Low‐Voltage Controlled Coercivity in Nanocrystalline Fe‐Ni Films by Magneto‐Ionic Effects

ABSTRACT Magneto‐ionic control of metal oxide/metal films provides a pathway to voltage‐tunable magnetoelectronic devices with high energy efficiency. So far, magneto‐ionic research mainly focuses on Co‐based films, while Fe‐Ni alloy films, despite their high industrial relevance, have not been studied systematically. In this work, a combined in situ Kerr microscopy and electrochemical analysis demonstrates magneto‐ionic control of coercivity in nanocrystalline Fe‐Ni alloy films across the whole compositional range. The required voltage is low (∼1 V) and decreases with increasing Ni content, presumably relating to the nobler nature of Ni versus Fe. For Fe‐rich alloys, a large voltage‐induced change of coercivity and remanence is connected to an oxide‐to‐metal transformation, reducing domain wall pinning. For intermediate compositions, the magneto‐ionic effects are largest. Here, the potential induces a moderate increase, followed by a drastic reversible decrease in coercivity by ∼ −90%. This behavior is attributed to the enhanced electrochemical reactivity of ultrafine grains and the heterogeneous oxide present on mixed bcc/fcc Fe‐Ni films. For Ni‐rich films, the magneto‐ionic effects are small, but voltage‐induced magnetic softening is still achieved. The study introduces Fe‐Ni films as a promising magneto‐ionic material platform and highlights the potential of tailored, defect‐rich microstructures for boosting magneto‐ionic performance.