  Electric-field control of magnetism provides a promising route towards
ultralow power information storage and sensor technologies. The effects of
magneto-ionic motion have so far been prominently featured in the direct
modification of interface chemical and physical characteristics. Here we
demonstrate magnetoelectric coupling moderated by voltage-driven oxygen
migration beyond the interface limit in relatively thick AlOx/GdOx/Co (15 nm)
films. Oxygen migration and its ramifications on the Co magnetization are
quantitatively mapped with polarized neutron reflectometry under thermal and
electro-thermal conditionings. The depth-resolved profiles uniquely identify
interfacial and bulk behaviors and a semi-reversible suppression and recovery
of the magnetization. Magnetometry measurements show that the conditioning
changes the microstructure so as to disrupt long-range ferromagnetic ordering,
resulting in an additional magnetically soft phase. X-ray spectroscopy confirms
electric field induced changes in the Co oxidation state but not in the Gd,
suggesting that the GdOx transmits oxygen but does not source or sink it. These
results together provide crucial insight into controlling magnetic
heterostructures via magneto-ionic motion, not only at the interface, but also
throughout the bulk of the films.
