Controlling interface anisotropy in CoFeB/MgO/HfO2 using dusting layers and magneto-ionic gating

Abstract

In this work, we present the magneto-ionic response to ionic liquid gating in Ta/CoFeB/MgO/HfO2 stacks, where heavy metal dusting layers of Ta, W, and Pt are inserted at the Ta/CoFeB and CoFeB/MgO interfaces. Dusting layers of W inserted at the Ta/CoFeB interface increase perpendicular magnetic anisotropy (PMA) by more than 50%, while no significant changes are seen for Pt. In these samples, gating cannot break the PMA seeded at the CoFeB/MgO interface, only relatively small changes in the coercivity can be induced, about 20% for Ta and Pt and 6% for W. At the CoFeB/MgO interface, a significant quenching of the magnetization is seen when W and Ta dusting layers are inserted, which remains unchanged after gating, suggesting a critical deterioration of the CoFeB. In contrast, Pt dusting layers result in an in-plane anisotropy that can be reversibly converted to PMA through magneto-ionic gating while preserving the polycrystalline structure of the MgO layer. This shows that dusting layers can be effectively used not only to engineer magnetic properties in multilayers but also to strongly modify their magneto-ionic performance.

Non‐Oxidative Mechanism in Oxygen‐Based Magneto‐Ionics

“…As previously reported, [5] a clear spin reorientation transition from a dominant IPA to a dominant PMA is evidenced after applying negative gate voltages to the top ITO electrode of the gate, and the PMA state that is obtained can be subsequently weakened by applying positive gate voltages. This behavior is shown in Figure 1b through anomalous Hall effect (AHE) measurements conducted under a magnetic field perpendicular to the sample plane (μ 0 H z ).…”

“…The IPA state achieved after the application of positive gate voltages evolves toward a state with increased PMA over the following days. [ 5 ] In the present case, as the gate‐induced changes in magnetic properties occur via a non‐oxidative process, a relatively weak interaction between mobile oxygen species and the magnetic atoms is expected, which could naturally lead to the time evolution effects observed. The analysis of the Oxygen K‐edge reveals that the reversible gating effects seen in the magnetic anisotropy are well reflected in the chemical environment of the oxygen atoms in the layer, despite the absence of any features in the Co and Fe edges, confirming a mechanism that is oxygen‐based but non‐oxidative in nature.…”

“…It was shown in a previous study that in this material the MgO layer is textured before and after ionic-liquid gating. [5] The presence of Pt has been found to be crucial to the appearance of crystalline order in the MgO layer, as layers without a Pt dusting layer show an amorphous structure in transmission electron microscopy measurements (TEM, see Supporting Information).…”

“…It has been seen in TEM measurements that in both as‐grown DL and gated samples the HfO 2 layer does not show any crystalline structure. [ 5 ] Therefore, any HfO 2 ‐related diffraction signal is expected to be broad and to come from relatively small crystallites. This broad feature is still visible, with minor differences, in the integration curves taken at CA.…”

“…We investigate the Ta/CoFeB/Pt(dusting layer)/MgO/HfO 2 system, where the Pt dusting layer (DL) between CoFeB and MgO largely suppresses the oxidation naturally happening at this interface during growth. The presence of the dusting layer, in consequence, weakens the magnetic anisotropy that builds at the CoFeB/MgO interface, so that a spin‐reorientation transition can be induced using gate voltages [ 5 ] in both ionic liquid gating and solid state gating devices. The Pt dusting layer also favors the crystallization of MgO during growth.…”