Multiple magneto-ionic regimes in Ta/Co${}_{20}$Fe${}_{60}$B${}_{20}$/HfO${}_2$

Abstract

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.