Dual-Ion Based Magneto-ionic Effects in Nanoporous Pd₇₅Co₂₅ Alloy

Magneto-ionics is an energy-efficient approach to control the magnetic properties of a material by the application of a voltage. For instance, a voltage-induced switching between a ferromagnetic ON state and a magnetic OFF state can be achieved in Co by the reversible reduction/formation of Co (hydr-)-oxides. In this work, we demonstrate that, in addition to this magneto-ionic effect based on oxygen species, a magneto-ionic effect based on hydrogen can be obtained by changing from pure Co to a Pd-Co alloy. To this end, we prepared a nanoporous Pd₇₅Co₂₅ alloy with a porous structure in the 10 nm range by dealloying of Al₈₀(Pd₇₅Co₂₅)₂₀. Initially, the nanoporous alloy is mostly oxidized and thus generates only a very weak magnetic signal, corresponding to an OFF state. Inducing the reduction of the Co (hydr-)-oxides by applying a voltage results in a transition to a ferromagnetic ON state. Due to the macroscopic sample size, the ON state exhibits a considerable absolute magnetic moment in the order of 0.01 emu. This moment can be obtained within just a few minutes thanks to the nanoporous structure. A detailed electrochemical characterization reveals that the magnetization of the alloy is also significantly influenced by the absorption of hydrogen into the nanoporous material, which becomes apparent through an additional increase in the magnetization during the hydrogen desorption. This second magneto-ionic effect is faster and occurs within the time frame of seconds. Overall, the occurrence of these two magneto-ionic effects, which are based on different kinds of ionic species, opens up the possibility to adapt the magneto-ionic response by adjusting the composition of the Pd-Co alloy.