Resumen de Síntesi i propietats magnetoelèctriques de materials porosos basats en co-pt
This Thesis dissertation focuses on the electrochemical synthesis and investigation of the magnetoelectric properties of cobalt-platinum (Co–Pt) based porous materials. These materials are expected to minimize heat dissipation and power consumption in magnetically actuated devices.
Owing to the interfacial nature of magnetoelectric effects, porosity, roughness and nanostructuration in Co–Pt based materials are expected to promote or enhance their magnetoelectric response due to the increase in the surface-to-volume ratio. For this reason, electrodeposition from aqueous solutions containing P-123 micelle assemblies (referred to as micelle-assisted electrodeposition) was employed to fabricate mesoporous Co–Pt based materials consisting of Co–Pt alloy and Co oxides (Co–Pt+CoxOy). In combination with photolithography and the atomic layer deposition (ALD) of HfOx and AlOx nanolayers, arrays of circular microstructures and heterostructured films, respectively, were prepared. The magnetoelectric properties of the materials were studied by an electrolyte-gating approach in a polar, water-free, organic solvent. This approach exploits the generation of an electric double layer to create large electric fields at the electrolyte/sample interface.
A reduction of coercivity (by 88%) and an increase of Kerr signal (by 60%) were obtained at room temperature when mesoporous Co–Pt+CoxOy arrays of micron-sized disks were subject to electric field. The observed voltage-induced variations were attributed to charge accumulation at the surface of the ultranarrow pore walls of the mesoporous microdisks and voltage-drive oxygen ion migration (i.e., magneto-ionic effects).
In nanostructured Co–Pt+CoxOy/HfOx and Co–Pt+CoxOy/AlOx heterostructures, coercivity and magnetic moment at saturation were effectively modulated after biasing the heterostructured films with negative and positive voltages. The observed electric-field induced changes were ascribed to oxygen migration through the Co–Pt+CoxOy/ALD oxide interface.
In parallel, the impact of electrolyte processing on the mesoporosity of fully metallic Co–Pt thin films was investigated. A parametric study using various baths revealed that previous dissolution of the hexachloroplatinate salt and their storage for a few days before the other chemicals (the cobalt salt and the P-123 block-copolymer surfactant) are added is critical for the reproducible formation of the mesoporous network.
Finally, electrodeposition on colloidal crystal templated substrates was pursued to manufacture hard magnetic macroporous Co–Pt films. As-deposited films exhibited tightly packed pores of 200 nm in diameter (which matched the size of the parent colloids) and were structurally composed of a mixture of face-centered cubic A1-disordered nearly equiatomic Co‒Pt solid solutions. Upon annealing, partial transformation from A1-disordered into tetragonal L10-ordered phase was achieved, resulting in a significant increase of coercivity from 148 Oe to 1328 Oe. Remarkably, the macroporosity in the films was preserved after the thermal treatment.
