Self-polarized magnetic nanocomposites based on ferrimagnetic nanowires for non-reciprocal microwave devices

Résumé

This thesis work aim to develop a self-biased coplanar circulator. The chosen approach is based on the utilization of composite substrates based on self-polarized ferrimagnetic nanowires. It consists of integrating and orienting cobalt ferrite (CoFe2O4) nano-crystallites in porous alumina membranes using the technique of electrodeposition under magnetic field. Ferrimagnetic CoFe2O4 nanocomposites have been synthesized and characterized by several techniques. The performed SQUID-VSM measurements have revealed that the nanocomposite electrodeposited and cooled under a magnetic field of 0.6 T has the best magnetic properties, i.e. a large anisotropy field (Ha), moderate coercive field (Hc), and partial squareness ratio (Mr/Ms). This nanocomposite is selected as an interesting candidate for the fabrication of a miniaturized self-biased circulator. The study has started with the implementation of an analytical dimensioning process based on Bosma's theory, in order to model a stripline circulator under MATLAB software. The main geometrical dimensions generated are then transposed to the 3D coplanar structure using the HFSS numerical simulator. Based on interesting simulation results, a first prototype was fabricated in the clean room and characterized at high frequencies. The measurements of the S-parameters have highlighted a circulation phenomenon, which remains very weak due to the imperfect orientation of the CoFe2O4 nano-crystallites within the pores of the AAO membrane.

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