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On the magnetic and structural properties of Co and Co-based nanowire arrays

AuthorsGonzález Vivas, Laura
AdvisorFesenko Morozova, Oksana; Vázquez Villalabeitia, Manuel
KeywordsMagnetization reversal
Co-based nanowire arrays
Structural and magnetic properties
Issue Date22-Jun-2012
PublisherCSIC - Instituto de Ciencia de Materiales de Madrid (ICMM)
AbstractThe knowledge of magnetization reversal processes in nanoscale systems is relevant for the design of advanced technologies such as advanced magnetic recording media, spintronics and various sensing and reading devices. Magnetic nanowires can have important applications in advanced 3D recording media, logic and MEMS devices (Micro-Electro-Mechanical-Systems). Commonly, nanowires and their patterned arrays are prepared by different lithography techniques. However, the alternative electrochemical route profiting from self-assembled pores in anodic alumina templates has been proved to be very fruitful and a less-expensive reproducible method to prepare 3D ordered arrays of magnetic nanowires with tunable magnetic characteristics. The template synthesis has recently been demonstrated to be an elegant chemical approach for the fabrication of nanoscale materials and an alternative to other sophisticated methods such as molecular beam epitaxy and microlithography. Self assembled ordered hexagonal nanoporous alumina templates (AAO) are chosen in this work for the fabrication of the nanowires, as an effective mean to create customized materials with designed physical properties. AAO templates are obtained by common electrochemical processes, with the aim to control their geometry (pore diameter, length and inter-pore distance). The latter is achieved by controlling the anodization parameters. The present work deals with the magnetic study of Co and Co-based nanowire arrays prepared by template-assisted electroplating growth. The role played by the crystalline structure in the magnetic properties of Co and Co-based nanowire arrays is emphasized, and it is shown that the crystalline structure depends on the particular deposition and geometrical parameters. Modifying the electrolytic bath acidity, the pH, in the electrodeposition of Co nanowires, it is possible to induce different hcp Co phase textures, resulting in different types of magnetic behavior. Additionally, an alternative way to tune the magnetic anisotropy without extreme changes in magnetic saturation moment is through the addition of other metallic elements, such as Ni and Pd. On the other hand, by modifying the dimensions (i.e. length and diameter) of Co nanowires their crystalline structure is also modified. Particularly, for reduced lengths of nanowires, fcc-crystal phase is dominant which together with the shape anisotropy usually results in longitudinal magnetization easy axis. For longer wires, hcp-crystal phase is formed, which determines the appearance of a magnetocrystalline anisotropy with nearly transverse orientation. In this latter case there is a competition between the shape and the crystalline anisotropies. For the case of Co nanopillars, it was found that for 35 nm of pore diameter the dominant crystal fase is the fcc one. Above this diameter the crystal structure is dominated by the hcp phase and a reorientation of the c axis is observed from out of plane to in plane for a pore diameter higher than 44 nm. The controlled preparation of ordered arrays of magnetic nanowires with welldefined magnetic anisotropy constitutes a route to control the magnetization reversal process. This is discussed on the basis of analytical models, where different reversal modes, particularly propagating transverse or vortex-like domain walls were considered as alternatives to homogeneous coherent magnetization rotational mode. The analytical models were specially developed for the comparison with the experimental angular dependence of coercivity of Co and Co-based nanowire arrays. Also the temperature dependence of magnetic properties of Co and Co-based nanowire arrays were addressed in this study, with the aim to reinforce the discussion of the interplay of the different magnetic anisotropies.
DescriptionManuscript submitted to reach the degree of Ph D in Physics, on 22nd June 2012
Appears in Collections:(ICMM) Tesis
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