Electrochemical reactions represent a promising approach to control magnetization via electric fields. Favorable reaction kinetics have made nanoporous materials particularly interesting for magnetic tuning experiments. A fully reversible ON and OFF switching of magnetism in nanoporous Pd(Co) at room temperature is demonstrated, triggered by electrochemical hydrogen sorption. Comprehensive magnetic characterization in combination with high-resolution scanning transmission electron microscopy reveals the presence of Co-rich, nanometer-sized clusters in the nanoporous Pd matrix with distinct superparamagnetic behavior. The strong magneto-ionic effect arises from coupling of the magnetic clusters via a Ruderman-Kittel-Kasuya-Yoshida-type interaction in the Pd matrix which is strengthened upon hydrogen sorption. This approach offers a new pathway for the voltage control of magnetism, for application in spintronic or microelectromagnetic devices.We demonstrate a highly reproducible room temperature switching between magnetic states in npPd(Co) which enables full voltage control of magnetism.
The use of hydrogen atoms for magneto-ionic applications has only been explored recently. Benefits of hydrogen compared to other ionic species for tuning magnetism are high switching speed and large changes in magnetic moment. Here, we test the influence of hydrogen intercalation on magnetism in nanoporous Pd(1−x)Cox, with Co being located in superparamagnetic clusters, building upon a previously suggested material system. Tailoring the Co concentration and distribution allows the magnitude of the magneto-electric effect to be influenced as well as to gain a deeper understanding of the interaction of hydrogen with magnetic clusters. In situ magnetization measurements are conducted to directly observe the variation in magnetic moment upon hydrogen-charging in nanoporous Pd(1−x)Cox. Temperature-dependent magnetization curves show that interstitial hydrogen atoms lead to an increase in magnetic anisotropy energy, a coupling of individual Co-rich clusters, and the concomitant blocking of their magnetic moments. The large obtained magnetic switching effects upon hydrogen-charging at room temperature (αC,V > 400 Oe V−1; ΔM = 1.5 emu g−1) open up new possibilities to use magneto-ionic effects for real-life applications in magnetic devices.
In situ superconducting quantum interference device magnetometry provides insights into the electrochemical dealloying mechanism of a CoPd alloy. Charge-dependent measurements of magnetic moment allow the separation of primary and secondary dealloying contributions. Coercivity evolution revealed the transition from collective ferromagnetism to superparamagnetism of small alloy clusters evolving in the dealloying process, which is interpreted as an “inverse” magnetic percolation problem. Temperature-dependent magnetization curves enable a qualitative comparison of magnetic cluster size distributions in the nanoporous Pd framework, which are found to be strongly influenced by dealloying potential. The study underlines the potential of electrochemical dealloying as a promising method for the preparation of tailor-made magnetic nanostructures.
The mechanical strain response of nanoporous palladium (npPd) upon electrochemical hydrogenation using an in situ dilatometric technique is investigated. NpPd with an average ligament diameter of approximately 20 nm is produced via electrochemical dealloying. A hydrogen-induced phase transition from PdHβ to PdHα is found to enable internal-stress plasticity (or transformation-mismatch plasticity) in nanoporous palladium, which leads to exceptionally high strains without fracture as a result of external forces. The high surface stress in the nanoporous structure in combination with the internal-stress plasticity mechanism leads to a peculiar strain response upon hydrogen sorption and desorption. Critical potentials for the formation of PdHα and PdHβ in npPd are determined. The theoretical concepts to assess the plastic strain response of nanoporous samples are elucidated, taking into account characteristics of structure and deformation mechanism.
Electrochemical dealloying has become a standard technique to produce nanoporous network structures of various noble metals, exploiting the selective dissolution of one component from an alloy. While achieving nanoporosity during dealloying has been intensively studied for the prime example of nanoporous Au from a AgAu alloy, dealloying from other noble-metal alloys has been rarely investigated in the scientific literature. Here, we study the evolution of nanoporosity in the electrochemical dealloying process for both CoPd and AgAu alloys using a combination of in situ grazing-incidence small-angle X-ray scattering (GISAXS), kinetic Monte Carlo (KMC) simulations, and scanning transmission electron microscopy (STEM). When comparing dealloying kinetics, we find a more rapid progression of the dealloying front for CoPd and also a considerably slower coarsening of the nanoporous structure for Pd in relation to Au. We argue that our findings are natural consequences of the effectively higher dealloying potential and the higher interatomic binding energy for the CoPd alloy. Our results corroborate the understanding of electrochemical dealloying on the basis of two rate equations for dissolution and surface diffusion and suggest the general applicability of this dealloying mechanism to binary alloys. The present study contributes to the future tailoring of structural size in nanoporous metals for improved chemical surface activity.
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