Control of the magnetic anisotropy in multirepeat Pt/Co/Al heterostructures using magnetoionic gating

Controlling magnetic properties through the application of an electric field is a significant challenge in modern nanomagnetism. In this study, we investigate the magnetoionic control of magnetic anisotropy in the topmost Co layer in $\mathrm{Ta}$/$\mathrm{Pt}$/[$\mathrm{Co}$/$\mathrm{Al}$/$\mathrm{Pt}$]${}_n$/$\mathrm{Co}$/$\mathrm{Al}$/$\mathrm{Al}\mathrm{O}$${}_x$ multilayer stacks comprising $n+1$ Co layers and its impact on the magnetic properties of the multilayers. We demonstrate that the perpendicular magnetic anisotropy can be reversibly quenched through gate-driven oxidation of the intermediary $\mathrm{Al}$ layer between $\mathrm{Co}$ and ${\mathrm{Al}\mathrm{O}}_x$, enabling dynamic control of the magnetic layers contributing to the out-of-plane remanence—varying between $n$ and $n+1$. For multilayer configurations with $n=2$ and $n=4$, we observe reversible and nonvolatile additions of 1/3 and 1/5, respectively, to the anomalous Hall-effect amplitude based on the applied gate voltage. Magnetic imaging reveals that the gate-induced spin-reorientation transition occurs through the propagation of a single ${90}^{\circ }$ magnetic domain wall separating the perpendicular and in-plane anisotropy states. In the five-repetition multilayer, the modification leads to a doubling of the period of the magnetic domains at remanence. These results demonstrate that the magnetoionic control of the anisotropy of a single magnetic layer can be used to control the magnetic properties of coupled multilayer systems, extending beyond the gating effects on a single magnetic layer.

Figure 1

(a),(b) Schematic of the (a) Ta/Pt/[Co/Al/Pt]${}_n$/${\mathrm{Co}/\mathrm{Al}/\mathrm{Al}\mathrm{O}}_x$ multilayer stacks and (b) the magnetoionic devices allowing for the control of the magnetic anisotropy in the topmost Co layer. (c),(d) Schematics of voltage-gate control of the oxidation front in the ${\mathrm{Al}/\mathrm{Al}\mathrm{O}}_x$ capping layers, which can be displaced downward or upward using negative (c) or positive (d) gate voltages applied to the top ITO electrode.

Figure 2

(a) Out-of-plane hysteresis curves obtained by Kerr microscopy for ${\mathrm{Ta}/\mathrm{Pt}/\mathrm{Co}/\mathrm{Al}/\mathrm{Al}\mathrm{O}}_x$ multilayer stacks, where the nominally deposited Al capping-layer thickness $y$ ranges from 0.6 to 1 nm, inducing significant changes in the magnetic anisotropy. (b) Normalized Kerr signal for the $\mathrm{Ta}$/$\mathrm{Pt}$/$\mathrm{Co}$/$\mathrm{Al}$(0.8 nm) stack in the as-grown state, and after the application of gate voltages of $-1.5$ V for 5 min and subsequently $+3$ V, for 5 min showing that gate voltages produce an equivalent effect in anisotropy as a variation in $y$. The inset in (b) shows the same data at higher magnetic fields, providing a better view of the saturation field after positive gate voltage. (c),(d) Normalized remanent Kerr signal (c) and saturation field ${\mu }_0{H}_{\text{sat}}$ (d) in the out-of-plane direction as a function of $y$. For the sample with $y=0.8$ nm, the values after negative ($-1.5$ V) and positive ($+3$ V) gate-voltage applications are also indicated. The results corresponding to a reference sample of Ta/Pt/Co/Al(3 nm)/Pt are shown as a dotted curve in (a), and as gray areas in (c),(d).

Figure 3

(a) Magnetoionic device composed of a ${\text{Ta/Pt/[Co/Al/Pt]}}_2{\text{/Co/Al/AlO}}_x$ ($y=0.8$ nm) multilayer stack. (b) Anomalous Hall resistance $\mathrm{\Delta }R=R-R_0$, measured in the as-grown state and after the application of alternating negative and positive gate voltages. (c) Evolution of the normalised remanent Hall resistance $\mathrm{\Delta }{R}_{\text{rem}}/R_{\text{sat}}$ with the gate voltage application time $t$ for positive and negative gate voltages. (d) OOP normalized magnetisation curves obtained by Kerr microscopy for the as-grown state, as well as after the application of positive and negative gate voltages. These later measurements are performed after the removal of the gate.

Figure 4

(a)–(c) Kerr microscopy images of the domain configuration during the application of positive gate voltage of $+2.5$ V during 5 min (a), 7 min (b), and 10 min (c), showing the transition behavior of the ${\text{Ta/Pt/[Co/Al/Pt]}}_2{\text{/Co/Al/AlO}}_x$ ($y=0.8$ nm) multilayer stack. The bottom insets show an enlargement at the domain wall between PMA and IP magnetic anisotropy domains in the topmost Co layer moving at a velocity of $61\text{μ}\mathrm{m}/$min. (a) The schematic magnetization configuration of the three Co layers is displayed over each different Kerr contrast. (c) Due to the progression of the IP domain compared to the fixed OOP ones, some of the contrast observed in the right-most part of the image appears on the other side of the large OOP domain with magnetization pointing down in the middle of the image. (d) Hysteresis loops measured at the top-down corner (blue), and over the entire field of view (red) of the image in the inset. The inset in (d) shows an extended view of the magnetic state in (c) at remanence.

Figure 5

(a) Magnetoionic device composed of a ${\text{Ta/Pt/[Co/Al/Pt]}}_4{\text{/Co/Al/AlO}}_x$ ($y=0.8$ nm) multilayer stack. (b) Anomalous Hall resistance $\mathrm{\Delta }R=R-R_0$ measured after the application of several gate voltages. (c) Kerr microsopy images taken at the boundary between the as-grown (bottom) and gated (top) areas under an OOP magnetic field of 9 mT showing the propagation of domains from the as-grown region into the gated region after 12, 14, 17, and 23 s. (d),(e) Kerr microsopy images of the final domain pattern at the boundary between the as-grown (bottom) and gated (top) areas obtained under an OOP magnetic field of 9 mT (d) and 18 mT (e). The inset in (d) shows the continuity of the domain pattern at the boundary.