Resumen de Enhanced magnetoelectric effects in electrolyte gated nanoporous metallic alloy and dense metal oxide films
This Thesis covers the study of the magnetoelectric response in nanoporous metallic alloy and transition metal oxide dense films.
The interfacial nature of magnetoelectric processes, independently of its origin, has limited its study to ultrathin film configurations (usually 1-2 nm). Here we propose a novel approach to overcome this thickness limitation, thus achieving magnetoelectric response in materials whose overall thickness is larger than 100 nm. To accomplish this, we have employed nanoporous materials, with pore walls and ligands of very few nanometers, which are characterized by a large surface to volume ratio.
These materials have been synthesized by micelle assisted electrodeposition. Micelles get trapped during the electrodeposition process thus acting as a soft templating agent, allowing us to synthesize nanoporous copper-nickel alloy films with tunable composition and morphology.
Voltage application has been performed through electrolyte-gating, taking advantage of the generation of an electrical double layer in aprotic organic electrolytes which helps to avoid spurious oxidation processes. This method allows for the application of electric fields as high as hundreds of MV/cm. Thanks to the high electric field achieved, together with the ultrahigh surface area of nanoporous materials, a 32 % reduction in the coercivity of a Cu25Ni75 nanoporous film has been achieved. Ab-initio simulations attribute this large effect to changes in the magnetic anisotropy energy due to charge accumulation in the sample|electrolyte interface.
In a second approach, the voltage control of redox processes has been studied in aqueous electrolytes (1M NaOH). After positive bias application up to a 33 % reduction in the magnetization has been achieved in a Cu20Ni80 nanoporous sample thanks to the selective Cu oxidation. The controlled oxidation process resulted in an enriched Ni alloy which possesses a larger magnetic moment.
Moreover, we have demonstrated the suitability of atomic layer deposition to conformally coat the nanoporous alloys, preserving the morphology and structure, thus setting the basis for future solid state applications.
In the last part of this Thesis, it has been demonstrated that, upon electric field application, a ferromagnetic response arises in a paramagnetic single Co3O4 layer, at room XII temperature. The applied voltage promotes the ionic diffusion, resulting in oxygen rich and cobalt rich regions, being the latter the responsible of the induced magnetic signal.
This experiment is one of the first evidences of ionic motion at room temperature without the assistance of oxygen buffer layers such as Gd2O3 or HfO2.
