  In Ta/CoFeB/HfO2 stacks a gate voltage drives, in a nonvolatile way, the
system from an underoxidized state exhibiting in-plane anisotropy (IPA) to an
optimum oxidation level resulting in perpendicular anisotropy (PMA) and further
into an overoxidized state with IPA. The IPA$\,\to\,$PMA regime is found to be
significantly faster than the PMA$\,\to\,$IPA regime, while only the latter
shows full reversibility under the same gate voltages. The effective damping
parameter also shows a marked dependence with gate voltage in the
IPA$\,\to\,$PMA regime, going from 0.029 to 0.012, and only a modest increase
to 0.014 in the PMA$\,\to\,$IPA regime. The existence of two magneto-ionic
regimes has been linked to a difference in the chemical environment of the
anchoring points of oxygen species added to underoxidized or overoxidized
layers. Our results show that multiple magneto-ionic regimes can exist in a
single device and that their characterization is of great importance for the
design of high performance spintronics devices.
