In-situ magnetometry of nanoporous metals

This project is dealing with magnetic studies of nanoporous metals and alloys made by so-called dealloying, an (electro-)chemical process in which the less noble component is removed from an alloy by selective etching. Their high surface-to-volume ratio makes this class of materials not only attractive for applications such as sensing or catalysis but also predestined for property tuning by electrochemical charging. The idea is that by imposing high surface charge densities at surface- electrolyte interfaces, overall macroscopic materials properties, such as the magnetic moment, can be modified in a controlled manner. Based on our recent progress on the development of in-situ techniques for studying variations of magnetic moments with high sensitivity in a magnetometer under full control of ongoing electrochemical processes, the focus of the present project is on in-situ magnetometry of nanoporous metals during dealloying and charging. Palladium and Pd-rich Pd-Co alloys are chosen as model system, because owing to the electronic properties of Pd a particular high sensitivity of the magnetic moment is anticipated with respect to charging. The project aims to answer the question of the primary mechanism which underlies the charging- induced process. The challenging question will be tackled whether magnetism can be made switchable, i.e., whether a charging-induced transition between the paramagnetic and ferromagnetic state can be accomplished by proper adjusting the chemical composition of Pd-Co. The in-situ method will further be used to study the dealloying process itself by continuously magnetic monitoring the electrochemical dissolution of Co. For the purpose of reference, comparative studies on well-defined thin-film model system will be performed. The project is breaking new ground in research, as well as with respect to the class of materials, the scientific questions, and the applied methods. The planned in-situ studies will contribute to a better understanding of the dealloying process of this highly interesting class of materials. Particular attractive with respect to magnetism are the prospects of switchable magnetism by an applied voltage. An important method-oriented aspect of this project will be the successful further implementation and application of the magnetic in-situ measuring technique as well as its extension to novel issues, such as in-situ dealloying. This kind of in-situ magnetic monitoring of electrochemical processes at surfaces and interfaces offers widespread applications in innovative materials science, ranging from ultrathin magnetic films to energy materials, such as battery materials.

In-situ magnetometry of nanoporous metals during dealloying and charging This project succeeded in showing that the magnetism of a nanoscale highly porous alloy can be reversibly switched by electrochemical loading and unloading of hydrogen. Voltage-control of magnetism has become an attractive research field as it offers the perspective of an energy-efficient switching of magnetisation which is of high interest for many applications. A promising candidate of magneto-electric materials for that purpose are magneto-ionic material systems, which affect magnetic properties via voltage-controlled electrochemical reactions. High-surface area electrodes are beneficial for such an approach, as a large fraction of atoms is affected by chemical surface reactions, while also kinetics for reactions, such as hydrogen incorporation, is enhanced. Particular well suited for that purpose are metals with a high fraction of nanoscale porosity, so-called nanoporous metals. Those are prepared by electrochemical dealloying synthesis, a selective dissolution process of one component from an alloy. Composition, pore size, and specific surface area of the nanoporous electrodes can be adjusted in the manufacturing process. With this project nanoporous, dealloyed metals are introduced in the field of magneto-ionics. As material system Co-rich CoPd alloy was chosen, on the hand, because the magnetic properties of nanoporous Pd prepared from that can be adjusted by the remaining residual Co concentration after dealloying. On the other hand, because the nanoporous Pd matrix provides an efficient host lattice for hydrogen atoms, which can be loaded electrochemically from aqueous electrolytes providing the basis for a magneto-ionic tuning effect. With respect to the method, this project is based on the special expertise developed at the involved institutes in studying variations of magnetic moments with high sensitivity in a magnetometer under full control of ongoing electrochemical processes, so-called in-situ magnetometry. One project part is devoted to the electrochemical dealloying process itself, in particular by studying the evolution of the magnetic properties in-situ in a magnetometer. It turns out that the nanoporous Pd-structure contains Co-rich clusters, which exhibit a particular magnetic, a so-called superparamagnetic behaviour. The cluster size can be adjusted by the production process and by heat treatment. In the central project part, the magnetic tuning by electrochemical charging was studied. By loading and unloading of hydrogen a fully reversible magneto-ionic switching effect of 100% could be achieved. As such a phenomenon has not been observed in the literature so far, all influences on the magnetic properties were carefully analysed and finally a novel magneto-ionic switching mechanism was presented. This mechanism is based on the magnetic coupling between the superparamagnetic clusters, the strength of which increases with hydrogen between the clusters. The presented novel magneto-ionic switching scheme opens up highly interesting future perspectives.