Non‐Oxidative Mechanism in Oxygen‐Based Magneto‐Ionics

The Ta/CoFeB/Pt/MgO/HfO2 system is investigated, whose magnetic anisotropy can be controlled through magneto-ionic gating, using both ionic liquid and solid state gating, via a non-oxidative mechanism combining reversible and irreversible gating effects. Analysis of X-ray absorption spectroscopy at the Co and Fe edges reveals no indications of oxidation after gating, while a reversible change at the oxygen K edge suggests the involvement of oxygen species in the magneto-ionic process. In addition, X-ray diffraction measurements reveal that gating can irreversibly increase the crystalline volume of MgO, through an increase in the MgO/Mg(OH)2 ratio. This is in line with measurements in solid state devices showing that in a series of 150 gating cycles a reversible effect combines with a progressive increase in the strength of the perpendicular magnetic anisotropy contribution that saturates after extensive cycling. Consequently, the observed gate-induced changes in magnetic anisotropy can be attributed to the combined effects of Mg(OH)2 dehydration into MgO (irreversible) and most likely a gentle reordering of oxygen species at the CoFeB interface (reversible) leading to a non-oxidative magneto-ionic mechanism. This study provides valuable insights into the underlying mechanisms governing the complex magneto-ionic phenomena, including the coexistence of both reversible and irreversible effects, and a pathway to voltage-control of crystalline order in spintronics materials.