Microwave devices based on unbiased tunable ferromagnetic nanowire arrays

Microwave devices are currently used in various areas of telecommunications, from wireless systems to the aerospace industry. Their rapid development over the last few years has brought some issues as the need of enlarging the working range to upper frequencies and the realization of miniaturized and highly integrated components. The purpose of this thesis is to develop and fabricate microwave devices based on ferromagnetic nanowire arrays embedded in anodic aluminum oxide (AAO) membranes. The magnetic nanowired substrates (MNWS) present several advantages over the ferrites and hexaferrites currently used in microwave devices, in terms of self-biased magnetic materials, design and operation of frequency tunable devices and planar integration. Moreover, the fabrication process, based on electrodeposition, is easy to control and it allows to vary the geometrical and material parameters. The intrinsic properties of MNWS are directly dependent on those geometric and material parameters, such as the length and diameter of the nanowires, the density of the arrays and the choice of the magnetic metal and alloy, which allows a large tunability. We successfully developed three microwave devices with outstanding properties. Non-reciprocity of the devices is achieved by taking advantage of the anisotropic permeability of the MNWS. First, a nonreciprocal microstrip topology, with specific configurations of nanowire areas in MNWS, is used as an isolator to prevent possible reflections in a extit{rf} circuit, or as a differential phase shifter used in the phase arrays antennas. Secondly, a nonreciprocal three-ports junction based on the MNWS is developed to fabricate circulators and isolators for airborne or aerospace applications. Finally, microwave planar inductors based on MNWS are a solution for the compactness and the integration requirements of microelectronics industry. Numeric optimizations on geometric parameters of the MNWS are obtained thanks to the good agreement of theoretical models with the experimental results. The MNWS were characterized using ferromagnetic resonance (FMR) measurements in order to understand their architecture-magnetic property relationship and, therefore, to pave the way for the design of tunable microwave devices. A model is proposed to explain the large shifts in the FMR absorption of Ni nanowire arrays embedded in polymer membrane due to variations in temperature. Moreover a two-stage strain mechanism is proposed for Ni nanowires grown in small diameter AAO template to understand the observed unexpected enhanced magnetic anisotropy. Finally, we present a new route for the fabrication of magnetic-ferroelectric nanocomposites and an electric-field control of the magnetic properties and resonance frequencies of the MNWS.