Multiple magneto-ionic regimes in Ta/Co\(_{20}\)Fe\(_{60}\)B\(_{20}\)/HfO\(_{2}\)

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.