  Controlling magnetic properties through the application of an electric field
is a significant challenge in modern nanomagnetism. In this study, we
investigate the magneto-ionic control of magnetic anisotropy in the topmost Co
layer in Ta/Pt/[Co/Al/Pt]$_n$/Co/Al/AlO$_\text{x}$ multilayer stacks comprising
$n +1$ Co layers and its impact on the magnetic properties of the multilayers.
We demonstrate that the perpendicular magnetic anisotropy can be reversibly
quenched through gate-driven oxidation of the intermediary Al layer between Co
and AlO$_\text{x}$, enabling dynamic control of the magnetic layers
contributing to the out-of-plane remanence - varying between $n$ and $n +1$.
For multilayer configurations with $n = 2$ and $n = 4$, we observe reversible
and non-volatile additions of 1/3 and 1/5, respectively, to the anomalous Hall
effect amplitude based on the applied gate voltage. Magnetic imaging reveals
that the gate-induced spin-reorientation transition occurs through the
propagation of a single 90$^{\circ}$ magnetic domain wall separating the
perpendicular and in-plane anisotropy states. In the 5-repetition multilayer,
the modification leads to a doubling of the period of the magnetic domains at
remanence. These results demonstrate that the magneto-ionic control of the
anisotropy of a single magnetic layer can be used to control the magnetic
properties of coupled multilayer systems, extending beyond the gating effects
on a single magnetic layer.
