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Arrays of interacting nanostructures: Ferromagnetic and superconducting cases
Arrays of ultra high-density (1.2 Tera/in2) vertical ferromagnetic cobalt nanowires with aligned shape and crystal magnetic-anisotropy axes are fabricated by electro-deposition in nanoporous polymer templates. The nanoporous films are derived from self-assembling PS-b-PMMA diblock-copolymers, which can provide lateral structure dimensions on the order of 10 nm, small enough to make cobalt nanowires in magnetic single-domain regime. An optimized fabrication procedure is developed for arrays of cobalt nanowires with enhanced perpendicular-to-plane magnetic anisotropy, by combining effects of shape anisotropy with perpendicularly oriented uniaxial magneto-crystalline anisotropy of hcp Co. Special conditions of electrolyte pH (pH>5.1) are required to obtain the desired c-axis crystal orientation, and pulse electrodeposition conditions are found to improve the overall perpendicular magnetic anisotropy, resulting in array-coercivities as large as 2.7 kOe at 300 K. X-ray diffraction and SQUID magnetometry measurements are used to characterize their structural and magnetic properties. A numerical model for the magnetization dynamics of an array of single-domain particles with vertically-aligned anisotropy axes is presented, which includes finite temperature effects, and magnetostatic interactions in a mean-field approximation. The model can qualitatively describe most features of the magnetization behavior, such as the temperature dependence of array-coercivity, "shearing" of the magnetization curves and the reduction of array-coercivity due to array demagnetization effects. Qualitative agreement is also obtained with the observed slow-relaxation magnetization decay of the cobalt nanowire arrays. To investigate particle non-uniformity effects, the model was further extended to include a distribution in energy barriers and particle volumes. The last part is concerned with the experimental investigation of the collective behavior of arrays of superconducting lead (Pb) nanowires with diameters smaller than the coherence length, which are coupled to each other by Andreev reflection at the S-N point contact interfaces with an underlying normal metal film. The system is characterized by magnetization and electrical transport measurements and is found to behave like an effective-medium type II superconductor with vortex pinning.
Ursache, Andrei E, "Arrays of interacting nanostructures: Ferromagnetic and superconducting cases" (2007). Doctoral Dissertations Available from Proquest. AAI3275777.