior. The use of a template assisted method has arouse great interest and shown many potentialities, as it allows the low-cost and high-yield growth of highly ordered nanomagnet arrays. Additionally, the combination of different filling techniques, such as sol-gel and electrodeposition methods, increases the diversity of nanostructures that can be produced [homogeneous, concentric or multisegmented heterostructured nanowires (NWs) and nanotubes (NTs)], opening the possibility to tune a larger variety of magnetic responses. In this thesis we describe the synthesis and characterization of nanoparticles (NPs), NWs, NTs and their hexagonally ordered arrays, by sol-gel and potentiostatic electrodeposition techniques, using nanoporous alumina templates (NpATs). First, we study in detail the optimized conditions for the potentiostatic electrodeposition of Ni NWs inside NpATs with pore diameters of 50 nm, by tuning the applied potential. In this way we are able to enhance the deposition rate to ∼ 10μm min−1, which is ten times larger than usually reported values. We also present a systematic comparative study on the fabrication of Ni NW and NT arrays with small diameters (∼ 50 nm), by monitoring the deposition current density transients during potentiostatic electrodeposition in NpATs. A new fabrication step is introduced in the membranes’ preparation procedure, prior to the NTs’ electrodeposition process, allowing us to produce small diameter NT arrays (∼ 30 nm). The magnetic properties of highly ordered hexagonal arrays of Ni and Co NWs and NTs are also presented. In particular, we describe a systematic comparative study on the magnetization reversal processes in ordered NW and NT arrays with different diameters (30 − 65 nm). For this study we performed analytical calculations and experimental measurements of the angular dependence of the coercivity. From the good agreement between the two, we can determine the reversal modes in NW and NT arrays. The magnetization of Ni and Co NW and small NT (< 50 nm) arrays is found to reverse by the nucleation and propagation of a transverse domain wall. On the other hand, for Ni and Co NT arrays, a non-monotonic behavior occurs above a diameter of ∼ 50 nm, revealing a transition between vortex and transverse reversal modes. The magnetic interactions between the NWs and NTs grown in the NpAT were measured using firstorder reversal curves (FORCs). This method is being currently employed for the study of interactions between individual nanoelements of an array, such as NPs or NWs. In this thesis, we used the FORC method to characterize the magnetic interactions present in Co NW and NT arrays. The diameter of the NWs/NTs was varied between 40− 65 nm, while the center-to-center distance was kept constant at 105 nm. In both NW and NT arrays, the magnetic interactions were found to increase with the diameter of the NWs/NTs, exhibiting higher values for NW arrays.